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Changes in the structure and physiology of a perennial ryegrass sward when released from a continuous stocking management: Implications for the use of exclusion cages in continuously stocked swards
Abstract
A perennial ryegrass sward was managed by continuous stocking with sheep (April–September) for 4 successive years after sowing. The sward was grazed to maintain a leaf (lamina) area index (LAI) close to 1.0. Areas of the sward were released from grazing on three occasions: once during summer in the third year after sowing, and twice during spring and summer in the fourth year after sowing. There were marked changes in the structure and physiology of the continuously stocked sward following release from grazing. After several successive years of continuous stocking, the sward comprised a large population of small tillers and the small LAI resulted in consistently low rates of photosynthesis. Following release from grazing, photosynthesis increased markedly as the LAI increased but this change was associated with the loss of a large proportion of the population of tillers. There were seasonal differences in the pattern of changes in photosynthesis and tiller numbers following release from grazing which were not apparent under continuous stocking. The changes in the structure and physiology of the sward following release from grazing suggest that the net accumulation of herbage in areas of sward from which the animals are excluded, for instance using cages, may be an unreliable estimate of production under continuous stocking.
... It has been argued that morphological and physiological characteristics of the pasture canopy may be excessively modified when the continuously stocked canopy (or a portion of it) is excluded from grazing. As a result, the FA in these excluded areas might not be a reliable estimate of the FA of the pasture (Parsons, Collett, & Lewis, 1984). ...
... It has also been proposed that the canopy condition at the time of cage placement may underestimate or overestimate FA (Barker et al., 2010;Carvalho et al., 2000;Parsons et al., 1984;Sbrissia et al., 2001). Evaluating Cynodon pastures kept at 5-, 10-, 15-, and 20-cm canopy heights, Carvalho et al. (2000) and Carnevalli et al. (2001) estimated the FA of 'Tifton' 85 and 'Coastcross-1' bermudagrasses (Cynodon spp.), respectively, and argued that the use of this technique in short canopies results in a rapid increase in the size of protected tillers during the exclusion period before the onset of intraspecific competition and the consequent reduction tiller population density (TPD), resulting in overestimated FA. ...
... This suggests that lesser TPD would be expected inside the cages, similar to what was measured on pasture in the 30-cm canopies (Table 3). In contrast with the results of Parsons et al. (1984), who reported that the use of exclosure cages results in a decrease in TPD on perennial ryegrass (Lolium perenne L.) pastures, this was not found in the present study. This contrast may be related to length of the exclusion period, since the results of Parsons et al. (1984) were from 42-and 50-d exclusion periods. ...
Forage accumulation (FA) is a key response in grazing experiments, and estimates of FA in continuously stocked pastures are often obtained using exclosure cages. This study evaluated how 14‐, 21‐, and 28‐d exclusion periods using cages affected morphological, physiological, and morphogenetic responses of continuously stocked ‘Mulato II’ hybrid brachiariagrass (Brachiaria spp. syn. Urochloa spp.) pastures maintained at 20‐ and 30‐cm canopy height and how this affected estimates of FA. Grazing exclusion resulted in greater rates of leaf elongation (LER; 1.62 vs. 1.37 cm tiller⁻¹ d⁻¹), stem elongation (SER; 0.16 vs. 0.07 cm tiller⁻¹ d⁻¹), leaf growth (101 vs. 86 kg dry matter [DM] ha⁻¹ d⁻¹), and stem growth (3.4 vs. 1.8 kg DM ha⁻¹ d⁻¹) inside the cages compared with outside. Greater LER resulted in greater final leaf length and leaf area index (LAI) in longer exclusion periods. With greater LAI, photosynthesis was greater within the exclosure cages than on pasture (54.5 vs. 43.9 μmol CO2 m⁻² s⁻¹). Greater leaf growth rate, together with same tiller population density and leaf senescence rate (LSR) across exclusion periods, resulted in overestimated FA by 14, 26, and 24% for 14‐, 21‐, and 28‐d exclusions, respectively. There was no effect of canopy height on FA, because greater LER and SER values in 30‐cm canopies were compensated by greater LSR. Choice of sites for cage placement by visual appraisal and reduced number of sampling sites resulted on “negative” values of calculated FA for the 14‐d exclusion, making the choice of sampling sites a major source of error in estimation of actual FA. Increasing the number of sampling sites and using indirect measurements of forage mass (FM) may increase the accuracy of FM and FA estimates.
... Microclimate differences between inside and outside the cages can also occur because exclosure cages allow the growth of vegetation that was previously subjected to grazing, likely impacting the canopy structure. Parsons et al. (1984) reported an increase in leaf proportion in the biomass, in canopy leaf area index (LAI), and in canopy photosynthesis caused by exclosure cages on perennial ryegrass canopies maintained with LAI close to 1. Considering that in photosynthesis, water is lost from the leaf to the atmosphere; the exclusion from grazing may increase the relative humidity (RH) in caged vegetation. Water has high specific heat (amount of energy needed to change temperature), and increased transpiration may reduce the air temperature (T) inside the cage. ...
... Errors in the estimate of biomass accumulation from the use of exclosure cages have been associated with the canopy condition (e.g., canopy height) at the time of exclusion, supposedly because during the exclusion period morphophysiological changes in the canopy depend on the vegetation condition at the moment of cage placement (Parsons et al. 1984;Carvalho et al. 2000;Carnevalli et al. 2001;Barker et al. 2010). We found no studies addressing the issue of whether these possible errors are also associated with microclimate changes inside the exclosure depending on the canopy condition at the time of exclusion. ...
Exclosure cages are often used for estimating biomass accumulation on continuously stocked pastures in grazing experiments. The microclimate inside the cages may affect the estimates of biomass accumulation, but this has not been previously identified or quantified. We evaluated how the exclusion from grazing for 21 days in Mulato II brachiariagrass (Brachiaria brizantha × Brachiaria decumbens × Brachiaria ruziziensis) pastures affected canopy air temperature (T) and relative humidity (RH) and how this related to biomass accumulation. We also evaluated the effect of the exclosure cage on wind speed (WS) and incoming solar radiation (SR), and how these impacted evapotranspiration (ET) and estimates of biomass accumulation on grazed canopies maintained at 20- and 30-cm height under continuous stocking. Regardless of canopy height, changes in canopy structure during the exclusion period up to 21 days did not affect T and RH (averages of 24.3 °C and 88.7%, respectively), indicating that the air circulation was not affected by the exclusion. The cage structure reduced SR by 5%, although there were times during clear days when SR was slightly greater inside the cage than outside. The cage also reduced WS by 4.4%. Smaller SR and WS resulted in less ET inside the cages than outside, although with close values (2.9 vs. 3.0 mm day−1; P = 0.0494). The biomass accumulation rate was greater inside than outside the cages for both canopy heights. This overestimation would be 5.8 and 9.7% greater if the structure of the cage did not reduce the SR, WS, and ET.
... The clipping sequence was designed to encompass both the growing season and the main periods of livestock use throughout a whole year, and is detailed fully in Omer (2003). It is known that the presence of the cage can alter net herbage accumulation, as the exclusion of grazing affects plant growth rates (Parsons et al, 1984;Frame, 1993;Hopkins, 2000). Therefore the time intervals selected between clippings were designed to be long enough to obtain measurable differences in vegetation growth, but short enough to minimize the effect of differential growth in the paired grazed and ungrazed plots (normally > 4 weeks ;Omer, 2003). ...
... Where use was very low (during summer in Bunji and Minapin), high spatial variability in total biomass and use between individual caged and uncaged sample quadrats resulted in negative mean values. This is a common problem in the use of the paired cage plot technique where vegetation use is low and vegetation is highly heterogeneous (Parsons et al, 1984;Frame, 1993;Hopkins, 2000). ...
... The clipping sequence was designed to encompass both the growing season and the main periods of livestock use throughout a whole year, and is detailed fully in Omer (2003). It is known that the presence of the cage can alter net herbage accumulation, as the exclusion of grazing affects plant growth rates (Parsons et al, 1984;Frame, 1993;Hopkins, 2000). Therefore the time intervals selected between clippings were designed to be long enough to obtain measurable differences in vegetation growth, but short enough to minimize the effect of differential growth in the paired grazed and ungrazed plots (normally > 4 weeks ;Omer, 2003). ...
... Where use was very low (during summer in Bunji and Minapin), high spatial variability in total biomass and use between individual caged and uncaged sample quadrats resulted in negative mean values. This is a common problem in the use of the paired cage plot technique where vegetation use is low and vegetation is highly heterogeneous (Parsons et al, 1984;Frame, 1993;Hopkins, 2000). ...
Construction of the Karakoram Highway (KKH) connecting Pakistan with China has brought about changes in the economic, social and cultural systems of the Northern Areas of Pakistan. The authors examined the impacts of this development on pasture resource use by comparing a transect of villages connected to the KKH with a transect still relatively remote within the Gilgit–Ghizer region. Patterns of livestock density and forage use were shown to differ between individual villages, but no direct impacts of the KKH communication infrastructure development were found. It is hypothesized that, although regional communication infrastructure changes facilitate many other social changes, their impacts on livestock systems are less direct than is commonly assumed, due to individual differences in rates of social change between villages, leading to a diversity of change within the livestock systems in the region.
... The size and shape of cage, type of mesh, length of time the sward is protected, and the time of year, all influence the micro-environmental conditions within the cages (e.g. temperature, air flow, humidity and light intensity), which determine the rates of production, senescence, litter fall and decomposition, and hence affect net herbage accumulation (Parsons et al., 1984;Frame, 1993). The rate of growth and structural characteristics of swards under continuous stocking differ from those of swards released from grazing (Parsons et al., 1984;Frame, 1993). ...
... temperature, air flow, humidity and light intensity), which determine the rates of production, senescence, litter fall and decomposition, and hence affect net herbage accumulation (Parsons et al., 1984;Frame, 1993). The rate of growth and structural characteristics of swards under continuous stocking differ from those of swards released from grazing (Parsons et al., 1984;Frame, 1993). By removing large herbivores is not only the effect of grazing removed, but also the effects of trampling, dung deposition and urine input are also removed (Brown and Evans, 1973;Frame, 1976;Frame, 1993). ...
The aims of this study were to examine the effect of three grazing treatments (year-round stocking rates of 0·8 ewes ha−1, 0·5 ewes ha−1 and 0·5 ewes ha−1 plus grazing cattle in summer), imposed for 4 years, on the herbage mass and surface height of a Nardus stricta-dominated grassland in western Scotland and to obtain estimates of annual productivity of this grassland. Nardus stricta-dominated grassland comprised proportionately 0·20 of the grazing area. Stocking rate of sheep had no significant effect on the herbage mass of the grassland in the first 2 years of the experiment, although mean summer pasture heights were significantly higher under the lower stocking rate of sheep. The pasture on the treatment with cattle grazing in summer had a significantly lower herbage mass and lower surface height than the two sheep-only grazing treatments. Year-to-year variation in the herbage mass and surface height of herbage in summer was greater than the effect of treatments. Despite changes in surface height, the structural diversity of the grasslands was not increased by the treatments. The annual production of vascular plant material ranged from 417 g DM m−2 in 1994 to 628 g DM m−2 in 1996.
... Because measurement of HA occurred every 2 wk instead of weekly, it is possible that use of the cage tech- nique may have inflated HA somewhat for the 1-wk, 4-cm SH treatment ( Parsons et al., 1984). However, their work may not apply directly to the current study because continuously stocked perennial ryegrass (Lolium perenne L.) was used, and herbage mass was quantified in exclusion cages released for 5-wk periods from long-term contin- uous stocking. ...
Forage–livestock systems in the southeastern United States are based on N-fertilized perennial grass pastures, with minimal legume contribution. The legume rhizoma peanut (RP, Arachis glabrata Benth.) can persist and spread in grazed mixtures with C4 grasses, and Ecoturf RP is of particular interest because it is relatively decumbent and may vary its growth habit in response to defoliation management. The objective was to quantify effects of grazing frequency and intensity of Ecoturf on herbage accumulation (HA), canopy characteristics, storage organ mass, and total nonstructural carbohydrate (TNC) concentration. Treatments were the factorial combinations of three levels of regrowth interval (RI; 1, 4, and 7 wk) between grazing events and two levels of post-grazing stubble height (SH; 4 and 8 cm). For the 4-cm SH, HA increased from 8.4 to 10.5 Mg ha⁻¹ in 2015 and from 9.8 to 13.6 Mg ha⁻¹ in 2016 as RI decreased from 7 to 1 wk. The effect of RI was less pronounced for the 8-cm SH. When grazed to a 4-cm SH, herbage bulk density and post-grazing leaflet mass increased linearly as RI decreased from 7 to 1 wk in both years. Changes in root-rhizome mass and TNC pool during 2 yr of grazing were not affected by SH or RI. Ecoturf adapted to frequent, close grazing by increasing herbage bulk density and positioning leaves close to the soil surface, allowing rapid regrowth after defoliation without depleting reserves. Ecoturf is tolerant of a range of grazing strategies, showing promise for use in pastures. © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved.
... As expected, herbaceous standing biomass (which did not include oak biomass) was significantly higher in the grassland than in the oak savanna (312 g m À2 vs. 130 g m À2 , P , 0.01). These amounts are lower than the true primary production because squirrels had access to cage interiors and because cages generally provide relative rather than absolute estimates of primary production (Parsons et al. 1984). ...
Understanding the impacts of livestock grazing on wildlands is important for making appropriate ecosystem management decisions. Using livestock exclosures, we examined the effects of moderate cattle grazing on the abundance of California ground squirrels (Spermophilus beecheyii Richardson) and the spatial distribution of active burrows within their colonies in grassland and blue oak (Quercus douglasii Hook. Arn.) savanna habitats in the coastal range of California over a 3-year period (1991-1994). Overall, relative population densities of California ground squirrels declined significantly throughout the experiment, but did not differ between grazed and ungrazed colonies or between habitats. There was also no significant interaction between these 2 factors. The spatial distribution of burrows, as measured by the mean nearest neighbor distance of active entrances within a colony, did not differ significantly between grazed and ungrazed colonies or between habitats, nor was the interaction significant. Thus, low to moderate levels of cattle grazing did not appear to have a strong effect on the population dynamics of California ground squirrels, and grazing may be compatible with maintenance of ground squirrel populations. Based on multivariate analysis of variance of 1994 data, live plant cover, native plant cover, and standing biomass were lower where the number of burrows was higher on grazed colonies but were little affected on ungrazed colonies. Ground squirrels may increase the impact of livestock grazing and thus reduce the capacity of the land to support other activities. However, it is clear that the effects of livestock grazing are complex and that detailed studies of potential mechanisms by which grazing impacts California ground squirrel populations are necessary.
... Primero, al método de medida en sí. La obtención de valores negativos con el uso de las jaulas de exclusión no es raro, ya que las condiciones de crecimiento de la hierba dentro y fuera de la jaula no son iguales (Parsons et al., 1984). En pastoreo rotacional, donde no se emplean jaulas de exclusión, si bien es difícil que ésto ocurra, debido a que el período entre aprovechamientos es largo (aproximadamente un mes), es posible que suceda si las condiciones (temperatura, humedad) no son adecuadas para el crecimiento del pasto. ...
... Segundo Cano et al. (2004a), os pastos de capim-Tanzânia, em regime de lotação contínua usadas em alturas do pasto entre 20 e 80 cm, permitem quantidades elevadas de taxas de acúmulo de matéria seca. Porém, cuidados devem ser tomados quando feito essas avaliações, pois segundo Parsons et al. (1984), o número de dias usado para a estimativa da taxa de acúmulo de MS pode ocasionar valores subestimados ou superestimados, em decorrência da evolução do índice de área foliar, no interior das gaiolas de exclusão do pastejo. ...
Objetivou-se avaliar o acúmulo de massa de forragem e a composição morfológica do pasto de
capim-Tanzânia adubado com nitrogênio ou consorciado com estilosantes Campo Grande. Utilizou-se um delineamento
experimental em blocos ao acaso com parcelas subdivididas, com três repetições. Os tratamentos foram constituídos por:
Tanzânia + Estilosantes; Tanzânia + 75 kg N ha-1; Tanzânia + 150 kg N ha-1; Tanzânia + 225 kg N ha-1. A maior
porcentagem de lâmina foliar verde foi observada na primavera e nos pastos com 150 e 225 kg de N/ha. A porcentagem de
colmo+bainha verde foi maior no verão e no outono, assim como, nos pastos adubados com N. A porcentagem de material
morto foi mais elevada no outono e inverno. Com a aplicação de 225 kg de N/ha, os pastos apresentaram maior acúmulo
diário de massa de forragem em relação aos outros tratamentos. A circunferência das touceiras foi semelhante entre os
pastos consorciados e adubados com N. A quantidade de perfilhos vivos foi maior quando utilizou-se doses mais elevadas
de nitrogênio. Os pastos consorciados com estilosantes ou adubados com 75 e 150 kg de nitrogênio apresentam respostas
semelhantes para as características estruturais do pasto, e para o acúmulo de forragem.
... The techniques most commonly used to estimate pasture production under continuous grazing in New Zealand have revolved around harvesting of herbage, which is protected from grazing animals and accumulates on a defined plot area over a period of time. Such approaches introduce artificiality into the assessment, and previous studies have suggested that the actual rates of net pasture production occurring in grazed pastures are substantially different from those estimated using harvested plot techniques (Field et al. 1981;Parsons et al. 1984;Laidlaw et al. 1994;Piggot 1997). ...
Estimates of pasture production are critical for comparing treatment effects in research trials, as inputs to decision support models, and for on-farm use in constructing grazing plans. We compared two techniques for measuring pasture production, over 2 years in 36 paddocks continuously grazed with sheep, on hill country at the Ballantrae Research Station. One technique (Cut method) involved harvesting pasture regrowth after trimming to a standard height. The other (Model method) calculated pasture production as the sum of grazing animal intake (estimated using a spreadsheet-based model with inputs including animal performance and pasture quality) and changes in pasture cover (above-ground biomass). Annual pasture production estimates were reasonably well correlated (R 2 = 0.66), but those using the Cut method were 55% greater than using the Model method. Agreement was best in spring and summer, and poorest in autumn and winter. Factors contributing to these discrepancies included the contrasting starting conditions for the pasture at the start of each measurement period (short-trimmed for the Cut method, and patch-grazed for the Model method); the effect of grazing in the Model method maintaining pastures in a more vegetative state; and the greater losses from senescence and decay in the Model method pastures. The potential for errors using the Model method was high where measurement periods were short, as accuracy of measurement of pasture cover can be suspect. The Model method probably better quantified net pasture production in grazed pastures.
... As correlações significativas da taxa de acúmulo de MS com algumas das características do pasto podem ser, também, parcialmente atribuídas ao período usado para o corte da forragem, no interior das gaiolas de exclusão do pastejo. Segundo Parsons et al. (1984), o número de dias usado para a estimativa da taxa de acúmulo de MS pode ocasionar valores subestimados ou superestimados, em decorrência da evolução do índice de área foliar, no interior das gaiolas de exclusão do pastejo. Experimentos com altos rendimentos de MS em pastos de Panicum maximum, em conseqüência das melhorias na fertilidade do solo e no manejo, podem ser analisados em Muir & Jank (2004) e Euclides et al. (2007). ...
The objective of this experiment was to evaluate different sward height (20, 40, 60 e 80 cm) in Tanzania grass (Panicum maximum Jacq.) pastures managed under continuous stocking. The animals used were Nellore steers, and the control of sward height was done with put-and-take techniques. Evaluations were made for: forage mass, green leaf mass, leaf:stem ratio, morphological composition and dry matter accumulation rate. The experimental design was completely randomized with two replications. Forage mass increased linearly with sward height with overall mean of 2,767, 3,105, 3,657 and 4,436 kg ha-1 at sward heights 20, 40, 60 and 80 cm, respectively. Rates of dry matter accumulation decreased with increasing sward heights and were 104, 108, 90 and 81 kg ha-1 per day for sward heights 20, 40, 60 and 80 cm, respectively. Leaf:stem ratio decreased linearly with sward heights. Tanzania grass pastures, under continuous stocking, should be managed between 40 and 60 cm heights in the final of spring and during summer station.
... As expected, herbaceous standing biomass (which did not include oak biomass) was significantly higher in the grassland than in the oak savanna (312 g m À2 vs. 130 g m À2 , P , 0.01). These amounts are lower than the true primary production because squirrels had access to cage interiors and because cages generally provide relative rather than absolute estimates of primary production (Parsons et al. 1984). ...
Understanding the impacts of livestock grazing on wildlands is important for making appropriate ecosystem management decisions. Using livestock exclosures, we examined the effects of moderate cattle grazing on the abundance of California ground squirrels (Spermophilus beecheyii Richardson) and the spatial distribution of active burrows within their colonies in grassland and blue oak (Quercus douglasii Hook. & Arn.) savanna habitats in the coastal range of California over a 3-year period (1991- 1994). Overall, relative population densities of California ground squirrels declined significantly throughout the experiment, but did not differ between grazed and ungrazed colonies or between habitats. There was also no significant interaction between these 2 factors. The spatial distribution of burrows, as measured by the mean nearest neighbor distance of active entrances within a colony, did not differ significantly between grazed and ungrazed colonies or between habitats, nor was the interaction significant. Thus, low to moderate levels of cattle grazing did not appear to have a strong effect on the population dynamics of California ground squirrels, and grazing may be compatible with maintenance of ground squirrel populations. Based on multivariate analysis of variance of 1994 data, live plant cover, native plant cover, and standing biomass were lower where the number of burrows was higher on grazed colonies but were little affected on ungrazed colonies. Ground squirrels may increase the impact of livestock grazing and thus reduce the capacity of the land to support other activities. However, it is clear that the effects of livestock grazing are complex and that detailed studies of potential mechanisms by which grazing impacts California ground squirrel populations are necessary.
... As correlações significativas da taxa de acúmulo de MS com algumas das características do pasto podem ser, também, parcialmente atribuídas ao período usado para o corte da forragem, no interior das gaiolas de exclusão do pastejo. Segundo Parsons et al. (1984), o número de dias usado para a estimativa da taxa de acúmulo de MS pode ocasionar valores subestimados ou superestimados, em decorrência da evolução do índice de área foliar, no interior das gaiolas de exclusão do pastejo. Experimentos com altos rendimentos de MS em pastos de Panicum maximum, em conseqüência das melhorias na fertilidade do solo e no manejo, podem ser analisados em Muir & Jank (2004) e Euclides et al. (2007). ...
O objetivo deste experimento foi avaliar alturas de manejo do pasto (20, 40, 60 e 80 cm) em capim-tanzânia (Panicum maximum Jacq.), em regime de lotação contínua, nas características do dossel e acúmulo de matéria seca. Os animais utilizados foram novilhos Nelore (Bos indicus), e a taxa de lotação foi variável. Foram avaliados: a massa de forragem, a massa de lâmina de folha verde, a razão folha:colmo, a composição morfológica e a taxa de acúmulo de matéria seca. O delineamento experimental foi inteiramente casualizado, com duas repetições. A massa de forragem aumentou linearmente com o aumento da altura do pasto. As médias de massa de forragem foram 2.767, 3.105, 3.657 e 4.436 kg ha-1, respectivamente, para as alturas de 20, 40, 60 e 80 cm. As taxas de acúmulo de matéria seca, a 20, 40, 60 e 80 cm, foram, respectivamente, 104, 108, 90 e 81 kg ha-1 por dia, o que indica que houve redução dessas taxas com a elevação da altura do pasto. A razão folha:colmo decresceu linearmente com o aumento da altura do pasto. Pastagens de capim-tanzânia, sob lotação contínua ao final da primavera e durante o verão, devem ser utilizadas entre 40 e 60 cm de altura.
... The current study, however, used only the pseudostem of the ryegrass shoot where concentrations of endophyte mycelium and endophyte alkaloids are greatest. Pseudostem represents a large proportion of the green shoot biomass of grass in a sward and a site of accumulation of dead material (e.g., Parsons et al. 1984;L'Huillier et al. 1986). Allelochemicals can be leached from this plant material or released by decomposition of the dead residues (Rice 1984). ...
Two glasshouse experiments were conducted to evaluate possible allelopathic effects of perennial ryegrass/endophyte (Lolium perennel Neotyphodium lolii) associations on white clover (Trifolium repens). The influence of between‐species competition, environmental stress, and soil fauna was eliminated by the use of a sterile sand nutrient culture technique to sustain potted clover seedlings, to which aqueous extracts from moisture‐stressed perennial ryegrass pseudostem were applied. In Experiment 1, extracts from two ryegrass cultivars ('Grasslands Pacific’ and ‘Grasslands Nui'), each hosting in separate seed lines two endophyte strains (ES, WT), suppressed clover growth (mean suppression 22% at 100% concentration) relative to extracts from endophyte‐free lines of the same cultivars. The degree of suppression increased with increasing extract concentration, differed between cultivars, but did not differ between endophyte strains. In Experiment 2, extracts at 100% concentration from a third ryegrass cultivar ('Grasslands Ruanui'), hosting in separate seed lines three endophyte strains (AR4, ES, WT), suppressed clover growth by a mean of 27% relative to extracts from an endophyte‐free line of this cultivar (range 11 to 47%), with significant differences between strains. The endophyte strains differed in three known endophyte alkaloids, viz peramine, ergovaline, lolitrem B, but in this study none of these alkaloids could solely account for the allelopathic response. The suppression of clover by all ryegrass cultivar/endophyte strain associations tested indicates that clover suppression is not confined to the specific associations used in previous studies, and that further testing of new associations is warranted as variation in the degree of allelopathy existed, being dependant on endophyte strain and host ryegrass cultivar.
... ness to fertiliser, led to us confining our assessments of seasonal responses to the times of the year when pastures were actively growing and assessments of net growth were more reliable. Studies in Britain have shown that the use of pasture cages can overestimate growth due to rapid growth rates of pastures released from grazing (Parsons et al . 1984;King et al . 1988). The implications of these findings as they apply to our work have been discussed elsewhere (Cayley and Hannah 1995). ...
The response of pastures based on Lolium perenne L. and Trifolium subterraneum L. to single superphosphate was assessed at Hamilton, Victoria, by measuring the growth of pastures during winter, spring, and summer over 7 years from 1979 to 1987. The seasons were defined by the pattern of pasture production, rather than by calendar months. Winter was the period of constant growth rate following the autumn rain; spring was the period of accelerating growth rate until growth rate changed abruptly following the onset of dry summer weather. Pastures were grazed with sheep at a low, medium, or high grazing pressure, corresponding generally to stocking rates of 10, 14, or 18 sheep/ha. At each level of grazing pressure, single superphosphate was applied at 5 rates from 1979 to 1982; the highest rate, expressed as elemental phosphorus (P), was reduced from 100 to 40 kg/ha during this time. In addition there was an unfertilised treatment. In 1984, fertiliser was applied at 6 rates from 4 to 40 kg P/ha. No fertiliser was applied in the remaining years, including 1983. Pasture production was measured from 1979 to 1982 and from 1985 to 1987.
Total pasture dry matter (DM) accumulation per year at the highest stocking rate was less than the other treatments in 4 of the years. Averaged over all years and fertiliser treatments, the annual net production was 10·1, 10·1, and 9·0 t DM/ha (P < 0·05) for plots grazed at low, medium, and high stocking rates, respectively. The amount of fertiliser required to reach a given proportion of maximum yield response did not vary between winter and spring in any year, but the greater potential yield in spring (P < 0 ·001) meant that as more fertiliser was applied, the disparity between pasture grown in winter and pasture grown in spring increased. Differences in this disparity between extreme levels of P ranged from 1·4 t DM/ha in a drought to about 7 t DM/ha in a good season.
The implications for managing farms when pastures are fertilised at higher rates than currently practised by district farmers are that systems of animal production with a requirement for plentiful good quality pasture in spring, such as ewes lambing in spring, should be used. The benefit of spring lambing over autumn lambing was supported when the 2 systems were compared over 26 years using the GrassGro decision support system. Well fertilised pasture systems will also allow more scope for conserving pasture as hay or silage, and increase opportunities for diversification in the farming enterprise, such as spring-growing crops.
... Micro-climatic conditions were not affected by the change in management imposed by the enclosures, but cutting rather than grazing produced a visible change in the structure of the sward (see Parsons et al. 1984). This change was quantified by obtaining measures of tiller density of the vegetation (Hodgson et al. 1981). ...
Depletion of soil invertebrate availabilities by starling foraging during the breeding season was investigated by using exclosures to create areas where the birds could not feed. A field 'preferred' by the wild birds was compared with a 'non-preferred' field. Prey biomass was measured from soil cores and the foraging success of individuals feeding in experimental enclosures gave a measure of prey availability. Foraging by these birds resulted in significant short-term depletion after a few hours of foraging. Trials conducted after a 24-h interval showed that this depletion was due to removal of prey by the birds rather than invertebrate movements. Comparison with exclosure sites al the end of the chick-feeding period showed that foraging by the wild birds caused significant long-term depletion. This depletion was greatest in the 'preferred' field. which offered a higher availability of leather jackets Tipula paludosa. The 'non-preferred' field yielded lower capture rates, despite a higher earthworm biomass in the soil cores. These findings suggest that soil core analysis alone is insufficient to predict the foraging success of starlings and other birds feeding on soil invertebrates. The significant reduction in prey availability to parent starlings is discussed in relation to the number of parental foraging trips made to the two fields.
... It is well established that sward density affects seedling recruitment (Bullock, 2000). Previous studies have shown that sward density is highly variable and can be altered rapidly and significantly by sward management (Parsons et al., 1984;Orr et al., 1988), but there have been relatively few attempts to apply this knowledge to promote the establishment of introduced species. There have been studies on sowing wildflower seeds on arable land (Frame et al., 1993;Bullock et al., 2001;Pywell et al., 2002) and of including severe disturbance of the sward on grassland sites (Fenner and Spellerberg, 1988;Burke and Grime, 1996;Coulson et al., 2001). ...
In a field experiment, the effects of different management treatments on the botanical development and productivity of a previously species-poor, intensively managed, lowland permanent grass sward were investigated over a 4-year period. Fertilizer inputs were stopped and nine main treatments, based on three pre-sowing and three post-sowing cutting regimes, were applied. Half of each treatment plot was oversown with a mixture of forb (wildflower) species and half left unsown. The aims were: (1) to investigate the effect of cutting regimes on the establishment of introduced forb species, (2) to assess the development and dry-matter (DM) yield potential of these introduced species, and (3) to compare the DM yield and forage quality of swards with and without introduced species. Seedling establishment was recorded until the end of the second year. From the third year onwards, a common twice-yearly cutting regime was imposed on all treatments, and the development of the introduced species was recorded. The DM yield of the nine oversown sub-treatments was compared with the sub-treatments not oversown. Frequent cutting after oversowing resulted in the greatest number of established plants and three times greater harvested DM yield of introduced species, compared with infrequent cutting. Plantago lanceolata and Trifolium pratense were the best performing introduced species. By years 3 and 4, there was proportionately 0·15 and 0·23, respectively, additional herbage DM yield harvested from the oversown treatments compared with unsown treatments, and up to 0·60 more in some treatments. The introduction of forbs increased the crude protein concentration, but decreased the digestibility of the forage.
The hills and uplands lie within the poorer ‘less favoured’ soil and climatic areas which account for almost 10 M ha of the UK's 19 M ha of agricultural land. Rough grazing comprises 6 M ha. Dry matter (DM) production from native hill swards, governed by soil type, climate and species composition, typically ranges from 1 to 5 t per ha, and is highly seasonal. The best grade of native hill sward is the bent-fescue ( Agrostis/Festuca ) association. Upland permanent pasture sward production is strongly influenced by the proportion of perennial ryegrass and level of fertilizer N input; typically, DM production ranges from 3.5 to 91 per ha.
Constraints to herbage production include acid soils, impeded drainage and shortage of major plant nutrients particularly N and P. Deficiencies of some major nutrients and of trace elements in the plants can consequently have serious implications for livestock performance. Relationships among trace elements are particularly complex. The climate is severe and highly variable, with low temperatures, exposure to wind, high rainfall and frequent winter frosts being notable features. Bracken, rush, gorse, broom and scrub woodland communities are special vegetation problems which adversely affect pasture production and utilization.
Specific improvement techniques for the differing soil type — plant community associations are available but economic considerations are important when making the choice and deciding on the extent of improvement. Addition of lime and fertilizers, and the introduction of improved grass species and white clover, are the keys to enhanced sward production and quality. Maintenance of soil fertility is essential to ensure continued production. White clover is particularly important for animal nutrition and for its ability to contribute to available soil N via rhizobial N fixation. However, fertilizer N has a role in strategically extending the grazing season and for intensifying winter feed production (silage or hay) from suitable land; silage offers the better option.
In many cases where semi-natural grassland is abandoned a decrease in species diversity is found, accompanied by a large accumulation of litter from one or more species with a large standing crop (Harper, 1977; Grime, 1979). Increasing the species and community diversity is a major aim of the management of such areas for nature conservation purposes in the Netherlands. This can be achieved by hay-making or by grazing. The latter, when occurring at different intensities, is considered more advantageous in meeting this objective (Oosterveld, 1975; Harper, 1977).
Liveweight gain was assessed in lambs grazing perennial ryegrass (Lolium perenne) bred for an elevated water-soluble carbohydrate (WSC) concentration (experimental var. Ba11353) compared to an intermediate WSC control (var. AberElan). Both grass varieties had similar heading dates and overall forage yield characteristics. The two grasses were sown separately in each of three replicate sward plots (0.137 ha each) laid out in a randomised block design. A core group of five Welsh Mountain ewes and their singleton lambs (Bluefaced Leicester, sire), initial liveweights of 50 and 14 kg (s.e. 0.6 and 0.3), respectively, were continuously stocked on these plots over three consecutive 21 d experimental periods. Swards were maintained at a target surface height of 6-7 cm with the use of sward adjustment animals. Core sheep were weighed every three weeks and adjustment animals whenever they were added to or removed from the plots. Lamb liveweight gain (g.d(-1)) was calculated from the core group of lambs. Lamb production (kg liveweight. ha(-1) over each 21d period) was calculated from the liveweight change of all the lambs. Water-soluble carbohydrate concentrations were higher (P = 0.03) and neutral detergent fibre concentrations (NDF) were lower (P = 0.01) in the high WSC grass (HWSC) compared to the control. In vitro dry matter digestibilities (DMD) were higher for HWSC (P < 0.01). Lamb liveweight gain (P = 0.03) and production (P = 0.06) were greater on HWSC compared with the control by approximately 12 and 23%, respectively. The increased lamb production resulted from the combined effect of higher liveweight gains and carrying capacity on HWSC. Correlation matrices indicated strong relationships between liveweight gain and grass WSC concentration (r = 0.67; P < 0.05) and fibre concentration (r = -0.73; P < 0.05). The results show that grazing forages selected for elevated concentrations of WSC and reduced fibre, compared to a control increases the liveweight gain of suckling lambs.
The simplest way to discuss the effects of grazing on plant communities is to compare grazed and ungrazed situations. The results of a number of studies in which large herbivores were excluded from previously grazed landscapes/plant communities are summarized in Table 5.1. This list is certainly not complete but it allows some generalizations. Grazed areas harbour pioneer species, including annuals and biennials that have to establish from seedlings, low-stature species and rosette plants. A similar conclusion was reached by Scherfose (1993) in a literature review on the impact of grazing on plant species in salt marshes in the Wadden Sea area. The ratio of species suffering from grazing and species promoted by grazing turned out to be about 1: 1 at the lower and middle salt marsh. At the higher salt marsh more species were promoted by than suffering from grazing. After the exclusion of grazing, the aforementioned groups of species are often replaced by tall grasses and herbs accompanied by litter accumulation, and by shrubs and trees. The general pattern shows a higher above-ground standing crop M exclosures than in continuously grazed plots, as reported in a review by Milchunas and Lauenroth (1993).
The experiment was conducted in Tupanciretā, Planalto Médio region at the State of Rio Grande do Sul, in a pasture of Italian ryegrass cv. Estanzuela 284 (Lolium multiflorum Lam) and white clover cv. Yi (Trifolium repens L.), submitted to different levels of dry matter residues (DMR). A completely randomized experimental design was used, being studied the DMR levels maintained in the pasture of 1119, 1320, 1477, 1695, 2146, 2166, 2410, and 2483 kg of dry matter (DM)/ha. In this experimental were used crossbred lamb Ile de France x Corriedale, weaned at 80 days, per an experimental period from September 30 to December 8. The dry matter (DM) production, DM accumulation rate, percentage of interception of luminous radiation, contribution of the structural components of the pasture, botanical composition, pasture height, green leaf DM, organic matter in vitro digestibility (OMIVD) and crude protein content, were evaluated. There were relationship among the levels of DMR with percentage of interception of luminous radiation, contribution of structural components, pasture height, green leaf DM and OMIVD. There was not, however, relationship among DM production, DM accumulation rate and crude protein content with the studied forage residues levels. Even with the pasture in final period of use, the estimate DM production, average of all DMR levels was of 6754 kg of DM/ha.
The objective was to evaluate the accumulation of forage yield, composition and morphology ofTanzania grass pastures (Panicum maximum Jacq. cv. Tanzania-1) fertilized with nitrogen or intercropped with CampoGrande Stylosanthes. Were used a randomized blocks with split plots with three replications and having treatments asmain plots: Stylosanthes + Tanzania; Tanzania + 75 kg N ha-1; Tanzania + 150 kg N ha-1; Tanzania + 225 kg N ha-1 andthe subplots seasons. The highest percentage of green leaf blade were observed in spring and the pastures with 150 and225 kg N/ha. The percentage of green stem + sheath were higher in summer and autumn, as well as in pastures fertilizedwith N. The percentage of dead material was higher in autumn and winter. With the application of 225 kg N/ha, pastureshad higher daily accumulation of dry matter in relation to other treatments. The circumference of the stools was similarbetween the consortium and pastures fertilized with N. The number of live tillers was higher when we used higher doses ofnitrogen. Pastures intercropped with stylosanthes or nitrogen fertilized (75 and 150 kg) presents better morphology and daily accumulation of forage. © 2015 Universidade Federal de Uberlandia. All rights reserved.
This study was conducted to identify effective strategies for grazing managing of Panicum maximum Jacq cv. Tanzania, under continuous stocking with variable stocking rate, based on assessments of their morphogenetic and structural characteristics. Four forage allowances: 3, 7, 11, and 15% BW were studied. A randomized block design, with three replicates was used, with split plot, where FA 4 as main plots and subplots were established cages. The number of basal tillers per cage protected and unprotected, both remaining as the new, decreased with FA. The unprotected tillers had higher numbers than cage protected. The weight of tillers and roots showed curvilinear response function in the FA. There was no effect of period at tillering (P> 0.05) of Tanzania grass. The total length of sheets, protected or not by exclusion cage and ligule height of tillers per cage had unprotected quadratic fit function OF. The ligule height protected tillers showed linear response function in the OF. The total number of leaves, expanded and expanding, tiller protected or not by exclusion cage, showed no differences between the FA and cage exclusion. The use of forage allowance of 11% of body weight for tanzania grass managed under continuous grazing and variable stocking rate, seems to be more sustainable.
The gas exchange responses of Andropogon gerardii were followed after defoliation by either cattle grazing or clipping and compared with the response of nondefoliated plants. Grazed plants had significantly higher rates of photosynthesis than either clipped or control plants. The photosynthesis/transpiration ratio as well as stomatal sensitivity to humidity indicate that leaves of grazed plants may have developed in a higher light and lower moisture environment than that of their clipped counterparts. Microenvironmental differences caused by grazing may be crucial in determining the responses of grasses to clipping versus grazing. -from Author
The field performance of the native Pappophorum vaginatum, the naturalized Eragrostis curvula and various cultivars of the introduced Achnatherum hymenoides and Leymus cinereus was evaluated as potential forage resources in rangelands of arid Argentina during the warm seasons of 2007/2008 and 2008/2009. Plants of these grass species, obtained from seeds, were transplanted to the field in 2006, when they were 1 year old. During the study years, there were two defoliation managements: plants of all study genotypes either remained undefoliated (controls) or were defoliated twice a year during spring at 5 cm stubble height. Despite tiller number being lower (P 0·05) between defoliation managements by mid-spring, there were no differences (P > 0·05) in dry weight production between defoliated and undefoliated plants in all genotypes at the end of the second growing season. Plants of one or more of the introduced genotypes showed a similar (P > 0·05) or greater (P 0·05) or greater (P 39%) genotypes at the end of the first or second growing seasons.
New herbage varieties are often tested in 'simulated grazing' (i.e. frequent cutting) trials, but the relative yield measured under this regime may not be the same as the yield (herbage offtake) under grazing. To explore any differences that may exist between yield and seasonal growth patterns of grass that is cut by machine compared to grazed grass, 15 intermediate heading perennial ryegrass varieties, with 5 replicate areas of each, were sown in September 1997. Seasonal patterns of herbage production and total annual yield were measured in 1998 and 1999, either by cutting to 3 cm on 8 occasions or under continuous variable stocking with sheep to maintain 4 to 5 cm sward surface height. Grazing yield was calculated from daily stocking rate carried and herbage intake ewe-1, measured on 3 occasions during the grazing season. There was no significant correlation between ranking of varieties under the rotational frequent cutting and continuous stocking treatments in either year. Compared with the relatively even grass growth pattern for frequent cutting, seasonality of growth was quite different on the continuously-stocked plots with markedly higher growth rates measured in July and August. These results suggest there are limitations to assessing the relative performance of grass varieties destined for continuous stocking under rotational frequent cutting management and material valuable for sheep grazing may be overlooked.
Species-rich alpine grasslands with Nardus stricta are important communities for both animal production and environmental conservation in Europe. We selected two contrasting types of Nardus grasslands (mesic and wet) within a rangeland of northern Spain and measured annual above-ground net primary productivity (ANPP), botanical components, forage utilization and their respective seasonal patterns, during a 5-year period. We analysed their chemical properties and recorded soil moisture and temperature in order to construct models able to explain grassland productivity. Mean annual ANPP of mesic Nardus grassland was about half (216 g DM m2 year ; ±29 s.e.) that of the wet grassland (406 g DM m2 year ±54 s.e.), with significant intra and interannual variability. Mesic grassland, with a more important contribution of forbs and legumes over graminoids in its botanical composition, was the preferred forage source of grazing livestock and showed better chemical properties in spring and early summer. In summer and autumn, wet grassland had a higher utilization owing to its ability to maintain high biomass production. This was partially explained by soil moisture, a limiting factor of mesic grassland productivity. Our results provide new and relevant information on key aspects of species-rich alpine Nardus grasslands, potentially useful for the definition of management options for these habitats of priority conservation.
Despite its significance as an inexpensive feedstuff for ruminant production, in many temperate regions the grass crop is poorly utilized. Whether the aim is to increase total yield or to improve the economic efficiency of production, attempts to make better use of grass demand a detailed understanding of the limitations to production imposed by the environment, the seasonal development and the defoliation management of this crop.
Use of the self-thinning rule to describe size/density compensation (SDC) in defoliated swards is examined. It is shown that defoliation related variation in leaf area and associated morphogenetic changes in plant structure necessitate slope corrections, designated Ca and Cr , respectively. The theory predicts that reduced leaf area in more heavily defoliated swards will result in SDC at slopes more negative than -3/2 (variable leaf area SDC), and that there will be a transition to -3/2 (constant leaf area) SDC at higher herbage mass.
Empirical data from previous experiments with Lolium perenne L. and Medicago sativa L. are examined, and appear to confirm the theoretical predictions, including the slope change at the point of transition from variable to constant leaf area SDC. This transition point, designated di , is subject to interspecific variation and is related to the mature shoot size of a particular species. Some applications of this theory are discussed, and in particular a sward productivity index is proposed.Copyright 1995, 1999 Academic Press
The response to phosphatic fertilizer of a pasture based on perennial ryegrass, subterranean clover and phalaris was assessed over 4 years. The pasture was established on a previously unfertilized area. Single superphosphate was applied at five rates. In addition there was an unfertilized treatment. The highest rate of fertilizer, expressed as elemental phosphorus (P), was reduced from 100 kg/ha in years 1 and 2 to 60 and 40 kg/ha in years 3 and 4 respectively. Each year total pasture drymatter (DM) production was measured during a 6-month growing season from early winter to late spring under four systems of defoliation: mown monthly (MI), mown every 2 months (M2), mown every 3 months (M3) or set stocked with 1 year old sheep (G). Grazed plots were stocked at 10, 14 or 18 sheep/ha in 3 of the years, and at 8.75, 12.25 or 15.75 sheep/ha during the remaining year. The production of mown pasture generally decreased with increasing frequency of cutting, and was always less than the production of grazed pasture. Mean values for MI, M2, M3 and G were 2.85, 4.35, 5.44 and 6.86 t DM/ha respectively. The absolute and marginal responses to fertilizer (kg DM/kgP) were always greater for the grazed treatments. This suggests that data from mowing trials seriously underestimate the fertilizer response of grazed systems. The amount of fertilizer required to reach a given proportion of potential yield response did not differ between the systems in the first 3 years, but in year 4, more fertilizer was required by the grazed system to reach a given proportion of potential yield (P < 0.01. Strategies for correcting the response curves of the mown treatments are considered.
Herbage characteristics were monitored over 3 years (1985–1987) in three perennial ryegrass (Lolium perenne L.) varieties as grass/only (200 kg N ha−1) and grass/clover (Trifolium repens L.) (75 kg N ha−1) swards when continuously stocked with sheep. Although mean total annual herbage production was similar from the grass varieties, growth of Aurora, a very early-flowering variety, was higher than that of Aberystwyth S23 and Meltra (tetraploid) late-flowering types in spring. Herbage production from grass-only swards was 15% higher than from grass/clover swards.
In vitro organic matter digestibility (OMD) of Aurora and Meltra was 34 g kg −1 and 26 g kg −1 higher (P < 0·01) than that of S23. Averaged over varieties, the OMD of grass/clover was 25 g kg −1 higher (P < 0·05) than grass-only during the post-weaning period. Individual lamb digestible organic matter intake (DOMI) was 22% higher (P < 0·05) on grass/clover than on grass-only during this period. Water-soluble carbohydrates content of Aurora was 42% higher (P < 0·001) than that of both Meltra and S23.
Herbage characteristics were related to lamb performance reported previously. Lamb production was positively correlated with DOMI. However, the magnitude of differences in lamb performance between treatments was much greater than the extent of differences detected in herbage characteristics.
In many areas of western North America, populations of Rocky Mountain elk (Cervus elaphus canadensis) avoid snow at high elevations during winter by migrating to sagebrush grassland communities in mountain valleys, communities that are used by cattle in the spring and early summer. As a result of these patterns of habitat use, the impact of elk on forage supplies for cattle has emerged as an important issue in range and wildlife management throughout the West. We examined effects of variation in population density of elk on the availability and use of forage resources by cattle in a randomized complete block experiment conducted in sagebrush grassland during four years. We manipulated elk numbers to achieve four levels of population density (0, 9, 15, and 31 elk/km(2)), replicated each level three times, and observed responses of vegetation and cattle to these manipulations. At high densities (31 animals/km(2)), elk annually removed 57% of the standing crop of dead perennial grass and 12% of the total annual production of live perennial grass. Standing crops of dead perennial grass in early spring declined in direct proportion to increasing elk density (linear effect F-1,F-6 = 10.0, P = 0.02) from a mean of 8.7 g/m(2) in the controls (0 elk/km(2)) to 3.3 g/m(2) in the high density (31 elk/km(2)) treatment. Early spring standing crops of live perennial grass also declined as elk population density increased, but these trends only approached significance (linear effect F-1,F-6 = 3.4, P = 0.12). Effects of elk grazing on herbaceous aboveground net primary production was not significant (minimum P > 0.38), but the total supply of herbaceous dry matter available to cattle (standing dead + primary production) declined in linear relation to elk density (linear effect F-1,F-6 = 7.7, P = 0.03). Canopy cover of shrubs was least and canopy cover of grass was greatest at intermediate levels of elk density (quadratic effect F-1,F-6 = 9.4, P = 0.03). We found weak enhancing effects of elk populations on nutritional quality of spring forage. Elk grazing caused linear increases in the digestibility (F-1,F-6 = 5.0, P = 0.07) and nitrogen content (F-1,F-6 = 15.1, P = 0.008) of perennial grass available to cattle, Nitrogen content of cattle diets increased in the moderately grazed treatments (control vs. 15 elk/km(2), F-1,F-6 = 4.3, P = 0.06), but dietary digestibility and fiber content did not change significantly with treatment. Daily forage intake by cattle (kilograms of dry matter per cow per day) declined in direct relation to elk density (F-1,F-6 = 5.2, P = 0.06), primarily as a result of reductions in intake of standing dead grass. Consequently, cattle daily intake of digestible energy (linear effect F-1,F-6 = 5.1, P = 0.06) and nitrogen (control vs, others F-1,F-6 = 5.4, P = 0.06) declined as elk population density increased, The mechanism responsible for this decline was a Type II functional response of cattle to forage biomass. We conclude that effects of elk on cattle represent a composite of facilitative and competitive effects. When forage production is low and cattle density is high, competition is a much stronger force than facilitation.
Annual liveweight gain of beef cattle (steers) grazing grass pasture fertilized with 200 kg N/ha was compared over a period of 7 years (1989–95) with that of steers grazing grass/white clover pasture given no artificial N fertilizer at North Wyke, Devon, UK. Nitrogen lost by leaching over the ensuing winter drainage periods was monitored from both pastures. Nitrogen leaching loss from the fertilized pasture over an extended period of 13 years (1983–95) is also reported.
The average annual liveweight gain of the steers grazing the grass/clover pasture (0·81 t/ha) was 19% lower than that of the steers grazing the N-fertilized grass pasture (1·00 t/ha). The average annual loss of nitrate-N by leaching in winter drainage from the grass/clover pasture (13 kg/ha) was only 26% of that recorded from the fertilized grass (50 kg/ha). A possible reason for this difference may arise from the previous history of the grass/clover pasture which had been ploughed in 1982, causing a flush of N mineralization and consequently greater immobilization of N in the soil in subsequent years.
Losses of N each winter by leaching measured over a 13-year period from the fertilized grass were highly correlated ( P <0·001) with the preceding summer's soil moisture deficit, with the highest losses following dry summers. The nitrate-N concentration in the drainage water exceeded the European Union limit in drinking water (11·3 mg/l) in the initial 25 mm of drainage during 11 of the 13 autumns. The average loss of N each winter (53 kg/ha) was equivalent to 26% of the fertilizer-N applied annually. Immediate losses of N by leaching of fertilizer applied in early spring and throughout one very wet summer (1993) were minimal.
Tissue flow measurements of leaf material in Lolium
perenne (perennial ryegrass), Agrostis
capillaris
or Poa
annua, and Trifolium
repens (white clover) were made at three upland sites in Scotland
(Hartwood, Sourhope and Fasque) in 1992/93 to determine if there were differences in seasonal
growth, senescence and losses to herbivory between species in their response to more extensive sward
management. The measurements were made monthly from May until October in established
predominantly perennial ryegrass/white clover swards receiving three different treatments. One
treatment received a conventional annual fertilizer application of 140 kg N/ha plus maintenance P
and K and was grazed by sheep at a sward height of 4 cm (4F), whereas the other two treatments were
unfertilized and grazed to maintain a sward height of 4 cm (4U) or 8 cm (8U).
Two experiments were conducted to provide information on the precision of herbage sampling, the degree of replication and the logistics of managing rotational and continuous grazing experiments. In a third experiment the possibility of estimating herbage growth, during the rotational grazing of paddocks for 4 days, by linear extrapolation of measured growth estimates made during the interval between grazings was investigated. A fourth experiment was then conducted to investigate animal and herbage production responses to nitrogen applied at five levels over the range 80–900 kg N/ha with three replicates per treatment.
Results from Expts 1 and 2 showed that similar precision could be obtained under both rotational and continuous systems of grazing with the same number of replicates and samples of herbage. There was little advantage in having more than three replicates and four samples per paddock or two replicates and eight samples per paddock. Under continuous grazing a 3-weekly sampling and movement of exclosure cages was indicated. It was also found, from Expt 3 that, under rotational grazing, linear extrapolation of growth from 0–24 days did not reflect growth to 28 days in a consistent manner. Six animals per treatment, permanently in the paddocks, proved adequate for the estimation of live-weight gain.
Experiment 4 confirmed that response curves could be fitted with an acceptable degree of precision if there was one low, one very high, and two intermediate levels of nitrogen. Adjustment of the number of animals to maintain sward height at between 5 and 7 cm ensured similar sward conditions and rates of live-weight gain across all treatments.
The problems associated with the measurement of herbage production under grazing are discussed.
Masham ewes, stocked at 20 ewes ha−1, reared twins at pasture in 1979 and 1980 and were rotationally grazed around six paddocks. There were three lengths of grazing cycle (18, 30 or 42 d) and the ewes at pasture were either offered silage ad libitum (S) for 8 weeks of lactation or they were not supplemented with silage (N). By feeding silage, S ewes and lambs moved into greater herbage masses (3735 u. 3390 kg organic matter (OM) ha−1), but there was little effect on net herbage accumulation (10.5 v. 10.0 t OM ha−1) and herbage intakes per ewe were similar (2.31 v. 2.27 kg OM d−1) over the 7-month grazing season. During the period of silage supplementation, total intake was 19% higher for S than N ewes, lamb growth rates were slightly higher (280 v. 273 g d−1) and the ewes lost less weight (−78 v−96 g d−1).
For 18-, 30- and 42-d grazing cycles respectively the herbage masses before grazing were 2825, 3620 and 4240 kg OM ha−1; net herbage accumulations were 9.8, 10.4 and 10.6 t OM ha−1; mean daily herbage intakes by each unit (ewe+lambs) were 2.22, 2.35 and 2.30 kg OM and lamb growth rates from birth to weaning at 20 weeks were 205, 204 and 204 g d−1.
Frequent grazing of relatively light herbage masses or infrequent defoliation of heavier herbage masses made little difference to sward or animal performance. It is concluded that, at this stocking rate, different grazing cycles of 18, 30 or 42 d do not affect the performance of lactating ewes.
A field study was conducted from 1983 through 1986 to ascertain the effect of endophyte (Acremonium coenophialum), nitrogen fertility and tall fescue (Festuca arundiancea) genotype upon the soluble carbohydrate content of tall fescue from grazed paddocks. Non-grazed paddock exclosures were evaluated for 1984 and 1985. Plant tissues were extracted with 0·1 mol 1−1 H2SO4 to obtain acid-extractable carbohydrate (AEC). The AEC concentration and yield (concentration × dry matter) varied among and within years, generally was not influenced by endophyte, and was influenced by N fertility and cultivar. The AEC concentration from non-grazed exclosures was inversely related to AEC yield. Grazed paddock AEC concentration and yield were not significantly correlated. Grazed paddock AEC concentration remained fairly constant over the growing season, whereas AEC yield was greatest during periods of ample rainfall. The cultivar Johnstone generally had the greatest AEC concentration when compared with AU Triumph and KY-31. Even though endophyte is a carbohydrate sink, sward scale effects on AEC, as influenced by endophyte, were virtually undetected, suggesting that presence of endophyte is not detrimental to the fescue host. Grazed and non-grazed tall fescue AEC concentration and yield patterns differ and indicate the need to interpret non-grazed quality and management data carefully.
The relationship between the rate of canopy gross photosynthesis and LAI was investigated for ryegrass swards released from grazing after a period of continuous stocking to maintain the pasture at LAI 2–3. The regrowing swards were allowed to increase to about LAI 5 over a period of about 2 weeks while the rate of photosynthesis was measured and compared with that for continuously stocked swards maintained at LAI values between 2 and 6. Gross photosynthesis was found to increase more rapidly than LAI on the regrowing swards than on the steady-state control pastures. However, this response appeared to be influenced by seasonal factors. The potential increase in net COi uptake was estimated to range from 0 to 22%. The response was attributed to an increasing proportion of young photosynthetically-efficient leaves near the top of the canopy, modified by seasonal changes in the rate of tillering. The agronomic implications of the results are iscussed.
Rates of N accumulation were studied on sheep grazed grass swards maintained at a constant height of 5 cm for two growing seasons (1985 and 1986) and receiving no N fertilizer using a tiller tissue turnover technique. Grazing with normal excretal returns resulted in an 85‐105% increase in the estimated rates of N accumulation by laminae compared with similarly grazed swards where excretal returns were prevented. Generally, increases in tiller numbers in plots receiving excreta were mainly responsible for the increased rates of N accumulation, rather than increased rates of N accumulation per tiller. Rates of N remobilization from laminae and senescence (flux of N to standing‐dead litter pool) per tiller, and rates of leaf extension and leaf appearance/disappearance were unaffected by excretal returns, in urine patches increased tiller numbers and increased rates of accumulation per tiller both resulted in greater estimates of N accumulation compared with tillers not visibly affected by excreta. However, at any one time only about 11% of the plot area was affected by excreta and increases in N accumulation in excreta patches could not account for the overall plot increases in rates of N accumulation.
The increased tillering in plots receiving excreta may have increased exploitation of the soil N. With an average stocking rate of 15 sheep ha ⁻¹ , soil nitrate‐N, but not total N (Kjeldahl) nor bulk density, was increased during the winter of the second year as a result of excretal returns.
The model comprises three submodels, which together give an integrated picture of nitrogen pools and fluxes in grassland under grazing or cutting. The first submodel represents the interaction of the grazing animal with the sward through intake and the production of excreta: the second is concerned with the growth of the vegetative grass crop and its response to light, temperature and nitrogen; these two submodels are interfaced with a submodel of soil carbon and nitrogen pools and processes, including dead shoot and root material, dead and live soil organic matter, and three pools representing mineral nitrogen. No account is taken of water, which is assumed to be non-limiting, or the possible effects of soil pH and soil aeration. The model is used to simulate a range of management strategies as applied to stocking density and fertilizer application, examining both steady-state and non-steady-state conditions. The model highlights the long time scales associated with grassland systems, the role of the grazing animal in modifying carbon and nitrogen flows, and the importance of soil conditions to grassland productivity and fertilizer response. The productivity of grazed swards may be greater or less than that of cut swards depending on stocking density and fertilizer application, although nitrogen recovery (as calculated here) is always lower in grazed swards. The model is able to stimulate mineralization and immobilization, and place these in the context of plant processes and the grazing animal.
The effects of application of sodium fertilizer on the turnover and defoliation of leaf tissue were investigated in a perennial ryegrass (Lolium perenne) pasture grazed by dairy cows. Eight plots were allocated to treatments either with or without sodium fertilizer, with the former receiving 32 kg Na ha–1 applied in five applications of NaNO3 over the grazing season. An equivalent amount of nitrogen was given to the controls as ammonium nitrate, the application of which was reduced in the sodium treatment to equate nitrogen fertilizer applications for the two treatments. In nine periods between April and September, marked tillers were recorded to measure leaf turnover, leaf lamina growth and specific leaf weight and, when combined with tiller density measurements, gave an estimate of herbage flux for the sward. The defoliation and net growth of the marked tillers were monitored at 3-day intervals and the data were combined with tiller density and specific leaf weight data to determine the intake of the expanding, penultimate and oldest live leaf laminae. Sodium fertilizer application did not affect the rate at which leaves appeared, but it retarded their rate of disappearance. The extension rate and the specific weight of green laminae were both increased by sodium fertilizer application and therefore the net gravimetric growth rate was increased. Tiller density was not affected by sodium fertilizer application and hence the estimated herbage growth and net herbage flux were increased by sodium fertilizer application. Application of sodium fertilizer did not affect lamina length, and in both treatments the penultimate laminae were approximately twice as long as expanding and oldest live laminae. Defoliation frequency decreased from the expanding to the oldest live laminae in the control treatment without sodium. Sodium fertilizer application increased the frequency of defoliation of the oldest live leaf and also increased the length of the expanding leaf that was defoliated. For penultimate leaf laminae sodium fertilizer application reduced the defoliation frequency and length of foliage grazed. The dry-matter (DM) intake of the oldest live laminae was increased by the application of sodium fertilizer. It is concluded that sodium fertilizer application increases net herbage growth both by increased extension rate of leaf laminae and specific leaf weight and by delayed laminae senescence, and that it increases herbage DM intake by increasing the defoliation frequency of the oldest live leaf laminae.
The possibility of increasing the herbage utilized over a grazing season was investigated in a study comparing continuously stocked steady-state swards maintained at optimum height (3.5 cm) with intermittently grazed swards. The intermittent systems were designed (a) to allow periodic increase in leaf area and hence growth rate, (b)to ensure that the accumulated herbage was eaten before it senesced, and (c) lo retain high tiller density by alternating periods of herbage accumulation with periods of continuous stocking.
Two treatments (no animals or animal numbers reduced to half those on the 3.5 cm steady-state treatment) were used during the 17-18-d periods of herbage accumulation. Grazing down was completed in 3–4 d, after which two treatments (14 d or 28 d) were used for the intervening periods of continuous stocking when sward height was maintained at 3.5 cm.
Herbage production was estimated using the tissue turnover technique, with tiller population densities and rates of growth, senescence and net production per tiller measured at frequent intervals.
Intermittent grazing treatments where animals were removed during herbage accumulation resulted in changes in tiller size and number, and in growth rates, but not senescence rates, per tiller such that short-term deviations in the net rate of herbage production occurred compared with the continuously stocked control. The periods of advantage during phases of herbage accumulation were counterbalanced by those of disadvantage during the subsequent steady-state phases. Where animal numbers were reduced during herbage accumulation, sward conditions differed little from those of the continuously stocked control, implying that intake per individual animal was increased.
It was concluded that intermittent grazing systems offered no advantage over simpler continuous stocking systems, provided that a flexible approach to conservation was incorporated to allow control of sward conditions on the grazed area.
Two experiments, each lasting approximately 12 months, were carried out at North Wyke, Devon, in 1982‐83 (A) and 1983‐84 (B), to investigate various sward managements following oversowing of white clover (Trifolium repens , cv. Grasslands Huia) at 4 kg ha ‐1 with a Hunter Rotary Strip‐Seeder in June or July into the stubble of a permanent grass sward following conservation. Experimental managements comprised cutting, grazing with wether sheep or grass suppression by herbicide, as appropriate, in late summer/autumn (Phase I), winter (Phase II) and spring/early summer (Phase III).
During Phase I, there was no differential effect on clover stolon development of lenient grazing at approximately 4‐weekly intervals or topping at the same frequency to a similar height. Early in Phase II of Experiment A, grazed paddocks became so badly poached that no differences occurred between grazing either to early January or throughout the winter. Under drier conditions in Phase II of Experiment B, continuous grazing at either five (L) or ten (H) sheep ha ‐1 had no immediate effect on clover stolon development, but in a silage cut in June, paddocks formerly stocked at the lower rate yielded 40% more DM than those at the higher rate.
Experiment A compared the use of a grass‐suppressing herbicide, propyzamide, applied at 0.6 kg a.i. ha ‐1 in either October or February; in Experiment B it was applied in October. Prophyzamide applied at either time in Experiment A increased the clover content of herbage regrowing after the end of the experimental period from 16% to 36% (s.e.d. ± 3.9). In Experiment B, October application raised the clover contents of herbage cut in June 1984 from 10% (H) and 17% (L) to 32% (s.e.d.±5.9), and stolon lengths per m ² at the end of the summer period from 33 (H) and 56 (L) to 86m (s.e.d. ± 11.7). However, the effect of spraying propyzamide on subsequent herbage yields was erratic, and appeared to depend on the incidence of frost after application.
In Phase III of Experiment A, continuous grazing was compared with a silage cut in June. At the end of the experiment there were 31 m m ‐2 of clover stolon in silaged areas compared with only 2.5 m m ‐2 following grazing (s.e.d.±6.6). Clover content and herbage yields were also significantly higher following conservation. In Experiment B in the same period, rotational grazing with a 14‐ or 35‐day recovery interval was compared with a silage cut in June, with or without 100 kg N ha ‐1 applied in March. Application of N to the conservation treatment reduced clover stolon length per unit area, and in the regrowth in the post‐experimental period the conservation treatment without N had the largest clover content (31% compared with 16‐23% for other treatments, s.e.d. ± 3.6)
Continuous stocking with sheep at high stocking rates may reduce the content of white clover (Trifolium repens) in mixed grass-clover swards. The present experiment was carried out to investigate the effects on sward production and composition of resting a perennial ryegrass (Lolium perenne)- white clover sward from grazing and taking a cut for conservation.
Swards were set-stocked with 25 and 45 yearling wethers ha−1 either throughout a grazing season, or on swards that were rested for a 6-week period and then cut in early, mid- or late season. In an additional treatment swards were cut only and not grazed.
Net herbage accumulation was higher at the lower of the two stocking rates and was marginally increased by the inclusion of a rest period at the high but not the low stocking rate. Clover content was higher at the lower stocking rate and was increased by the inclusion of a rest period by 30% at 45 sheep ha−1and by 11% at 25 sheep ha−1 The effect was most marked at the end of the rest period before cutting. When rested from grazing the tiller density of ryegrass decreased although tiller length increased, and clover stolon length, petiole length and leaflet diameter increased though leaf and node number per unit length of stolon decreased; the reverse applied when the sward was returned to grazing after cutting.
At the high stocking rate, rest periods in mid-season or later maintained the greatest clover content and marginally increased total net herbage accumulation. At the low stocking rate the timing of the rest period had no significant effect on total net herbage accumulation or on clover content.
These results show that the combination of grazing and cutting is of benefit where the stocking rate is high enough to threaten clover survival and limit sheep performance. However, at such a stocking rate, feed reserves are at a minimum throughout the grazing season and so opportunities for resting the sward are probably low.
The long-term influence of sward height from April to July (Phase 1) and from July to early October (Phase 2) on total herbage and white clover production was measured over four years (1988–1991) as herbage accumulation. A subsidiary experiment to determine the influence of leaf area index (LAI) on gross canopy photosynthesis was conducted to aid interpretation of growth from herbage accumulation data. Target sward heights in 0·5 ha plots in two blocks were 5,7 or 9 cm in Phase 1 and 7 or 9 cm in Phase 2, although mean actual heights per phase were slightly higher. Net herbage accumulation (NHA) was measured within mobile exclosure areas over successive two-week intervals. Gross photosynthesis was measured in circular turves removed from the trial area representing a range of LAIs with an at least reasonable clover content. Despite wide differences in mean sward height and herbage mass, NHA and net clover accumulation for a given phase were not generally affected by treatments. Positive effects of grazing at 5 cm in Phase 1 on NHA and clover accumulation later in the year, and of grazing at 7 cm in Phase 2 on NHA in the following spring were sometimes apparent. Gross canopy photosynthesis (g CO2 m−2 h−1) at 1500 μE m−2 s−1 and 18–21°C was linearly related to LAI described by 1·003 + 1·165 LAI over the LAI range 0·7 to 4·5. Total herbage and clover growth, interpreted from NHA by a previously described model, was predicted to be marginally lower in shorter swards. Similarity in NHA and clover accumulation between treatments was considered to be because of lower senescene and decomposition, and a higher proportion of new tissue being assigned to lamina growth, despite lower LAI and gross photosynthesis in the shorter swards. It was concluded that stocking intensity in swards continuously stocked with cattle did not have a strong influence on net total herbage and clover growth.
Comparative herbage characteristics and sheep production (ewes and lambs until weaning and lambs thereafter) from swards of Aurora (very early flowering), Frances (early flowering), Talbot (intermediate flowering) and Melle (late flowering) varieties of perennial ryegrass (Lolium perenne) were studied under a continuous variable stocking management based on sward surface height guidelines. The varieties were assessed as grass-only (215 kg N ha−1 fertilizer) and grass/clover (83 kg N ha−1 fertilizer) pastures.
Over 2 years (1985-86) total annual lamb production per hectare from grass-only swards of Aurora was 19% more than that from Frances despite similar herbage productivity. Differences between the varieties in lamb output were more pronounced as grass/clover swards, with Aurora producing 29 and 18% more lamb than Melle and Talbot respectively and with Frances also giving 16% more than Melle. Overall lamb production from grass/clover swards was 10% more than that from the grass-only pastures, mainly due to 69% advantage in individual lamb growth rates after weaning. Herbage organic matter digestibility, during the post-weaning period, was higher on grass/clover than on grass-only swards, and similarly with Aurora versus the other varieties. Under a frequent cut simulated grazing regime the relative herbage productivity of the four varieties differed, with Aurora 13% less productive than Frances.
The results are discussed in relation to the limitations of assessing performance under cutting, and the significance of the interaction in animal productivity between the two sward types.
The objective of this study was to characterize changes in morphology and shoot/root total nonstructural carbohydrate (TNC) relationships in Caucasian bluestem (Bothriochloa caucasica) under grazing. Pastures were continuously grazed by steers weighing about 225 kg from mid-May to late September of 1983 and 1984. Heavily grazed (H) swards produced more tillers m−1 than lightly grazed (L) swards. Number of tillers declined in both treatments during the midsummer drought period; however, the reduction was much less in H than in L swards. Above normal precipitation in August 1983 allowed number of tillers to rapidly recover (by approximately 4000 and 8000 m−1 for L and H treatments, respectively) reaching 10 700 and 17 800 tillers m−1 respectively, in late September. After 2 years of grazing, H swards initiated twice as many roots on an area basis; however, ability to penetrate soil to lower depths may have been reduced. Heavily grazed Caucasian bluestem had lower concentrations of total non-structural carbohydrate (TNC) in shoots (P < 0.04) in both years, but had higher (28% at the end of 1984) TNC concentration in roots compared to lightly grazed (P= 001) plants. Greater root mass was found in lightly grazed swards; therefore, we suggest that root growth and TNC concentration tend to be inversely related. Total amount of TNC was higher in L swards, but during the course of the study total above and below-ground TNC in H swards remained stable. Total above-ground TNC pools are subject to short-term variation and dilution from senescent herbage, thus TNC pools in individual plant parts may be a better indication of plant response.
Utilized Metabolizable Energy output was calculated and herbage utilization evaluated in two contrasting years on five profitable farms representing a range of soil types.
Annual UME output was 72 GJ ha-1 on average, with a range from 47 to 91 GJ ha-1. Grazed grass provided 66% of the utilized metabolizable energy, and conserved grass 34%. Higher utilized metabolizable energy output was not always obtained at higher fertilizer N inputs, even when soil moisture conditions favoured herbage growth.
The efficiency of utilization of herbage conserved (almost always as silage rather than hay) was calculated by expressing utilized metabolizable energy output as a proportion of the quantity of herbage cut, measured by swath weighings; the mean value was 64%, with a range from 55 to 73%. Cellulose analyses indicated that mean dry matter losses via CO2 and effluent were 10%; the remaining 26% loss appeared to be due to physical losses in the field, surface waste and wastage at feedout.
For grazed herbage the utilized metabolizable energy output was expressed as a proportion of herbage accumulation measured over 28-day periods. The resultant apparent efficiency of utilization averaged 67%, with a wide range from 51 to 83%. The lowest values were on badly drained farms.
It is suggested that:
(i) there is considerable potential for increasing output from grazing on dairy farms; higher grazing pressure and more flexible management would be needed. Targets should probably be set lower on badly drained soils;
(ii) there is great potential for increasing the efficiency of utilization of conserved forage, by careful application of existing technology;
(iii) on the farms studied the utilized metabolizable energy output from grazed and conserved fields appeared to be similar.
Herbage growth was measured in two contrasting years on five farms representing a range of soil types. On fields cut for conservation in 1982 DM production varied between farms from 12.3 tha-1 to 14.1 tha-1 in line with fertilizer N levels, which ranged from 220 to 333 kg ha-1. In 1983 production was from 10.3 tha-1 to 12.3 tha-1; this again corresponded with fertilizer N, except on farm B which had the shallowest soils. On fields used for grazing all farmers used rotational management. Herbage accumulation was measured by trimming to 25 mm and harvesting after 4 weeks. As on conservation fields, herbage accumulation was generally greater on farms using more N, at least up to 300 kg ha-1.
In 1982 growth consistently exceeded predictions based on plot experiments. In 1983 growth was on average less than predicted on grazing fields but slightly more than predicted on cutting fields-We conclude that such predictions of annual production are a valuable aid to farm planning, and need not be reduced to allow for on-farm conditions, although they are least good at low levels of N. Predictions of individual cuts were not satisfactory, with a strong tendency to overestimate first cut and underestimate second cut.
There was no indication that annual herbage production was less on badly drained than on well drained soils. Indeed, in the dry summer of 1983 badly drained land appeared to have an advantage, despite considerable poaching damage in the spring. Furthermore, there was no evidence that production was deficient on swards dominated by Holcus and Agrostis spp.
Populations of Lolium perenne were sown at a wide range of densities and allowed to develop without defoliation under full daylight and under reduced light intensities. The multiplication of tillers, the growth in their mean weight and the death of genetic individuals (genets) acted together to regulate the character of the population and to determine that the density of tillers per unit area became independent of sowing density. The rate of elimination of genets from the population was related to the rate of growth of the survivors according to the 3/2 thinning law, but under low light intensities the thinning process was radically altered in a way that suggests that the density stress within the grass populations was caused by mutual self-shading.
Swards of S24 perennial ryegrass (Lolium perenne L.) were grown in the field and harvested by repeated cutting. The rate of photosynthesis of the swards and of single leaves of plants taken from them was measured throughout the growing season. Leaf area and light interception of the sward were also measured.
In each growth period, sward photosynthesis increased with increasing leaf area and then declined. In the first growth period, during which the crop flowered, the maximum rate of sward photosynthesis was higher and was reached later than in succeeding vegetative growth periods.
The photosynthetic capacity of successive youngest expanded leaves was high throughout the first growth period, and only declined due to leaf age when no further leaves were produced after flowering. However, in the succeeding vegetative growth periods, although the first leaves to expand had high photosynthetic capacities, later leaves had progressively lower capacities.
The extent to which changes in the photosynthetic capacity of single leaves accounts for the changes in sward photosynthesis observed in the field is examined using a mathematical model.
The rate at which the net photosynthesis of grass leaves grown in bright light (119 W m⁻²) decreased as they aged was increased by severe shading (to 21 W m⁻² or less). However, less severe shading (light intensities of 36 W m⁻² or more) had no effect. The decrease in photosynthesis was unaffected by whether the whole plant was shaded or only the leaf whose photosynthesis was measured.
In both shaded and unshaded leaves, photosynthesis measured in bright light fell faster as the leaf aged than did photosynthesis in dim light. Both mesophyll and stomatal diffusion resistances rose as the leaf aged but the former rose faster. The chlorophyll content fell only towards the end of the life of the leaves.
(1) Perennial ryegrass swards were continuously grazed by sheep. One sward was grazed to maintain a Leaf (lamina) Area Index (LAI) of close to 1·0 ('hard'-grazed); a second sward was grazed to maintain an LAI of close to 3·0 ('leniently'-grazed). (2) In the hard-grazed sward, the growing leaves were efficient in photosynthesis and, together with the youngest fully expanded leaves, contributed 77% to the net photosynthesis of the sward canopy. Sheaths contributed very little-less than 5%-even though the area of sheath was almost as great as the area of young laminae. (3) Despite the high photosynthetic efficiency per unit leaf area in the hard-grazed sward, canopy photosynthesis in this sward was substantially less than in the leniently-grazed sward. (4) Calculated over the grazing season, April to September, the amount of gross photosynthetic uptake accumulated under lenient grazing was only marginally less than under infrequent cutting. (5) The consequence to yield of the difference in photosynthetic uptake between the hard- and the leniently-grazed swards is discussed.
(1) Measurements were made to determine the balance between photosynthesis, animal intake and the losses of matter in swards maintained by 'lenient' continuous grazing by sheep at a Leaf (lamina) Area Index of 3·0 and by 'hard' continuous grazing at an LAI of 1·0. (2) Gross photosynthetic uptake was greater in the leniently-grazed sward than in the hard-grazed sward. In both swards, a similar proportion of photosynthetic uptake was lost in respiration or by partition to non-harvestable parts. Thus, shoot production under lenient grazing was also greater than under hard grazing. (3) Despite the lower gross photosynthetic uptake in the hard-grazed sward, animal intake in this sward was greater than in the leniently-grazed sward. This was because, in the hard-grazed sward, a far greater proportion of the shoot produced was harvested-a far smaller proportion remained unharvested to be lost to death. (4) It is a major limitation to production under continuous grazing that the high gross photosynthetic uptake and high rate of shoot production seen in the leniently-grazed sward cannot be associated with a high efficiency of harvest and so give rise to high harvested yield. Maximum intake per hectare is therefore achieved in a sward maintained at an LAI which is substantially below the optimum for photosynthesis.
Experiments designed to assess differences between treatments in pasture productivity as measured by the production of animals are complicated by the stocking rates chosen. The paper attempts toderive a simple model relating production per animal and production per unit area to stocking rate for meat animals, based on biological considerations.
A method is proposed which would allow meaningful grazing experiments to be carried out employing only one stocking rate, thus considerably reducing the expense of this type of experimentation.
The model is applied to some recent data obtained by Hodgson (1966) and Appleton (1967, personal communication).
The causes of tiller death in a 2-year-old perennial ryegrass sward were examined between April and August 1977. Physiological causes accounted for most tiller deaths and grazing by slugs and rodents was more important than the damage caused by stem-boring larvae. Tillers which died were mainly small and vegetative, although some flowering tillers died prematurely. Low nutrient status delayed but did not prevent tiller death.
Using 14CO2 it was shown that small tillers fixed relatively less radiocarbon than did larger tillers and they did not receive much support for their carbon economy. Selective defoliation showed that in April defoliated tillers imported radiocarbon from undefoliated tillers but that in July at anthesis an undefoliated reproductive tiller retained most of the carbon it fixed, despite its vascular association with defoliated tillers. It appears that much of the tiller death during the period April-August is due to the failure of the more favourably placed tillers to support other tillers which are heavily shaded.
Autoradiographs were made of plants of Lolium multiflorum Lam. after 14CO 2 had been fixed by selected leaves. The results showed that labelled compounds were not translocated to other tillers but were moved to the whole root system. This pattern of distribution was changed when all or some of the tillers on the plant were defoliated. Where a single undefoliated tiller remained, it initially supplied the cut tillers with 14C-containing products, thus reintegrating a system of apparently independent tillers. When all the tillers were partially defoliated, labelled compounds were no longer translocated to the root system. A further experiment suggested that root reserves were not mobilized for regrowth following defoliation. These results are discussed in terms of the integration of a grass plant in the vegetative state.
The leaf growth, tiller production, light interception, and dry weight increase of a simulated sward of S24 perennial ryegrass ( Lolium perenne ) were followed during the development of the sward from a collection of two-leaved seedlings to a closed canopy with an LAI of 23, of which 15 consisted of green leaf laminae. The dry weight of live shoots increased exponentially at first, but then entered a long linear phase of increase. This was equivalent to a crop growth rate of 200 Kg ha−1 day−1 and a conversion efficiency of radiant energy (400–700 nm) of 7.2 per cent. Towards the end of the growth period the rate of increase of live shoots declined rapidly to zero and a ceiling yield was reached equivalent to 10 metric tons ha−1. Leaf growth continued at a high rate, but was equalled by the rate of leaf death, so that the weight of live leaf tissue remained constant. By this time the swards had achieved a stable tiller population (about 1 cm−1), each tiller bore a constant number of live leaves (about three), and the length of each newly expanded leaf equalled the length of the old leaf it replaced (about 70 cm). The swards were grown in Perlite so that in the absence of soil fauna dead leaves accumulated at the base of the sward where, after 12 weeks, they accounted for 19 per cent of the total weight of dry matter produced.
Measurements of CO2 exchange were used to construct a detailed account of the carbon economy of established simulated swards of perennial ryegrass during 10 week periods in spring and autumn. Changes in sward dry weight estimated from gas exchange measurements closely matched observed changes in dry weight.
In spring, light energy increased, the photosynthetic potential of the canopy increased, and together these factors led to a pattern of increasing photosynthetic uptake. In autumn, decreasing light energy and decreasing canopy photosynthetic potential led to decreasing photosynthetic uptake. During the periods investigated, the changes in light energy receipt played the major role in determining the pattern of photosynthetic uptake.
A simple model of crop growth was used to illustrate the effect of such characteristic seasonal differences in the pattern of photosynthetic uptake on the subsequent loss of carbon during respiration and tissue death, and consequently on the production of live tissue. The model describes how a reproductive crop in spring may accumulate more living dry matter than a vegetative crop in autumn from the same total gross photosynthetic uptake of carbon.
COi-assimilaie wiihin vegetative planis o( Lolium penmnt at low and high densiiy Lamb production and seasonal production of pastures Tissue turnover in the sward
- Ci Lvillk
- Marshallc 1 Curll
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196S) The design and interpre-tation of experiments to study animal production from grazed pastures Carbon exchange and assimilate partition-ing
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The use ofcutting treatments designed to simulate defoliation by sheep Journalofihe Brilish Grassland Socieiy, 25, 198-206 Continuous stocking by sheep on the structure and physiology of swards The physiology of tiller death in grasses. I. The influence of liller age. size and position
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Milk from grassesSeminar Integrating the Cutting and Grazing of Grass, held at ICI House
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Effects or defoliation and shading on short‐rotation ryegrass
- Mitchell K. J.
Milk from grassesSeminar Integrating the Cutting and Grazing of Grass held at ICI House Millbank
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Enclosure method for measuring photosynthesis, respiration and transpiration of crops in the field
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