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Seed Biology and Population Dynamics: Expanding Horizons
Abstract
Seed biology is important for emergence in the field and for future weed infestations. This chapter focuses on seed biology, germination, dormancy and efforts in predicting weed emergence from seeds from a European perspective. It presents a brief overview of population dynamics in time and space, the factors influencing the dynamics and how population dynamics can be modelled. Emergence from the seed-bank starts with germination, pre-emergence growth and finally emergence. In addition to seeds, vegetatively propagated material is briefly mentioned. Dormancy influences under what conditions that germination can occur and regulates timing of germination. Population dynamics are important for understanding the whole system and are often based on the life-cycle of weeds: seed-bank, seedlings, adult plants, seed production and dispersal. Challenges in modelling emergence and population dynamics are large, due to differences between and within populations of species, variability in species response and there being many weed species in the same field with contrasting characteristics.
... The existence of large weed seed banks in agricultural soils have roots in the seed production of the past, leading to weed invasion in the current year and in the not-sodistant future years [1]. Weed seeds, as a valuable trait underlying the fate of weed populations, exhibit different levels of seed dormancy, ensuring their survival under uncertain conditions. ...
... Soil seedbanks store viable and non-germinated (dormant) weed seeds to maximize survival and reproduction in the face of environmental hazards. The soil seedbank has various implications for weed population dynamics, particularly through maintaining adequate insurance for population persistence and populations occupying habitats exposed to stochasticity [1,2]. Environmental conditions may impede germination from seedbank so that germination and seedling emergence can occur when conditions are suitable for germination and seedling emergence, thereby promoting survival after dispersal. ...
In every agricultural setting, weed seeds can be found in every cubic centimeter of soil. Weed seeds, as a valuable trait underlying the fate of weed populations, exhibit differing levels of seed dormancy, ensuring their survival under uncertain conditions. Seed dormancy is considered as an innate mechanism that constrains germination under suitable conditions that would otherwise stimulate germination of non-dormant seeds. This work provides new insight into changes in germination patterns along the dormant to nondormancy continuum in seeds with physiological dor-mancy. Notable findings are: (1) germination synchrony can act as a new parameter that quantitatively describes dormancy patterns and subsequently weed population dynamics, (2) germination synchrony is dynamic, suggesting that the more dormancy decreases, the more synchrony is obtainable , (3) after-ripening and stratification can function as a synchronizing agent that regulates germination behavior. Brassica napus showed the most synchronous germination with the value of 3.14, while lower level of germination asynchrony was for Sinapis arvensis, with the asynchrony value of 2.25. After-ripening and stratification can act as a synchronizing factor through decreasing asynchrony level and increasing synchrony. Weed establish a firm relationship between dormancy cycling and germination synchrony patterns, ensuring their survival and reproductive strategies. By germinating in synchrony, which is accompanied by cycling mechanisms, weeds have more opportunities to persist. The synchrony model used in the present study predicts germination behavior and synchrony along the dormant to nondormancy continuum in weed seeds with physiological dormancy, suggesting a useful method for quantification of germination strategies and weed population dynamics.
... Seed germination is a complex biological process that is influenced by different environmental physical factors, including temperature, water potential, salinity, pH and light, as well as intrinsic genetic factors (Ansari et al., 2016;Torresen et al., 2017). The ability to predict the timing of seed germination and seedling emergence using soil temperature and water potential could enhance effectiveness of seedling management (Leblanc et al., 2004;Ansari et al., 2016). ...
Seed germination is a complex biological process that is influenced by different environmental physical factors including temperature, water potential, salinity, pH and light, as well as intrinsic genetic factors. In such environments, the water needed for germination is available for only a short time, and consequently, successful crop establishment depends not only on rapid and uniform germination of the seedlot, but also on its ability to germinate under low water availability. All of these attributes can be analyzed through the hydrotime model (HT). Millet ( Panicum miliaceum L.) is cultivated in arid and semi-arid regions of Iran. Therefore, in this study, using the hydrotime modeling approach, germination response of millet to priming (water and gibberellin 50 ppm at 15°C for 24 h) and water potential (0, -0.3, -0.6, -0.9, and -1.2 Mpa) was studied. Hydrotime (HT) model were fitted to cumulative germination of seeds and recorded in germination tests carried out at different water potentials (0, -0.3, -0.6, -0.9 and -1.2 MPa) and priming treatments (control, hydropriming and hormone priming). Results showed that, germination of millet decreased significantly with reduction of osmotic potential. Results indicated that the hydro-time constant (θH) for control, hydro-priming and hormone priming were 0.89, 0.79 and 0.67 MPa d, the water potential (Ψb(50)) for control, hydropriming and hormone priming were -0.89, -0.94 and -1.11 MPa, respectively. Results indicated that the use of hydrotime model in germination prediction could be useful to provide more accurate estimates for the timing of sowing and management of millet.
Önen, H. 2021. YABANCI OTLARIN ÜREME BİYOLOJİSİ: Aseksüel (Vejetatif) Üreme, 5. Bölüm, 1.Kısım. “Herboloji (Yabancı Ot Bilimi): İlkeler, Kavramlar ve Uygulamalar / Weed Science: Theory and Practice” içinde (61 sayfa). Adana, Doi: 10.13140/RG.2.2.35669.96480
-------------------------------------------------------------------------------------------------------------Kitabın 5. bölümünün ilk kısmında yabancı otların aseksüel (vejetatif) üreme stratejileri olabildiğince bütün yönleriyle ele alınacaktır. Ancak çok yıllık yabancı otlar hem otsu hem de odunsu olabilmektedirler. Nitekim dünya genelinde tarım ve tarım dışı alanlarda; akasya türleri (Acacia spp.), kokarağaç (Ailanthus altissima), çetiotları (Prosopis spp.) ve böğürtlen türleri (Rubus spp.) gibi çalı, yarı çalı veya ağaç formundaki bazı bitkilerin yanında bazı kaktüs türleri (Opuntia spp.) de son derece önemli ekolojik ve ekonomik sorunlara yol açabilmektedir. Ancak tarımsal ekosistemlerde otsu bitkilerin daha büyük sorun oluşturduğu dikkate alınarak bu kısım tarım alanlarında ciddi sorunlara yol açan "otsu çok yıllık yabancı otların" üreme stratejilerine ayrılmıştır. Kitapta odunsu bitkiler ise farklı bir bölümde ele alınacaktır.
Attempts to discuss the various aspects of plant dormancy can be bewildering due to the excessive number of nonphysiological, independent terms that have arisen over the years. In the context of field observations and orchard management, this terminology has often been adequate. However, in the complex realm of scientific description of the processes that constitute dormancy, the terminology has not been able to keep pace with physiological investigation. In 1985, a set of alternative terms, endodormancy , ectodormancy , and ecodormancy , were suggested to improve the situation (14). During the past 2 years, R. Darnell, J. Early, G. Martin, and I have reviewed the dormancy literature to evaluate the strengths and weaknesses of new and previous terms. At various times, N. Arroyave, R. Biasi, R. Femandez-Escobar, G. Stutte, and others from around the world have contributed greatly to discussion and critical analysis of the requirements for a physiological nomenclature. In 1986, ectodormancy was replaced by paradormancy (16) due to the former’s spoken and written similarities to ecodormancy . This paper summarizes the communicative burden presented by the current terminology, the evolution of the new terms, the universal classification system in which the terms are used, and the implications for future dormancy research. These topics are presented in greater detail elsewhere (15).
A 40-yr-old soil seed bank was investigated. Soil samples were taken from the surface and down to 30 cm in a former agricultural area which was planted with spruce 40 yr before the investigation. The site was locate near Linköping, S Sweden. Viable seeds of, among others, Erysimum cheiranthoides and Galeopsis spp. were documented.
Weed management is a challenge in all agroecosystems. Given the negative consequences associated with herbicide-based weed management, it is important to consider integrated weed management options with emphasis on strategies such as biological control. Postdispersal weed seed predation by granivorous and omnivorous carabid beetles results in substantial natural suppression of weed populations. Although the role of ground beetles as “generalist predators” in various agroecosystems is known, their contribution to weed management is not well recognized. In this context, this review presents an account of carabids and their granivorous nature, the importance of a seed diet in the life histories of different carabid groups, factors affecting granivory, and their potential role in weed seed management. Below, we discuss the interrelationships among various factors influencing weed seed consumption by carabids, its consequences for weed management, and the need for future research.
1. A metapopulation neighbourhood model of the seed bank of an annual plant, that included the effects of heterogeneity in space and time, of stochastic local extinction and of dispersal, was modified using data reported previously, to examine control of the arable weed Avena sterilis. 2. In the absence of herbicide, for spatially homogeneous environments, few differences were found in the modelled mean predicted population for two levels of dispersal (strong and moderate), although the rate of spread and the variance of the number of seeds per cell were greater for the higher level of dispersal. For spatially heterogeneous environments, with strong dispersal, an increase of the spatial scale of patchiness increased the variance, whereas moderate dispersal had the opposite effect. The introduction of temporal heterogeneity did not affect the results greatly; nor did the inclusion of variation in the fecundity parameter. 3. With the introduction of a herbicide with spatially variable efficacy, the modelled metapopulation in all cases declined exponentially and became globally extinct in approximately 20 years; strongly dispersed populations with large-scale spatial heterogeneity were slightly more persistent. However, in all cases, it was usually possible to decrease the population within the model to acceptable levels (<10 seedlings m-2) within a period of between 3 and 5 years. Spatial variability was considerable and extreme patch persistence was occasionally observed.
(1) A mathematical model for simulating the population dynamics of Avena sterilis ssp. ludoviciana (Dur.) Nyman has been constructed using previously reported data. The model considers the age structure of the population of seedlings as well as the effects of density on plant survivorship and reproduction. (2) The model is used to describe the behaviour of the population in the absence of control practices and to predict the effects of various control strategies. In the absence of control, and under continuous winter cereal cropping, the population grows hyperbolically, reaching equilibrium at a density of 535 plants m-2 Annual application of herbicides with < 85% control results in moderate reductions in the equilibrium level. To obtain a negative growth of the population it is necessary to apply herbicides annually with a control level of > 90%. Fallowing the land for 1 in every 2-3 years gave a practical method of containing the populations of A. sterilis. However, to eradicate this weed it was necessary to combine crop rotation with application of herbicides. (3) The effects of changing the values of the parameters on the output of the model were generally minor. The two processes most sensitive to parameter variation were dispersal and mortality of seeds after reproduction and the fecundity of the first cohort of plants. The contribution of late emerging plants to the overall dynamics of the population was rather small and could be disregarded. (4) The model was validated by comparing simulation results with those from long-term field studies. Model predictions closely matched expermental results from herbicide trials, but gave only a crude description of the population dynamics under various crop rotations.
AlertInf is a recently developed model to predict the daily emergence of three important weed species in maize cropped in northern Italy (common lambsquarters, johnsongrass, and velvetleaf). Its use can improve the effectiveness and sustainability of weed control, and there has been growing interest from farmers and advisors. However, there are two important limits to its use: the low number of weed species included and its applicability only to maize. Consequently, the aim of this study was to expand the AlertInf weed list and extend its use to soybean. The first objective was to add another two important weed species for spring-summer crops in Italy, barnyardgrass and large crabgrass. Given that maize and soybean have different canopy architectures that can influence the interrow microclimate, the second objective was to compare weed emergence in maize and soybean sown on the same date. The third objective was to evaluate if AlertInf was transferable to soybean without recalibration, thus saving time and money. Results showed that predictions made by AlertInf for all five species simulated in soybean were satisfactory, as shown by the high efficiency index (EF) values, and acceptable from a practical point of view. The fact that the algorithm used for estimating weed emergence in maize was also efficient for soybean, at least for crops grown in northeastern Italy with standard cultural practices, encourages further development of AlertInf and the spread of its use.
Non-linear regression (NLR) techniques are used widely to fit weed field emergence patterns to soil microclimatic indices using S-type functions. Artificial neural networks (ANNs) present interesting and alternative features for such modelling purposes. In the present work, a univariate hydrothermal-time based Weibull model and a bivariate (hydro-time and thermal-time) ANN were developed to study wild oat emergence under non-moisture
restriction conditions using data from different locations worldwide. Results indicated a higher accuracy of the neural network in comparison with the NLR approach due to the improved descriptive capacity of thermal-time and the hydro-time as independent explanatory variables. The bivariate ANN model outperformed the conventional Weibull approach, in terms of RMSE of the test set, by 70·8%. These outcomes suggest the potential applicability of the proposed modelling approach in the design of weed management decision support systems.
Hydrothermal time (HTT) is a valuable environmental index to predict weed emergence. In this paper, we focus on the problem of predicting weed emergence given some HTT observations from a distribution point of view. This is an alternative approach to classical parametric regression, often employed in this framework. The cumulative distribution function (cumulative emergence) of the cumulative hydrothermal time (CHTT) is considered for this task. Due to the monitoring process, it is not possible to observe the exact emergence time of every seedling. On the contrary, these emergence times are observed in an aggregated way. To address these facts, a new nonparametric distribution function estimator has been proposed. A bootstrap bandwidth selection method is also presented. Moreover, bootstrap techniques are also used to develop simultaneous confidence intervals for the HTT cumulative distribution function. The proposed methods have been applied to an emergence data set of Bromus diandrus.
The modification of the weed seed bank in the top soil (0–20 cm) during a six-year period including a sequence of annual crops and a perennial grass-clover ley in the rotation, were analysed in an organic cropping system after conversion to organic farming. The seed bank of dicotyledonous annual species varied from a minimum of 7200 seeds m, the year following a three-year period with perennial ley, to a maximum of 17600 seeds m, the year following a three-year period with annual crops. The number of species in the seed bank was essentially constant at 18–21 during the rotation, but the number of emerged species in the field decreased from about 19–20 during the annual crop period to a minimum of eight in third year ley, indicating a reduced input of seeds to the seed bank during the ley period. Dicotyledonous perennial weed seeds in the soil constituted only 2–3% of the seed bank. The correspondence between the soil seed bank and emerged weed plants was not straightforward. For Capsella bursa-pastoris and Chenopodium album, only 41% of the calculated correlations were significantly positive, indicating that an increase in the seed bank only occasionally can predict an increase of the actual weed flora.
Weed seeds in the soil go through annual dormancy-nondormancy cycles, exhibiting a continuum of changes in physiological responses. These gradual transitions may be related to changes in membrane properties. Seeds of winter and summer annuals respond in opposite ways to seasonal temperature changes; in both groups the responses are important life cycle adaptations to the habitat.
This book deals with the appearance and management of plants with different traits occurring as weeds on arable land. It describes and discusses matters and relationships that are important as a basis for understanding the varying occurrence of weeds in different crops and cropping systems and, at the same time, the response of different weeds to specified management measures for integrated weed management. The following topics are covered: ecological plant classification, annual and perennial crops, weed communities in vegetation succession, diverse life forms, germination, emergence and establishment, competition, plant adaptation to environmental conditions, soil tillage effects on weeds, chemical weed control, special management measures (including cover crops, mulches, biological control, plant breeding and physical weed control), and understanding the occurrence and rational management of weeds. This book will be of interest to teachers, postgraduate and undergraduate students and weed science researchers.
What are the ecological attributes of weeds that confer the ability to interfere with human activities? Roger Cousens and Martin Mortimer place weed management within an ecological context, with the focus on the manipulation of population size. The dynamics of abundance and spatial distribution are considered at both geographic and local scales. The basic processes of dispersal, reproduction and mortality are described, together with the factors that influence them. Management is shown to modify patterns of behaviour that are intrinsic to populations. Attention is given to the evolution and management of resistance to herbicides. This book provides weed science with the conceptual basis that has previously been lacking. It also gives ecologists access to the extensive database on the population ecology of weeds.
Unique long-term historical emergence records were used to assess the association between weed seedling emergence and various elements of meteorological data. These elements included both temperature-based and rainfall-related variables in the 7-d periods before and during which emergence occurred. Five weed species (Stellaria media, Chenopodium album, Capsella bursa-pastoris, Matricaria perforata, and Veronica hederifolia) with contrasting emergence patterns were studied in disturbed soil. Logistic regression analysis was used to identify meteorological variables of interest and allowed their relative importance to be assessed and ranked. Logistic regression was further used to associate probabilities of emergence with observed levels of important individual meteorological elements. This approach enabled prediction of the probability of emergence following given meteorological conditions and hence an assessment of the risk of omitting weed control measures. Predictions were made based on single meteorological variables and compared with observed data. Results indicated that temperature was the dominant factor in predicting emergence. Soil moisture, while also important, was a secondary factor only becoming important once the species-specific temperature requirement had been satisfied. The potential for further development of the model is discussed.
Bromus tectorum L. (cheatgrass) is an invasive winter annual whose seeds lose dormancy through dry afterripening. In this paper a thermal after-ripening time model for simulating seed dormancy loss of B. tectorum in the field is presented. The model employs the hydrothermal time parameter mean base water potential (ψb(50)) as an index of dormancy status. Other parameters of the hydrothermal time equation (the hydrothermal time constant θHT, the standard deviation of base water potentials σψb, and the base temperature Tb) are held constant, while ψb(50) is allowed to vary and accounts for changes in germination time-course curves due to stage of after-ripening or incubation temperature. To obtain hydrothemal time parameters for each of four collections, seeds were stored dry at 20°C for different intervals, then incubated in water (0 MPa) or polyethylene glycol (PEG) solutions (−0.5, −1.0, −1.5 MPa) at 15 and 25°C. Germination data for the thermal after-ripening time model were obtained from seeds stored dry in the laboratory at 10, 15, 20, 30, 40, and 50°C for 0 to 42 weeks, then incubated at two alternating temperatures in water. Change in ψb(50) was characterized for each collection and incubation temperature as a linear function of thermal time in storage. Measurements of seed zone temperature at a field site were combined with equations describing changes in ψb(50) during afterripening to make predictions of seed dormancy loss in the field. Model predictions were compared with values derived from incubation of seeds retrieved weekly from the field site. Predictions of changes in ψb(50) were generally close to observed values, suggesting the model is useful for simulating seed dormancy loss during after-ripening in the field.
Changes in dormancy and viability of Amaranthus retroflexus seeds were examined after placement in pots that were buried in the field. Seeds were placed in woven nylon envelopes on the soil surface or buried at depths of 2.5, 5 or 10 cm. After 1, 3, 6, 9 and 12 months seeds were exhumed and their germinability was tested to assess changes in dormancy and viability. Depletion of seed stocks placed on the soil surface was partly because of in situ germination that did not exceed 21% and did not vary significantly over the 12-month study period. Less germination of buried seeds occurred in situ, and seeds that did not germinate appeared to acquire dormancy. Exhumed seeds germinated readily; germinability was linearly related to the depth of burial, with those retrieved from the surface germinating least. Cyclical changes in germinability occurred during the 12-month burial period, but this influence was identical for seeds buried at all depths. Germinability was greatest after periods with the lowest mean monthly temperatures and least during the hottest periods. The stimulation of remaining ungerminated seeds exhumed at each period, by the addition of ethephon to the germination medium, provided further evidence of a seasonal acquisition of dormancy, and it was concluded that other unknown factors besides cyclical changes in seasonal temperature were responsible. Irrespective of placement, all seeds lost viability at an exponential rate over time. However, the decline was most rapid for those placed on the surface, whereas the loss in viability became less with increased depth of burial. Possible explanations for this adaptation of enhanced survival when buried an discussed.
WEEDSIM is a bioeconomic decision aid for management of annual weeds in corn and soybean. It was field-tested for 4 yr in Minnesota. The decision aid has two categories of management recommendations: soil-applied plus postemergence (PRE+), based on estimated weed seedbank composition and density; and postemergence (POST), based upon observed weed seedling composition and density. Weed densities, weed control, herbicide use, environmental impact of herbicide use, weed management costs, crop yields, and economic returns that resulted from PRE+ and POST recommendations were compared to those associated with herbicide management systems (HERB) that were standard for the region. After 4 yr of applying WEEDSIM recommendations to the same plots, there were no increases in annual weed densities (seedbanks, seedlings, established plants, or seed production) or decreases in weed control or crop (soybean, rotation corn, and continuous corn) yields, compared to HERB. WEEDSIM recommendations resulted in average annual herbicide applications of 1.1 kg ai ha-1 for PRE+ and 1.0 kg ai ha-1 for POST, compared to 3.5 kg ai ha-1 for HERB. Environmental impact indices associated with PRE+, POST, and HERB were 0.75, 0.71, and 0.54, with the lowest value indicating greater environmental risk than the two higher values. Similarly, average weed management costs were $24, $33, and $77 ha-1 for PRE+, POST, and HERB, respectively. Based on crop prices of $94 Mg-1 for corn and $220 Mg-1 for soybean, the average gross margins over weed control costs were higher for PRE+ ($509 ha-1) and POST ($522 ha-1) than for HERB ($455 ha-1). In general, WEEDSIM appeared to make management recommendations that adequately controlled weeds, maintained crop yields, reduced herbicide use, decreased environmental risk, lowered weed management costs, and increased gross margins over weed control costs compared to the use of herbicides standard for the region.
Research was conducted to determine the suitability of the Fermi-Dirac distribution function for modeling the seedling emergence of downy brome, johnsongrass, and round-leaved mallow, as influenced by burial depth. Six sets of previously published emergence data were used to formulate the model and test its adequacy. Two independent johnsongrass emergence data sets were used to validate the model. Constant temperature growth chamber studies were conducted to evaluate the effects of temperature and moisture on the model parameters. The Fermi-Dirac distribution function was found to adequately describe the seedling emergence of downy brome, johnsongrass, and round-leaved mallow as indicated by a good visual data fit, narrow confidence intervals for the model parameters, and regression analysis of observed vs. modeled data. Although this function is a model used in physical science, its parameters can be related to abiotic factors such as soil texture, temperature, and moisture.
Seedbanks and seedling emergence of annual weeds were examined in arable fields at eight locations in the Corn Belt. Seed densities were estimated by direct seed extraction from each of several soil cores in each sampled plot. Average total seedbank densities ranged from 600 to 162 000 viable seed m ⁻² among locations. Coefficients of variation (CV) typically exceeded 50%. CV for seed densities of individual species usually exceeded 100%, indicating strongly aggregated distributions. CV were lower for species with dense seed populations than those with sparse seed populations. Variance of total seedbank densities was unstable when < 10 cores were examined per plot, but stabilized at all locations when ≥ 15 cores were analyzed, despite a 12-fold difference in plot size and 270-fold difference in seed density among locations. Percentage viable seed that emerged as seedlings in field plots ranged from < 1% for yellow rocket to 30% for giant foxtail. Redroot pigweed and common lambsquarters were the most frequently encountered species. Emergence percentages of these species were related inversely to rainfall or air temperatures in April or May, presumably because anoxia and/or high temperatures induced secondary dormancy in nondormant seed. From 50 to 90% of total seed in the seedbank were dead. This information can be employed by bioeconomic weed management models, which currently use coarse estimates of emergence percentages to customize recommendations for weed control.
A bioeconomic model was tested as a decision aid for weed control in soybean at Rosemount, MN, from 1991 to 1994. The model makes recommendations for preplant incorporated and preemergence control tactics based on the weed seed content of the soil and postemergence decisions based on weed seedling densities. Weed control, soybean yield, herbicide use, and economic return with model-generated treatments were compared to standard herbicide and mechanical control systems. Effects of these treatments on weed populations and corn yield the following year were also determined. In most cases, the model-generated treatments controlled weeds as well as a standard herbicide treatment. Averaged over the 3 yr, the quantity of herbicide active ingredient applied was decreased by 47% with the seedbank model and 93% with the seedling model compared with a standard soil-applied herbicide treatment. However, the frequency of herbicide application was not reduced. Soybean yields reflected differences in weed control and crop injury. Net economic return to weed control was increased 50% of the time using model-recommended treatments compared with a standard herbicide treatment. Weed control treatments the previous year affected weed density in the following corn crop but had little effect on weed control or corn yield. The bioeconomic model was responsive to differing weed populations, maintained weed control and soybean yield and often increased economic returns under the weed species and densities in this research.
In a semiarid Mediterranean site in central Spain, field experiments were conducted on a Calcic Haploxeralf (noncalcic brown soil), which had been managed with three crop rotations and two tillage systems (no-tillage and conventional tillage) since 1987. The crop rotations consisted of barley → vetch, barley → sunflower, and a barley monoculture. The study took place in two growing seasons (1992-1994) to assess the effects of management practices on the weed seedbank. During this period, spring weed control was not carried out in winter crops. In the no-tillage system, there was a significant increase in the number of seeds of different weed species: anacyclus, common purslane, corn poppy, knotted hedge-parsley, mouse-ear cress, spring whitlowgrass, tumble pigweed, venus-comb, and Veronica triphyllos. Conversely, the presence of prostrate knotweed and wild radish was highest in plots under conventional tillage. These results suggest large differences in the weed seedbank as a consequence of different soil conditions among tillage systems, but also the necessity of spring weed control when a no-tillage system is used. With regard to crop rotations, the number of seeds of knotted hedge-parsley, mouse-ear cress, and spring whitlowgrass was greater in the plots under the barley → vetch rotation. Common lambsquarters dominated in the plots under the barley → sunflower rotation, whereas venus-comb was the most frequent weed in the barley monoculture. Larger and more diverse weed populations developed in the barley → vetch rotation rather than in the barley → sunflower rotation or the barley monoculture.
The ability to predict time of weed seedling emergence relative to the crop is an important component of a mechanistic model describing weed and crop competition. In this paper, we hypothesized that the process of germination could be described by the interaction of temperature and water potential and that the rate of seedling shoot and radicle elongation vary as a function of temperature. To test these hypotheses, incubator studies were conducted using seeds and seedlings of common lambsquarters. Probit analysis was used to account for variation in cardinal temperatures and base water potentials and to develop parameters for a new mathematical model that describes seed germination and Shoot and radicle elongation in terms of hydrothermal time and temperature, respectively. This hydrothermal time model describes the phenology of seed germination using a single curve, generated from the relationship of temperature and water potential.
The introduction of genetically-modified (GM) crops is often accompanied by other changes in cultural practices. The objective of the present study was to evaluate these changes with the simulation model FlorSys which quantifies the effects of cropping systems and pedoclimate on weed dynamics as well as indicators of weed-related biodiversity (species richness and equitability, trophic resources for birds, insects and pollinators) and crop production loss (yield loss, harvest contamination, harvesting problems, field infestation). The study focused on two GM maize variety types, i.e. expressing Bacillus thuringiensis toxins against insects (Bt) and tolerating the non-selective herbicide glyphosate (HT). Two contrasting maize-growing sites were studied: Aquitaine, a region in South-Western France, and Catalonia in North-Eastern Spain. Typical regional cropping systems containing maize were identified for each site from expert knowledge and the Biovigilance database recording French agricultural practices. GM scenarios were based on expert knowledge, literature and current Spanish practices. A total of 11 most probable scenarios (1 conventional, 3 Bt and 8 HT) were simulated over 28 years for each region, and repeated with 10 different regional random weather series. An additional series of 5–7 scenarios per region was run to decorrelate factors, and make it easier to identify the cultural practices responsible for changes in weed flora, biodiversity and production. The simulations showed that the changes accompanying the introduction of GM maize varieties affected weed flora as well as weed-related biodiversity and crop production loss, but that the consequences depended on local conditions. Most of these consequences were caused by simplifications in the cropping systems made possible by the GM varieties, rather than by the glyphosate associated to GM varieties. Simplified tillage or no-till increased weed harmfulness, particularly in Aquitaine where the weed flora was poorer and dominated by larger and more persistent species. Conversely, no-till cancelled part or all of the nefarious effects on biodiversity of simplified rotations (maize/wheat and maize monoculture), particularly by improving trophic resources offered by weeds to birds, insects and pollinators. Overall, biodiversity was less affected by simplified rotations in Catalonia where the initial weed flora was richer and more equitable. Delayed maize sowing reduced weed harmfulness and biodiversity, except food offer for insects and pollinators whose pertinent feeding period covered spring and summer (vs. winter for birds). Based on the two most affected weed-based biodiversity indicators, ex post monitoring after GM introduction should focus on birds in Aquitaine and on pollinators in Catalonia.
A weed seedling recruitment microsite is the location of a weed seed in the soil profile which affects germination, time of emergence and seedling establishment. The relationship between the recruitment of seedlings and their physical environment, including microclimate, soil, topography, and residue cover can provide the key to understanding the timing of seedling recruitment. The variability that exists in germination and establishment requirements within and among weed species raises important questions for recruitment research addressing multiple species, as well as regional models of genetic variability within species. Current weed recruitment research focuses mainly on summer annual species in annual cropping systems. However, with changes in cropping systems, climate, and weed biology, there will be an increasing demand for the management of both summer and winter annual weeds that develop very early in the spring. Many studies to date take an average of microsite conditions, particularly for soil depth, to describe the seedling recruitment zone. Whereas this practice of under-sampling expedites lengthy and difficult soil environmental measurements, it limits the description of the microsite for predictive purposes. Because soil disturbance disperses seeds to microsites throughout the vertical profile of the shallow seedling recruitment zone, seeds are subjected to gradients of temperature and water that create diverse microsites with depth in the recruitment zone. Research on the interactions of microclimate, field topography, soil properties and agronomic practices can further our knowledge base of the seedling recruitment microsite to better understand weed ecology and population dynamics generally, as well as enhance our ability to predict seedling recruitment specifically.
Seeds of Agrostis gigantea, Alopecurus myosuroides, Arrhenatherum elatius, Avena fatua, Bromus sterilis, Poa trivialis, Capsella bursa-pastoris, Galium aparine, Papaver rhoeas, Plantago major, Polygonum aviculare, Stellaria media, Tripleurospermum inodorum, Veronica arvensis, V. persica and Viola arvensis were sown in soil. Four patterns of emergence were recorded: 1) emergence in spring alone; 2) predominantly in autumn; 3) in both spring and autumn; 4) emergence indifferent to season. Most grasses emerged predominantly in autumn; the main period of emergence for many dicotyledons was in spring. Burial of small-seeded species both delayed and reduced seedling emergence, but germination and emergence of the larger-seeded species was increased by burial. Soil disturbance increased the total number of seedlings which emerged, but in general did not alter the relative periodicity of germination, nor was emergence promoted by cultivation at times outside the period of natural emergence. The number of viable but ungerminated seeds which remained after 2 yr varied between species: least for surface sown seed, greatest for buried undisturbed seed. Most frequent were A. myosuroides, Viola arvensis, P. rhoeas and P. aviculare. No seeds of A. elatius, B. sterilis or G. aparine remained after 12 months. Relatively few species emerged from <50 mm with the exception of several large-seeded species. The majority of annual grass-weeds will continue to be encouraged by current agronomic practices. In contrast, many annual dicotyledonous species will decline in importance, partly as a result of their susceptibility to herbicides, partly because their periodicity of germination does not coincide with the establishment phase of autumn-sown cereals.-from Authors
In three experiments, begun in successive years, seeds of twenty species of annual weeds were mixed with 15 cm of soil confined in cylinders sunk in the ground outdoors. The soil was either cultivated or left undisturbed, seedling emergence was recorded for 6 years, and each year determinations were made of the numbers of viable seeds remaining. The numbers of viable seeds of individual species and of the population as a whole decreased exponentially. In cultivated soil, the mean decrease was equivalent to 32% per year (half-life rather less than 2 years), with a range from 20-26% (Matricaria matricarioides, Tripleurospermum maritimum ssp. inodorum, Fumaria officinalis and Papaver rhoeas) to 44-48% (Senecio vulgaris, Veronica hederifolia and V. persica). In undisturbed soil the mean decrease was 12% per year (half-life 6 years), with a range of individual species of 6-21%. The mean numbers of seeds remaining viable after 6 years amounted to 5·9% of those initially added to cultivated soil and 27·5% where the soil was not disturbed. The species with the greatest proportion of seeds still viable after 6 years in undisturbed soil were Chenopodium album (53%) and Thlaspi arvense (48%). The species differed in the extent of initial seedling emergence, its distribution through the year, and in the total numbers of seedlings that emerged. Under a consistent cultivation regime the numbers decreased exponentially from year to year; in undisturbed soil emergence in each of the last three years amounted to only 1·3% of the seeds originally added. With all species a proportion of the seeds was not accounted for either as seedlings or as viable seeds after 6 years. This ranged from 20 to 60% in cultivated soil and from 33 to 70% where it was not disturbed. The results are discussed in relation to previous work on the dynamics of populations of weed seeds in the soil.
A simple model is developed in which the density of weed seedlings emerging in a field is related to (1) the ability of seedlings to emerge from various depths in the soil, (2) the survival of seeds at different depths, and (3) the depth of seed burial in no tillage, rotary tillage, and plow tillage. Other tillage regimes are considered by analogy. Literature is reviewed to determine biologically reasonable functions describing seedling emergence, seed survival, and distribution of seeds with depth, and parameters of these equations are estimated from data in the literature. Problems related to the mathematical description of these phenomena are discussed, and it is noted that some commonly held beliefs regarding survival of seeds in the soil are mutually incompatible. Although many studies have investigated the persistence of seeds as a function of depth in the soil, few have distinguished death from the production of seedlings. The model indicates that in the first year following input of seeds to the soil, no tillage will have more seedlings than tillage, but in later years no tillage will likely have fewer seedlings unless innate or induced dormancy is high or seed survival near the soil surface is unusually good. If seed return is allowed, no tillage or minimum tillage will have more seedlings perennially. Recovery of good weed control following a year with substantial seed input may be easiest if the soil is plowed deeply to bury the seeds, and then shallow or no tillage is used in subsequent years to avoid returning seeds to the surface. Much of the literature on the effects of tillage on weed density is difficult to interpret because little indication is given of the vertical distribution of seeds in the soil at the beginning of the experiment.
(1) Using a standardized procedure, a laboratory study was made of the germination characteristics of seeds collected from a wide range of habitats in the Sheffield region. Measurements were conducted on freshly-collected seeds and on samples subjected to dry storage, chilling and scarification. Responses to temperature and light flux were also examined. (2) The data have been used to compare the germination biology of groups of species classified with respect to various criteria including life-form, family, geographical distribution, ecology, and seed shape, weight and colour. (3) Marked differences were observed in the capacity of freshly-collected seeds for immediate germination. Of the 403 species examined, 158 failed to exceed 10% germination but 128 attained values greater than 80%. Germination was high in the majority of grasses and low in many annual forbs and woody species. With respect to initial germinability, major families could be arranged in the series Gramineae > Compositae > Leguminosae = Cyperaceae > Umbelliferae. Many small-seeded species were able to germinate immediately after collection and seeds of these species were often elongated or conical and had antrorse hairs or teeth on the dispersule. High initial germinability was conspicuous among the species of greatest abundance in the Sheffield flora. (4) In the majority of species, germination percentage increased during dry storage; this effect was most marked in small-seeded species. Among the seventy-five species which responded to chilling, some germinated at low temperature in darkness whilst others were dependent upon subsequent exposure to light or to higher temperature or to both. Responses to chilling were characteristic of the Umbelliferae. In all of the legumes examined, rapid germination to a high percentage was brought about by scarification. (5) Under the experimental conditions, all of the annual grasses showed the potential for rapid germination. High rates were also observed in many of the annual forbs and perennial grasses. Low rates of germination occurred in the majority of sedges, shrubs and trees, and were particularly common in species of northern distribution in Britain. Rapid germination was characteristic of the species of greatest abundance in the Sheffield flora. Rate of germination showed a progressive decline with increasing seed weight, and, with some exceptions, there was a positive correlation between rate of germination and the relative growth rate of the seedling. (6) In sixteen species, germination in the light was found to be dependent upon exposure to diurnal fluctuations in temperature. Under constant temperature conditions, the majority of grasses, legumes and composites germinated over a wide range of temperature, and the same feature was evident in species of ubiquitous or southern distribution in the British Isles. A requirement for relatively high temperature was apparent in sedges, in plants of northern distribution and in a majority of the marsh plants. The range of constant temperatures conducive to germination tended to be wider in grassland plants than in woodland species. Rapid germination over a wide range of temperature occurred in many of the species which attain greatest abundance in the Sheffield flora. (7) Although germination in most species was promoted by light, some were inhibited under relatively high light flux. In 104 species a marked reduction in germination occurred if seeds were kept in the dark, and in many species this inhibitory effect could be intensified by either or both excluding temperature fluctuations and abandoning the use of a green `safety' light. The capacity for germination in darkness was observed in all of the legumes and many of the grasses. Dark germination did not occur in the Cyperaceae and was uncommon in the Compositae. The inhibitory effect of darkness was characteristic of many of the species known to form reserves of buried seeds, but it occurred also in certain species with more transient seed banks. (8) There were recurrent associations between features of seed morphology and of germination, several of which coincided with particular ecological characteristics. (9) The functional significance of some of the germination characteristics observed in this study leads us to the conclusion that certain regenerative mechanisms in the field may be predicted from the laboratory characteristics of the seed.
Weeds are both harmful for crop production and important for biodiversity, while herbicides can pollute the environment. We thus need new cropping systems optimising all cultural techniques, reconciling agricultural production, herbicide reduction and biodiversity conservation. Here, we show how to (i) develop models quantifying the effects of cropping systems on weed dynamics, (ii) integrate interactions between weeds and other organisms, (iii) predict the impact on production and biodiversity and (iv) use the model for multicriteria evaluation and multiobjective design of cropping systems. Among the existing weed dynamics models, we chose the one closest to our requirements to illustrate these different steps, that is, FlorSys which predicts multispecific weed dynamics as a function of cultural techniques and pedoclimate. We have illustrated the development of interaction submodels with the example of a crop pathogen whose propagation is increased when infecting grass weeds. To evaluate the weed flora impact, predicted weed densities were translated into indicators of harmfulness (crop yield loss, technical harvest problems, harvest pollution, field infestation, crop disease increase) and biodiversity (weed species richness and equitability, trophic resources for birds, insects and pollinators). Simulations were run over several years and with different weather scenarios (i) to optimise cultural techniques to control harmful weeds, (ii) to analyse the impact of changing agricultural practices (e.g. simplified tillage and rotations, no-till, temporary crops) on weed density, species and trait composition and (iii) to evaluate cropping systems for their ability to reconcile agricultural production and biodiversity, thus identifying levers for designing sustainable cropping systems.
1.Spatially explicit weed population models are very useful to explore long-term weed management scenarios. The economic implications of the management practices have been rarely considered in those models. Their inclusion could enhance weed management decisions.2.A bioeconomic spatially explicit model was developed and validated using field data. The model integrated a weed population sub-model, a weed–crop competition sub-model and an economic analysis. It was employed to evaluate long-term management strategies to control Lolium rigidum Gaudin, a troublesome weed infesting cereals in Mediterranean climates. A total of eleven individual and integrated management strategies, including cultural and chemical control, were evaluated.3.The predicted and observed distributions of population growth rates were similar for the four experimental plots used in the validation. One plot was fully validated with most proposed validation methods, while in the remaining plots the predicted growth rates, in general, underestimated the observed growth rates.4.Our results showed that most individual management programmes were worse at controlling L. rigidum populations than integrated management ones. The programmes integrating cultural control strategies and herbicide at its full rate achieved the best long-term control. However, other strategies that were not as efficient at decreasing weed densities were more profitable economically, such as herbicide application at full rate. Economic returns were negative in all the strategies simulated. A sensitivity analysis showed that the seed bank and the economic outcomes were especially sensitive to fecundity and to the control exerted by management.5.Synthesis and applications. Our results suggest that the use of herbicide and low-input cultural control methods or full-dose herbicide applications are the most economically efficient strategies controlling Lolium rigidum populations. However, cultural control methods are not recommended as the only management strategies. Herbicide applications at full doses are preferable to applications at low doses, producing more successful weed control and, generally, less economic losses. The spatially explicit bioeconomic model developed has proved to be useful evaluating long-term weed management strategies in dryland cereal agrosystems. This kind of model would enhance weed management decisions for farmers based on an economic as well as an agronomic point of view.This article is protected by copyright. All rights reserved.
Distribution of seeds was not uniform in the soil profile, and fluctuated annually on ploughed plots. Seed decline, although not markedly different between treatments, was more rapid in ploughed soil. The rate of seed decline at different depths was similar for both uncultivated and ploughed plots. Successful germination alone could not account for the rate of seed decline reported, ie. losses from other causes were substantial. Some 20-30 species were recorded in the seed bank, but comprised the majority. A number of species present in the seed bank of uncultivated soil were absent from the seedling flora.-from Authors
Predicting the impact of climate change on the damage niche of an agricultural weed at a local scale requires a process-based modelling approach that integrates local environmental conditions and the differential responses of the crop and weed to change. A simulation model of the growth and population dynamics of winter wheat and a competing weed, Sirius 2010, was calibrated and validated for the most economically damaging weed in UK cereals, Alopecurus myosuroides. The model was run using local-scale climatic scenarios generated by the LARS-WG weather generator and based on the HadCM3 projections for the periods 2046–2065 and 2080–2099 to predict the impact of climate change on the population dynamics of the weed and its effect on wheat yields. Owing to rising CO2 concentration and its effect on radiation use efficiency of wheat, weed-free wheat yields were predicted to increase. The distribution of the weed was predicted to remain broadly similar with a possible northward shift in range. Local-scale variation in the impact of climate change was apparent owing to variation in soil type and water holding capacity. The competitive balance was shifted in favour of the deeper rooted crop under climate change, particularly on sites with lighter soils, owing to more frequent and severe drought stress events. Although the damage niche of A. myosuroides was predicted to reduce under climate change, it is likely that weeds with contrasting physiology, such as C4 species, will be better adapted to future conditions and pose a more serious threat.
Enhanced understanding of soil disturbance effects on weed seedling recruitment will help guide improved management approaches. Field experiments were conducted at 16 site-years at 10 research farms across Europe and North America to (i) quantify superficial soil disturbance (SSD) effects on Chenopodium album emergence and (ii) clarify adaptive emergence behaviour in frequently disturbed environments. Each site-year contained factorial combinations of two seed populations (local and common, with the common population studied at all site-years) and six SSD timings [0, 50, 100, 150, 200 day-degrees (d°C, base temperature 3°C) after first emergence from undisturbed soil]. Analytical units in this study were emergence flushes. Flush magnitudes (maximum weekly emergence per count flush) and flush frequencies (flushes year−1) were compared between disturbed and undisturbed seedbanks. One year after burial, SSD promoted seedling emergence relative to undisturbed seedbanks by increasing flush magnitude rather than increasing flush frequency. Two years after burial, SSD promoted emergence through increased flush magnitude and flush frequency. The promotional effects of SSD on emergence were strongest within 500 d°C following SSD; however, low levels of SSD-induced emergence were detected as late as 3000 d°C following SSD. Accordingly, stale seedbed practices that eliminate weed seedlings should occur within 500 d°C of disturbance, because few seedlings emerge after this time. However, implementation of stale seedbed practices will probably cause slight increases in weed population densities throughout the year. Compared with the common population, local populations exhibited reduced variance in total emergence measured within sites and across SSD treatments, suggesting that C. album adaptation to local pedo-climatic conditions involves increased consistency in SSD-induced emergence.
Dormancy is a common attribute of many weed seed populations and this usually hampers the task of predicting timing and extent of emergence of weeds. Both the number of established plants and the timing of emergence of a weed are strongly related to the dynamics of dormancy release of the seed population. In this paper, we discuss the different factors that affect dormancy in weed seed banks in soil, aiming to set a conceptual basis that will facilitate the construction of predictive models. From the long list of factors that are known to control dormancy under field conditions, we distinguish those that modify the dormancy level of the population (i.e. soil temperature and soil hydric conditions) from those that terminate dormancy or in other words, remove the ultimate constraints for seed germination once the degree of dormancy is sufficiently low (i.e. light, fluctuating temperatures, nitrate concentration). We also discuss the effect of agricultural practices on dormancy of weed seed populations, making reference to studies that have evinced clearly the factor(s) involved in determining a particular pattern of response. Overall, we stress the importance of clarifying, both qualitatively and quantitatively, the interaction between soil thermal and hydric conditions in the modification of the degree of dormancy of seed populations. Similarly, it is essential that we understand the extent to which such changes in dormancy comprise changes in sensitivity to factors that terminate dormancy.
Models quantifying the effects of cropping systems on weed life-cycles, in interaction with environmental conditions, are invaluable tools to evaluate and design integrated cropping systems. In the past, we have developed such a model called AlomySys for a single grass weed species. The objective of the present paper was to extend the monospecific emergence model to a multi-specific weed flora, by removing a few non-generic processes (e.g. maternal effects on dormancy and germination) and by adapting the germination sub-model to a larger diversity of species.The model is based on a daily simulation of weed seedbank mortality, seasonal dormancy variations, germination and pre-emergent growth until seedling emergence, in interaction with cropping systems and environmental conditions. In order to parameterize the model for numerous and contrasting species, we developed an innovative approach consisting in estimating 25 parameters from easily accessible species traits or from empirical expert knowledge. Relationships were established between seed traits (e.g. seed coat thickness, mass or shape) and seed mortality rates, dormancy levels, speed of germination as well as pre-emergent mortality. Base temperature for germination and dates of dormancy variations were related to usual dates of the species emergence in fields, obtained from expert knowledge. The generic model structure and the trait–parameter relationships make it possible to simulate a wide diversity of weed species and to predict their response to a large range of cropping systems. Thanks to a daily time-step and the integration of the interactions with soil and climatic conditions, FlorSys permits to optimize the choice of cultural techniques for weed management, as a function of field characteristics and previous technical operations. For instance, the model was used to compare the emergence of different weed species depending on tillage, in interaction with the preceding technical operations.
Summary In relatively short-term studies it is often difficult to identify the factors that are important in determining the patterns of emergence for different weed species. When results from longer-term studies are averaged over years, they demonstrate that some weed species follow charac- teristic, and potentially predictable, patterns of annual emergence. In recent years, our understanding of the seasonal changes in dormancy and germination beha- viour, and the interaction of these processes with the environment, has advanced considerably. In particular, the capacity for computer-based statistical analysis and the ability to handle large datasets has made it possible to use this information in a predictive way. Currently, there are several approaches to developing predictive weed emergence modelling. Some researchers have taken an empirical approach, seeking to identify correlations between environmental variables and observed emer- gence patterns. Others have taken a reductionist approach, subdividing the emergence process into its component stages of dormancy, germination and pre-emergence growth, to work towards achieving an eventual understanding of the physiological processes involved. Despite recent advances, a number of major challenges remain (e.g. population variability, dormancy and the quality of the input data) that must be overcome before these emergence models can be implemented in practice. The level of complexity and degree of paramete- rization incorporated within such models must also be addressed in relation to the intended use of the model.
The demography of the annual grass weed Phalaris brachystachys (short-spiked canarygrass) was studied during three consecutive years in winter wheat in south Spain. The average density was 16.70 seedlings m−2. The rate of seedling recruitment was 0.18. Average seedling survivorship was estimated with a value of 0.3 and was relatively uniform. Plant fecundity was calculated at 1232 seeds plant−1. The annual rate of population growth was estimated based on the above-mentioned parameters. The growth rate obtained was very high (λ=75.96), indicating the high potential of P. brachystachys to increase its populations in the absence of control measures. Sensitivity analysis indicated emergence, survivorship and seed losses were most sensitive to parameter variation. Management practices that reduce these parameters may result in reductions in weed populations.
The development of models that allow the prediction of timing and extent of weed emergence is essential for planning more effective weed control strategies in agricultural systems. Dormancy is a common attribute of many weed seed populations and this hampers the task of predicting emergence from weed seed banks under field conditions. In this paper, we present a conceptual framework as an attempt to understand how the different environmental factors affect dormancy in weed seed banks in soil. We also present examples showing how these concepts could be used to develop quantitative models to predict dormancy loss in summer annual weed species and discuss how predictions derived from these models could be used to maximise the effect of weed control techniques.
A mathematical model for simulating the population dynamics of annual ryegrass (Lolium rigidum) has been constructed using previous reported data. The model is used to describe the bahaviour of the population in the absence of control practices and to predict the effect of various control strategies. In the absence of control and under continuous winter barley cropping the population grew sigmodally, reaching an equilibrium at a density of 2357 seeds m−2 (1508 seedlings m−2). Annual application of herbicides at 100%, 50% and 25% the standard rate resulted in reductions in the equilibrium position of the population of 90%, 80% and 70%, respectively. Using various types of cultural control tactics (delayed seeding, crop competition, seed catching, crop rotation) resulted in partial control of this weed, maintaining relatively high population levels. Keeping the prevailing crop rotation (continuous barley) and integrating the use of various chemical and cultural tactics resulted in the best long-term results with a seed bank equilibrium level of 77 seeds m−2 (49 seedlings m−2). Results from the sensitivity analysis indicate that the size of ryegrass populations is particularly vulnerable to small changes in the reproductive capacity and the loss of seeds. These two processes should be studied with greater detail in order to provide possible clues for the control of this weed. Furthermore, control tactics specifically focused on those life cycle periods should be investigated.
Data on reproductive and vegetative weights of individuals from five species of agricultural weeds (Apera spica-venti L., Datura stramonium L., Abutilon theophrasti Medic., Sorghum halepense (L.) Pers., and Panicum miliaceum L.) were used to test Weiner's (1988) model of linear size-dependent reproductive output in plants. In general, the populations showed strong evidence (P < 0.001) of linear relationships between reproductive and vegetative weight. Linearity was most pronounced in cases where size differences were primarily due to competition. Generally, the linear relationships were consistent from population to population within a species. Many of the populations also showed positive x-intercepts, indicating a threshold size for reproduction. However, there were a number of populations with no apparent relationship between reproductive and vegetative weight, with departures from linearity, or with positive y-intercepts (negative x-intercepts). Key words: reproductive weight, vegetative weight, agricultural weeds, size-dependent reproduction.
Studies were conducted to calibrate and validate a mathematical model previously developed to predict common lambsquarters seedling emergence at different corn seedbed preparation times. The model was calibrated for different types of soil by adjusting the base temperature of common lambsquarters seedling emergence to the soil texture. A relationship was established with the sand mineral fraction of the soil and was integrated into the model. The calibrated model provided a good fit of the field data and was accurate in predicting cumulative weed emergence in different soil types. The validation was done using data collected independently at a site located 80 km from the original experimental area. There were no differences between observed and predicted values. The accuracy of the model is very satisfactory because the emergence of common lambsquarters populations was accurately predicted at the 95% probability level. This model is one of the first to take into consideration seedbed preparation time and soil texture. This common lambsquarters emergence model could be adapted to model other weed species whose emergence is limited by low spring temperature.