Effect of irrigation and plant density on yield, composition and in vitro digestibility of a new forage sorghum variety, Tal, at two maturity stages

Abstract

Most of the commercial varieties of forage sorghum belong to the tall types. Use of low types is limited, mainly due to their lower forage productivity. Recently a new low variety of forage sorghum, Tal, was developed in Israel. This study examined effects of irrigation level (IL) and plant density (PD) on Tal productivity and quality, as measured by field performance, chemical composition and in vitro digestibility. The optimal harvest stage for getting the best combination of yield amount and forage quality was explored. Irrigation included levels of 20, 100 and 180mm, and PD consisted of 200,000, 260,000 and 330,000plants/ha. Harvests were carried out at maturity stages of early heading (EH) and soft dough (SD). Tal resistance to lodging was high. High irrigation increased plant height and dry matter (DM) yield in both harvests, and enhanced the content of neutral detergent fiber (NDF) and lignin, at EH. In most cases, additional irrigation decreased DM content, DM ratio of leaves/stems, and in vitro DM digestibility (IVDMD). Plant density did not affect significantly plant height or DM yield at either harvest, but did affect DM digestibility at EH. Maturation from EH to SD increased considerably DM content under all irrigation levels, and DM yield only under high irrigation. Maturation increased DM allocation to the panicles and enhanced their DM digestibility. Tal has the potential to become a successful forage crop, which under sufficient irrigation attains the best digestible DM and NDF yields at SD.

Full text

Animal Feed Science and Technology
131 (2006) 120–132
Effects of irrigation and plant density on yield,
composition and in vitro digestibility of
a new forage sorghum variety,
Tal, at two maturity stages
Avner Carmi ∗, Yohav Aharoni, Menahem Edelstein,
Nakdimon Umiel, Amir Hagiladi, Edith Yosef,
Moses Nikbachat, Abraham Zenou, Joshua Miron
Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet-Dagan 50250, Israel
Received 25 July 2005; received in revised form 18 January 2006; accepted 8 February 2006
Abstract
Most of the commercial varieties of forage sorghum belong to the tall types. Use of low types is
limited, mainly due to their lower forage productivity. Recently a new low variety of forage sorghum,
Tal, was developed in Israel. This study examined effects of irrigation level (IL) and plant density
(PD) on Tal productivity and quality, as measured by field performance, chemical composition and
in vitro digestibility. The optimal harvest stage for getting the best combination of yield amount and
forage quality was explored. Irrigation included levels of 20, 100 and 180mm, and PD consisted
of 200,000, 260,000 and 330,000 plants/ha. Harvests were carried out at maturity stages of early
heading (EH) and soft dough (SD). Tal resistance to lodging was high. High irrigation increased plant
height and dry matter (DM) yield in both harvests, and enhanced the content of neutral detergent
fiber (NDF) and lignin, at EH. In most cases, additional irrigation decreased DM content, DM ratio
of leaves/stems, and in vitro DM digestibility (IVDMD). Plant density did not affect significantly
plant height or DM yield at either harvest, but did affect DM digestibility at EH. Maturation from EH
Abbreviations: CP, crude protein; DM, dry matter; EH, early heading stage; IL, irrigation level; IVDMD,
in vitro dry matter digestibility; NDF, neutral detergent fiber; PD, plant density; SD, soft dough stage; S.E.M.,
standard error of the means
∗Corresponding author. Tel.: +972 3 9683946; fax: +972 3 9604023.
E-mail address: Carmi@volcani.agri.gov.il (A. Carmi).
0377-8401/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.anifeedsci.2006.02.005

A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132 121
to SD increased considerably DM content under all irrigation levels, and DM yield only under high
irrigation. Maturation increased DM allocation to the panicles and enhanced their DM digestibility.
Tal has the potential to become a successful forage crop, which under sufficient irrigation attains the
best digestible DM and NDF yields at SD.
© 2006 Elsevier B.V. All rights reserved.
Keywords: New sorghum variety—Tal; Plant density; Irrigation level; Growth stage; In vitro digestibility; Forage
composition and yield
1. Introduction
Sorghum is becoming an increasingly important forage crop in many regions of the
world (Zerbini and Thomas, 2003). Its high resistance to drought makes it a suitable crop
for semi-arid areas (Tabosa et al., 1999), especially in light of its higher productivity under
dry conditions compared to corn (Tabosa et al., 1986). Expanding the use of sorghum as
a forage crop obliges to overcome its tendency to lodging that characterizes the tall types
especially under irrigation (Reddy et al., 1999; Miron et al., 2005). Another obstacle to
expanded use of tall forage sorghums is their insufficient accumulation of DM content, as
proper accumulation is a precondition for successful ensiling (Carmi et al., 2005; Miron et
al., 2005, 2006).
Improving the nutritive value of forage sorghum for productive ruminants is an important
goal. According to Casler (2000), the main selection criterions for improving forage nutri-
tional value must include increased in vitro dry matter digestibility (IVDMD) and reduced
content of lignin. Such improvements may be promoted by genetic breeding and selection,
choosing the optimal stage for harvest (Carmi et al., 2005; Miron et al., 2006) and improving
growth factors, such as irrigation level (IL) and plant density (PD) (Cusicanqui and Lauer,
1999; Singh and Singh, 1995).
Sorghum can respond to additional irrigation by stem elongation and increase of yield
(Saeed and El-Nadi, 1998; Singh and Singh, 1995). However, better water status can increase
lignin content and decrease sorghum digestibility (Amaducci et al., 2000).
Plant density can affect forage yield (Cusicanqui and Lauer, 1999) and quality (Defoor
et al., 2001). An increase in PD can reduce water availability to the individual plant and lead
to water deficiency (Berenguer and Faci, 2001), followed by yield decrease. This response
may be compensated by an improved IVDMD caused by the decrease in lignin content of
the denser canopy. Plant density can also affect plant morphology (Lafarge and Hammer,
2002), DM content (Rosenthal et al., 1993) and chemical composition (Widdicombe and
Thelen, 2002), parameters that affect forage quality.
There is little research concerning the integrated effects of IL and PD on yield, IVDMD
and chemical composition of forage sorghum. In particular, there is limited information
relating to low types of sorghum that have the potential for forage production. Recently, a
new low type of sorghum variety, Tal, was developed in Israel. This variety is characterized
by high resistance to lodging, high content of DM and high yield. Its IVDMD is comparable
to that of the commercial variety FS-5 (Carmi et al., preliminary study).

122 A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132
The objectives of this research were to study comprehensively the effects of IL and
PD on field performance, chemical composition and IVDMD of the new variety Tal,
and to determine its optimal harvest stage for getting the best combination of yield and
quality.
2. Materials and methods
2.1. Growth conditions and experimental design
The new low variety of forage sorghum, Tal (Sorghum bicolor) registered in Israel in
2004, is characterized by early flowering and grain maturation, rapid and high accumulation
of DM, and high resistance to lodging. Tal was raised in the summer of 2004 in Neve-
Yahar, in the northern part of Israel. The climate there is of the Mediterranean type, and
lacks summer rainfall. The field was in fallow during the previous winter, and received
approximately 600 mm of rainfall. The field was tilled to a depth of 15 cm, and was treated
pre-sowing with 2.4 l Atrazine/ha to prevent weed germination. A week after germination,
the field was re-treated against weeds, by using 2–4-D. A pre-sowing supply of 150 kgN/ha
was given in the form of urea. An additional amount of 100kg N/ha was given 14 days after
germination ended.
Sorghum seeds were sown in nine separate plots of 0.2 ha each, in a tri-factorial design,
which included nine combinations of three plant densities ×three irrigation levels ×two
maturity stages. Each plot (treatment) included 10 beds with four plant rows, a distance
of 30 cm apart and with a row length of 100 m. The distances between adjacent beds were
80 cm. Each plot was divided randomly into four sub-plots, 15 m in length, located along
four adjacent plant rows of the same bed. Plant densities of low, mid and high levels were
represented by 200,000, 260,000 and 330,000 plants/ha, respectively. Low, moderate and
high irrigation levels were 20, 100 and 180 mm of water, respectively, supplied by sprinklers.
Seeds were sown on March 23, 2004, and irrigated with 20 mm of water for germination.
Fourteen and 28 days later, moderate and high IL plots received two additional irrigation
doses of 40 mm each. High IL got two more irrigation doses of 40 mm each, at 42 and 56
days after germination.
In each sub-plot, plant samples were harvested from four adjacent rows of 5 m length,
which were taken at EH and SD and weighed in the field. Stem height, number of leaves
on the main stem and extent of lodging were measured in each sub-plot before cutting.
About a fifth of the harvested plants were divided into panicles, leaves and stems, and
dried at 60 ◦C for 96 h in an aerated oven. The dry separated organs were weighed and
their proportion of the whole plant DM was calculated. An additional 30% of the fresh
plants were sampled from each replicate and chopped (particles size was in the range of
1–3 cm). Part of this chopped material was kept at −20 ◦C for composition analysis of the
green forage. Another part was dried at 105 ◦C and used for calculation of DM content and
DM yield. The rest of the biomass was dried in an aerated oven at 60 ◦C for 72 h. Each
dried sample or organ to be used for composition analysis (AOAC, 1980; Van Soest et
al., 1991) or IVDMD measurement (Tilley and Terry, 1963) was ground through a 1 mm
sieve.

A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132 123
2.2. Chemical analysis
Triplicate samples of whole forage and Tal organs were dried over 24 h at 105 ◦C for
analysis of DM content, or over 4 h at 600◦C for residual ash determination (AOAC, 1980).
Other Tal samples and organs were dried at 60 ◦C for 72 h and analyzed for chemical compo-
sition and IVDMD. Content of crude protein (CP) was determined according to the Kjeldahl
method (AOAC, 1980). Neutral detergent fiber was determined without sodium sulfite and
with heat stable amylase (aNDFom). Acid detergent fiber (ADFom) and lignin (sa) were
determined by sequential analysis and expressed exclusive of residual ash (Van Soest et al.,
1991). Hemicellulose was estimated as aNDFom–ADFom, and cellulose as ADFom–lignin
(sa). Ankom apparatus (Ankom220, Fairport, NY, USA) was used for extraction and
filtering.
2.3. In vitro digestibility
In vitro digestibility of DM and NDF of air-dried whole plants, or their separated organs,
were determined in triplicate (three tubes). The procedure involved a 48h incubation of
0.5 g of plant material in a 100 ml glass tube containing 40 ml buffer and 10 ml rumen
fluid, followed by an additional 48 h incubation with 40 ml HCl 0.1N and pepsin 0.02%,
according to the two-stage fermentation technique of Tilley and Terry (1963). Rumen fluid
was obtained before morning feeding via rumen fistula from three dry cows fed with sorghum
silage. Residual NDF in the in vitro tubes were determined according to Van Soest et al.
(1991) and used for NDF in vitro digestibility measurements.
2.4. Statistical analysis
Tal sorghum was grown in a tri-factorial design which included 18 possible combinations
of three irrigation levels ×three plant densities ×two harvest stages, in four sub-plot repli-
cates for each combination. These 72 replicates (sub-plots) were used for factorial analysis
of variance (ANOVA). SAS software (SAS, 1996) was used to calculate the significance of
the influence of the main factors (irrigation level or plant density) on various parameters and
the significance of the interaction between the main factors at each maturity stage. Parame-
ters analyzed included: morphology, organs distribution, DM yield, chemical composition
and in vitro digestibility of the green forages and their separated organs.
3. Results
3.1. Morphology, DM content and DM distribution among plant organs
Measurements of morphological traits are presented in Table 1. Stem elongation was ter-
minated at EH and plant height stayed similar during maturation from EH to SD. Additional
irrigation increased stem height from 74 to 143 cm, while PD had no effect on plant height.
Neither IL nor PD influenced significantly the number of leaves along the main stem, in
most cases.

124 A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132
Table 1
Effects of plant density (PD), irrigation level (IL) and maturity stage at harvest, on plants height (cm), dry matter
(DM) content (g/kg), number of leaves and DM distribution among the different organs (fraction of plant DM) of
Tal sorghum
PD IL
(mm)
DM
content
Plant
height
Number of
leaves
Organs partitioning
Leaves Stems Panicles
1st cut early heading
Low 20 300d81.4d10.5a0.39ab 0.51c0.10e
Low 100 248ef 112c9.50ab 0.31cd 0.53abc 0.13e
Low 180 223fg 133ab 10.5a0.32cd 0.58ab 0.10e
Medium 20 301d74.4d10.5a0.39ab 0.52bc 0.09e
Medium 100 236f121bc 9.50ab 0.32cd 0.59a0.09e
Medium 180 202g143a8.75bc 0.33c0.57abc 0.10e
High 20 280de 79.9d9.75ab 0.42a0.51bc 0.07e
High 100 252ef 115c9.25ab 0.35bc 0.56abc 0.09e
High 180 223fg 137a9.50ab 0.36bc 0.54abc 0.10e
2nd cut soft dough
Low 20 430ab 81.4d10.5a0.27de 0.32e0.41ab
Low 100 392c112c9.50ab 0.27def 0.38d0.35bcd
Low 180 393c133ab 10.5a0.22f0.38de 0.40ab
Medium 20 400bc 74.4d10.5a0.34bc 0.38d0.27d
Medium 100 379c121bc 9.50ab 0.26ef 0.38d0.36abc
Medium 180 370c143a8.75bc 0.23ef 0.34de 0.43a
High 20 442a79.9d9.75ab 0.32cd 0.39d0.29cd
High 100 396c115c9.25ab 0.25ef 0.40d0.35bc
High 180 371c137a9.50ab 0.27de 0.37de 0.36abc
S.E.M.111.5 4.62 0.45 0.02 0.02 0.03
PD effect at 1st cut NS NS *NS NS NS
PD effect at 2nd cut *NS ****
IL effect at 1st cut *** **
NS
IL effect at 2nd cut *** ***
Interaction PD ×IL at 1st cut *NS *NS *NS
Interaction PD ×IL at 2nd cut *NS ****
Growth stage effect *NS NS ***
Means in the same column followed by different superscript letters (a–g) differ significantly at P<0.05.
1S.E.M., standard error of the means.
*Significant at P<0.05 or non-significant (NS) effect of treatment or interaction.
Plant density did not affect significantly the content of DM in the biomass, in most cases.
Additional irrigation, from low to moderate IL, reduced DM content considerably. Matura-
tion increased DM content dramatically in all treatments. Plant density affected DM distri-
bution among the leaves, stem and panicles, only at SD. Additional irrigation, from low to
moderate IL, during the early growth period (the period from germination to EH), promoted
DM allocation to the stems, at the expense of DM content in the leaves. During maturation
and under low IL, a change from low to mid PD increased significantly the allocation of DM
to the stems and the leaves, while reducing it in the panicles. The effect of maturation was
significant, with respect to DM content and its distribution between the leaves, stem and pan-
icles. In both maturation stages, a significant interaction between IL and PD was shown when
considering: DM content, number of leaves and DM distribution among the different organs.

A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132 125
3.2. Dry matter yield and contents of CP and cell wall constituents
The effects of PD and IL on DM yield and forage chemical composition are summarized
in Table 2. Plant density had no effect on DM yield, irrespective of IL. Additional irrigation
during the early growth period increased moderately DM yield at EH. Additional irrigation
during the growth from EH to SD increased DM yield more dramatically, from 7.3–8.5
to 17.5–20.9 t/ha. There was a significant interaction between PD and IL in both harvests
when considering DM yield.
The effect of PD on crude protein (CP) content was influenced by growth stage (Table 2).
Irrigation at EH led to a progressive rise in CP content, for all PD levels. However, at SD
the effects were less consistent. In both harvests a significant interaction between PD and
IL was observed when considering CP content.
Table 2
Effects of plant density (PD), irrigation level (IL) and maturity stage at harvest, on DM yield (t/ha) and chemical
composition (g/kg DM) of Tal sorghum
PD IL
(mm)
DM
yield
CP
content
NDF
content
Hemicellulose
content
Cellulose
content
Lignin
content
1st cut early heading
Low 20 6.97f70.9f635cdef 320fgh 278bcd 38.0fgh
Low 100 9.96def 75.9d637cdef 364abc 226i47.6bcd
Low 180 11.1cde 84.0b670ab 385a226i58.4a
Medium 20 8.26ef 73.6e615f320fgh 255efg 40.1efg
Medium 100 12.3cd 84.1b625ef 343cdef 237ghi 45.9cde
Medium 180 11.1cde 91.4a658bc 381a227i49.7bc
High 20 7.40f75.8d620f327fgh 250fgh 42.4def
High 100 11.2cde 76.4d633def 357bcd 224i51.8bc
High 180 11.7cde 85.2b662ab 378ab 231hi 53.4ab
2nd cut soft dough
Low 20 7.31f81.9c648bcde 311gh 297ab 39.8fg
Low 100 11.3cde 67.4g659bc 334defg 286abc 39.2fgh
Low 180 20.9a73.8e683a355bcde 280bcd 48.1bcd
Medium 20 8.27ef 63.7h633def 308h290abc 35.3gh
Medium 100 13.9c64.1h648bcde 338def 263def 47.1cd
Medium 180 19.0a72.8ef 658bc 341cdef 274cde 43.2def
High 20 8.51ef 67.5g657bc 321fgh 302a33.7h
High 100 14.0bc 72.2ef 652bcd 331efgh 278bcd 43.2def
High 180 17.5ab 71.1f656bcd 336def 276cd 43.7def
S.E.M.11.25 0.66 8.28 8.38 6.92 2.08
PD effect at 1st cut NS *NS NS **
PD effect at 2nd cut NS ** NS NS *
IL effect at 1st cut **** * *
IL effect at 2nd cut **** * *
Interaction PD ×IL at 1st cut **** * *
Interaction PD ×IL at 2nd cut **** * *
Growth stage effect **** * *
Means in the same column followed by different superscript letters (a–i) differ significantly at P<0.05.
1S.E.M., standard error of the means.
*Significant at P<0.05 or non-significant (NS) effect of treatment or interaction.

126 A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132
Neutral detergent fiber content in the biomass was not affected by PD in most treatments.
Additional irrigation increased NDF content at all levels of PD at EH, but just in the low PD
level at SD. Maturation increased NDF content in most treatments. An interaction between
PD and IL was observed with respect to NDF content.
Only at EH was PD observed to affect cellulose accumulation, which increased during
maturation. Notwithstanding, hemicellulose accumulation decreased during maturation in
some treatments, and was not affected by PD. Additional irrigation increased hemicelloluse
content in most cases. Interactions between PD and IL relating to cellulose and hemicellulose
content were significant in both harvests (P<0.05).
The data indicate that PD affected lignin accumulation in the forage, in most cases.
Additional irrigation was associated with considerable increases in lignin content at both
stages, but the effect was more prominent at EH. Maturation led to considerably decreased
lignin content in most treatments. There was a significant interaction at both growth stages
between IL and PD when considering lignin accumulation (P<0.05).
Table 3
Effects of irrigation level (IL) and maturity stage at harvest, on NDF and lignin content (g/kg DM) and in vitro
digestibility of the DM and NDF in the separated plant organs of Tal sorghum, grown under medium plant density
IL Plant
organ
NDF
content
Lignin
content
DM
digestibility
NDF
digestibility
1st cut early heading
20 mm Leaves 667f26.7jk 0.67c0.65a
100 mm Leaves 674de 32.5fg 0.61fg 0.59ef
180 mm Leaves 678d33.8f0.63def 0.61cde
20 mm Stems 583i29.4hi 0.74b0.61bcde
100 mm Stems 563j38.6e0.68c0.53jk
180 mm Stems 642h46.7bc 0.61fg 0.53jk
20 mm Panicles 640h49.1ab 0.67c0.63b
100 mm Panicles 654g49.6a0.63de 0.59def
180 mm Panicles 689bc 48.5abc 0.62efg 0.57fgh
2nd cut soft dough
20 mm Leaves 696a28.9ij 0.62efg 0.61bcd
100 mm Leaves 695a34.1f0.59h0.58fgh
180 mm Leaves 693ab 31.6fgh 0.62efg 0.62bc
20 mm Stems 653g36.8e0.64d0.56ghi
100 mm Stems 668ef 41.3d0.62efg 0.55ij
180 mm Stems 685c46.4c0.61g0.54ijk
20 mm Panicles 385k29.8hi 0.77a0.58fg
100 mm Panicles 342l30.9ghi 0.77a0.56hi
180 mm Panicles 344l25.7k0.76ab 0.52k
S.E.M.12.12 0.89 0.01 0.01
IL effect at 1st cut *** *
IL effect at 2nd cut *** *
Organs difference at 1st cut *** *
Organs difference at 2nd cut *** *
Growth stage effect *** *
Means in the same column followed by different superscript letters (a–l) differ significantly at P<0.05.
1S.E.M., standard error of the means.
*Significant at P<0.05 or non-significant (NS) effect of treatments, organs or interaction.

A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132 127
3.3. Nutritional characteristics of specific organs
Since PD had no effect on DM yield (Table 2) and digestible DM yield (Table 4),
nutritional examinations of the different organs were carried out only for the mid PD. The
amounts of NDF and lignin in the leaves, stems and panicles are presented in Table 3. The
amount of NDF in the leaves was higher than in the stems or panicles, at both stages. During
maturation a dramatic decrease in the NDF content of the panicles was observed. The effect
of IL on NDF content was significant (P<0.05) but minor. At EH, lignin content in the leaves
was considerably lower than in the stems or panicles. During maturation the lignin content
in the leaves remained level, in the stem it increased in most cases, while in the panicles it
decreased dramatically.
Table 4
Effects of plant density (PD), irrigation level (IL) and maturity stage at harvest, on the in vitro DM and NDF
digestibility and yields of digestible DM and NDF (t/ha) of Tal sorghum
PD IL
(mm)
DM
digestibility
NDF
digestibility
Digestible
DM yield
Digestible
NDF yield
1st cut early heading
Low 20 0.70bc 0.68bcd 4.87f2.95g
Low 100 0.68d0.65cdef 6.72cdef 4.12cdefg
Low 180 0.61h0.62fgh 6.80cdef 4.60cdef
Medium 20 0.74a0.72a6.08def 3.67defg
Medium 100 0.72ab 0.69ab 8.83bc 5.32c
Medium 180 0.61h0.62fgh 6.81cdef 4.55cdefg
High 20 0.70cd 0.68bc 5.13ef 3.10efg
High 100 0.69cd 0.66bcde 7.73cd 4.68cde
High 180 0.62gh 0.62fgh 7.29cde 4.85cd
2nd cut soft dough
Low 20 0.65e0.64defg 4.75f3.02fg
Low 100 0.64efg 0.60hi 7.23cde 4.50cdefg
Low 180 0.61h0.58ij 12.7a8.35a
Medium 20 0.65e0.62fgh 5.38ef 3.23efg
Medium 100 0.63efgh 0.57j8.78bc 5.10cd
Medium 180 0.63efgh 0.62fgh 12.1a7.86a
High 20 0.65e0.63efgh 5.51def 3.54defg
High 100 0.64efg 0.60hi 9.00bc 5.51bc
High 180 0.63fgh 0.61ghi 10.9ab 7.01ab
S.E.M.10.01 0.01 0.83 0.57
PD effect at 1st cut **
NS NS
PD effect at 2nd cut NS *NS NS
IL effect at 1st cut ****
IL effect at 2nd cut ****
Interaction PD ×IL at 1st cut ****
Interaction PD ×IL at 2nd cut ****
Growth stage effect ****
Means in the same column followed by different superscript letters (a–j) differ significantly at P<0.05.
1S.E.M., standard error of the means.
*Significant at P<0.05 or non-significant (NS) effect of treatment or interaction.

128 A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132
In vitro digestibility of the organs is shown in Table 3. At EH, IVDMD of the stems
was higher than that of the leaves or panicles. At SD, IVDMD of the panicles increased
considerably to a level greater than that of the stems. The IVDMD of leaves dropped during
maturation, but only under low and moderate IL. The in vitro digestibility of NDF was lower
than that of the whole DM, for the same organs. During maturation, the difference between
DM and NDF digestibility of the stems and panicles increased. Increased irrigation reduced
in vitro digestibility of DM and NDF in all organs and in most treatments.
3.4. Digestibility and digestible yield of DM and NDF
Data concerning in vitro digestibility and digestible yield per hectare of whole plant
DM and NDF are shown in Table 4. At EH, IVDMD ranged from 0.61 to 0.74 and at SD
from 0.61 to 0.65. Digestibility of DM and NDF decreased in response to maturation and
IL in most cases, while PD was generally not an influence at either growth stage. At both
maturation stages a significant interaction between PD and IL was observed, with respect
to the in vitro digestibility of DM and NDF (P<0.05).
Assessment of yield in terms of digestible DM and NDF is important to quantify the
nutritional value of the forage. At both harvest stages, PD had no effect on digestible DM or
NDF yields (Table 4). Additional irrigation increased the yield of digestible DM and NDF
in all PD treatments. Maturation increased digestible yields of DM or NDF only under high
IL. Interactions between PD and IL were observed at both growth stages, with respect to
digestibility of DM and NDF and their digestible yields per hectare.
4. Discussion
4.1. Effects of PD on forage characteristics
The absence of any PD effect on Tal plant height differs from previous data concerning
other sorghum varieties (Sticker et al., 1961). This phenomenon is valuable, since it enables
increasing the canopy PD of Tal to optimal levels without the negative consequences of
excessive stem elongation and lodging. At SD Tal plants had a high proportion of DM in
the panicles, up to 0.43 of the whole plant DM. This preservation of a high proportion of the
grains in the biomass even under non-sufficient irrigation is very important, since it ensures
the forage has high nutritional quality.
In this study, increasing PD did not affect DM yield per hectare, in contrast to the
observations of previous studies (Cusicanqui and Lauer, 1999; Staggenborg et al., 1999).
According to Staggenborg et al. (1999) an increase in PD leads to yield increase when under
conditions of sufficient water availability. It is possible that the lack of any effect of PD
on yield is due to the high range of PD used in the present study, which may be above the
influential level when considering Tal yield.
In this study, increasing PD had no effect on NDF content at EH, which is different
from the results with maize (Widdicombe and Thelen, 2002). Plant density also had no
affect on digestibility of DM at SD; this was true for all irrigation conditions. Widdicombe
and Thelen (2002) reported that an increase in corn PD reduces DM digestibility. Thus it

A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132 129
appears that the responses to PD of sorghum and corn are different concerning DM and
NDF digestibility.
4.2. Effects of irrigation on forage characteristics
Sorghum responds to additional irrigation by stem elongation (Saeed and El-Nadi, 1998).
This effect was observed in this study, but only at the EH stage (Table 1) due to termination
of stem elongation upon panicle emergence. The number of leaves along the main stem,
which reflects the number of nodes, was not affected by IL, PD or growth stage (Table 1).
This indicates that the increased ratio of stems to leaves due to additional irrigation (Table 1)
is a result of stem expansion and elongation rather than emergence of new highly digestible
nodes and leaves. These morphological responses likely influence forage digestibility, as
discussed later.
In the present work additional irrigation increased DM yield in most cases. The relative
differences in yields among the irrigation treatments were greater at SD as compared to
EH. This might be explained by how irrigation was managed, since in the high IL the
additional water was provided late in the growth period. Thus it appears that Tal responds
more strongly to irrigation, with respect to DM yield, during the period of grain filling that
takes place between EH and SD.
In this study additional irrigation increased lignin content in most cases, particularly at
EH. The capacity of a better water status to increase lignin content was observed previously
for sorghum (Amaducci et al., 2000) and for other forage crops (Goodchild, 1997). In other
sorghum varieties harvested at EH, lignin content and IVDMD have been found to correlate
negatively (Carmi et al., 2005; Miron et al., 2006). Therefore, it can be assumed that the
decrease in IVDMD in response to additional irrigation at EH (Table 2) observed in the
present study is attributable to the concomitant increase in plant lignin content. It is notable
that the effect of additional irrigation on IVDMD reduction in the leaves and stems was
lower at SD than at EH (Table 3). This concurs with a previous study (Wilson and Ng,
1975), where it was observed that a better water status in maturing plants reduced the extent
of the digestibility decrease in senescing leaves and stems.
4.3. Effects of maturity stage at harvest on Tal characteristics
Maturity stage affected significantly (P<0.05) DM content, DM partitioning among
organs, chemical composition, dry matter yield, and DM and NDF digestibility of Tal
plants (Tables 1, 2 and 4).
High DM content is extremely important for ensuring an efficient ensilage process,
and the minimum level of 246 g DM/kg is recommended to prevent effluents and decay of
organic materials in the silo (Castle and Watson, 1973). From this perspective the Tal variety
is very useful as its minimal DM content at SD exceeds 370 g/kg. The corresponding value
for corn is similar at SD; these DM content values are much higher than those obtained
for tall forage sorghums varieties at SD, which exhibit DM content values ranging below
280 g/kg (Miron et al., 2005, 2006; Carmi et al., 2005). During Tal maturation a considerable
increase of DM content was detected, from 252 to 397 g/kg, even under high IL. In contrast
to this, the tall sorghum varieties respond to high irrigation levels at SD by reducing DM

130 A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132
content and by increasing lodging (Miron et al., 2005, 2006). It is likely that non-stressed
Tal plants respond to an improved water status by extending the period of active physio-
logical activity, and consequently Tal produces a higher yield of forage and grains even at
high IL.
In most treatments cellulose content increased during maturation (Table 2). This increase
is likely connected to the build up of secondary walls, rich in cellulose, in the maturing tissues
of the stems and the leaves.
During maturation, lignin content decreased, mostly under the high irrigation levels
(Table 2). This may be explained by intensive growth of panicles with starchy grain mass,
and of stem tissue with soluble sugars, which would lead to a proportional reduction of
lignin content in the Tal plant. Furthermore, lignin synthesis and accumulation usually
occur during the formation and thickening of the secondary cell walls. Therefore, factors
that affect cell wall thickening, such as maturation and irrigation level, likely influence
lignin accumulation.
The data in Table 3 reveal that in most cases the decrease of lignin content in plant
organs was accompanied by an increase of IVDMD. It is possible to address the question
whether lignin content is the major factor affecting digestibility in light of the finding that an
increase of 1% in forage lignin content is accompanied by a reduction of 4% in its IVDMD
(Cherney et al., 1991). Calculations using the data in Table 3 indicate that such specific
quantitative relationships are evident also in the present study. The average increases of
0.5 and 1.6 g/kg DM of lignin in the leaves and the stems, respectively, observed in the
three irrigation treatments during maturation, were associated with average reductions of
3.1 and 5.3% in IVDMD values, respectively. Therefore, the ratios between lignin accumu-
lation and IVDMD reduction for the leaves and the stems were 0.16 and 0.30, respectively,
approximating the 0.25 ratio of whole DM found by Cherney et al. (1991). The greater size
of this ratio in the case of the stems, as compared to the leaves, reflects that the change in
stems’ IVDMD during maturation was less affected by lignin content, perhaps due to the
accumulation of digestible soluble sugars.
5. Conclusions
This study indicates that Tal has excellent potential to be a useful forage crop, in spite of
its relatively low height. The combination of low or medium Tal plant density with sufficient
irrigation and harvest at SD provides the highest yields per hectare of digestible DM and
NDF. Under the specific experimental conditions, IL was a more influential factor than PD,
concerning most parameters. However, interactions between IL and PD were found with
respect to most of the agronomic and nutritional parameters studied. Therefore, optimal
growth management of Tal should take into consideration the practical implications of such
interactions.
Practically, Tal may be superior to the commercial tall FS-5 hybrid used in Israel, due to
its lower height and smaller lodging, greater leaf/stem ratio, higher content of DM and grain
proportion in the forage, higher content of CP and lower content of lignin. With respect
to forage digestibility and digestible DM yield per hectare, both sorghum types are similar
(Miron et al., 2005, 2006).

A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132 131
Acknowledgement
This study was supported by a grant # 362-119 of the Israeli Milk Council.
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