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

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 180 mm, 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: [email protected] (A. Carmi). 0377-8401/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.anifeedsci.2006.02.005

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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 nutritional 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).

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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 NeveYahar, 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 kg N/ha was given in the form of urea. An additional amount of 100 kg 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.

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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 composition 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 48 h 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 replicates 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. Parameters 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 terminated 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.

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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

1st cut early heading Low Low Low Medium Medium Medium High High High 2nd cut soft dough Low Low Low Medium Medium Medium High High High S.E.M.1 PD effect at 1st cut PD effect at 2nd cut 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

IL (mm)

DM content

Plant height

Number of leaves

Organs partitioning Leaves

Stems

Panicles

20 100 180 20 100 180 20 100 180

300d 248ef 223fg 301d 236f 202g 280de 252ef 223fg

81.4d 112c 133ab 74.4d 121bc 143a 79.9d 115c 137a

10.5a 9.50ab 10.5a 10.5a 9.50ab 8.75bc 9.75ab 9.25ab 9.50ab

0.39ab 0.31cd 0.32cd 0.39ab 0.32cd 0.33c 0.42a 0.35bc 0.36bc

0.51c 0.53abc 0.58ab 0.52bc 0.59a 0.57abc 0.51bc 0.56abc 0.54abc

0.10e 0.13e 0.10e 0.09e 0.09e 0.10e 0.07e 0.09e 0.10e

20 100 180 20 100 180 20 100 180

430ab 392c 393c 400bc 379c 370c 442a 396c 371c 11.5 NS

81.4d 112c 133ab 74.4d 121bc 143a 79.9d 115c 137a 4.62 NS NS

10.5a 9.50ab 10.5a 10.5a 9.50ab 8.75bc 9.75ab 9.25ab 9.50ab 0.45

0.27de 0.27def 0.22f 0.34bc 0.26ef 0.23ef 0.32cd 0.25ef 0.27de 0.02 NS

0.32e 0.38d 0.38de 0.38d 0.38d 0.34de 0.39d 0.40d 0.37de 0.02 NS

0.41ab 0.35bcd 0.40ab 0.27d 0.36abc 0.43a 0.29cd 0.35bc 0.36abc 0.03 NS

*

*

*

*

*

*

*

*

*

NS

*

*

*

*

*

*

NS NS NS

*

NS

*

NS

*

*

*

*

NS

*

*

*

*

* * *

*

Means in the same column followed by different superscript letters (a–g) differ significantly at P<0.05. 1 S.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. Maturation increased DM content dramatically in all treatments. Plant density affected DM distribution 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 panicles. 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.

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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 1st cut early heading Low Low Low Medium Medium Medium High High High 2nd cut soft dough Low Low Low Medium Medium Medium High High High S.E.M.1 PD effect at 1st cut PD effect at 2nd cut 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

IL (mm)

DM yield

CP content

NDF content

Hemicellulose content

Cellulose content

Lignin content

20 100 180 20 100 180 20 100 180

6.97f 9.96def 11.1cde 8.26ef 12.3cd 11.1cde 7.40f 11.2cde 11.7cde

70.9f 75.9d 84.0b 73.6e 84.1b 91.4a 75.8d 76.4d 85.2b

635cdef 637cdef 670ab 615f 625ef 658bc 620f 633def 662ab

320fgh 364abc 385a 320fgh 343cdef 381a 327fgh 357bcd 378ab

278bcd 226i 226i 255efg 237ghi 227i 250fgh 224i 231hi

38.0fgh 47.6bcd 58.4a 40.1efg 45.9cde 49.7bc 42.4def 51.8bc 53.4ab

20 100 180 20 100 180 20 100 180

7.31f 11.3cde 20.9a 8.27ef 13.9c 19.0a 8.51ef 14.0bc 17.5ab 1.25 NS NS

81.9c 67.4g 73.8e 63.7h 64.1h 72.8ef 67.5g 72.2ef 71.1f 0.66

648bcde 659bc 683a 633def 648bcde 658bc 657bc 652bcd 656bcd 8.28 NS

311gh 334defg 355bcde 308h 338def 341cdef 321fgh 331efgh 336def 8.38 NS NS

297ab 286abc 280bcd 290abc 263def 274cde 302a 278bcd 276cd 6.92

39.8fg 39.2fgh 48.1bcd 35.3gh 47.1cd 43.2def 33.7h 43.2def 43.7def 2.08

*

*

*

NS

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

Means in the same column followed by different superscript letters (a–i) differ significantly at P<0.05. 1 S.E.M., standard error of the means. * Significant at P<0.05 or non-significant (NS) effect of treatment or interaction.

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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 1st cut early heading 20 mm 100 mm 180 mm 20 mm 100 mm 180 mm 20 mm 100 mm 180 mm 2nd cut soft dough 20 mm 100 mm 180 mm 20 mm 100 mm 180 mm 20 mm 100 mm 180 mm S.E.M.1 IL effect at 1st cut IL effect at 2nd cut Organs difference at 1st cut Organs difference at 2nd cut Growth stage effect

Plant organ

NDF content

Lignin content

DM digestibility

NDF digestibility

Leaves Leaves Leaves Stems Stems Stems Panicles Panicles Panicles

667f 674de 678d 583i 563j 642h 640h 654g 689bc

26.7jk 32.5fg 33.8f 29.4hi 38.6e 46.7bc 49.1ab 49.6a 48.5abc

0.67c 0.61fg 0.63def 0.74b 0.68c 0.61fg 0.67c 0.63de 0.62efg

0.65a 0.59ef 0.61cde 0.61bcde 0.53jk 0.53jk 0.63b 0.59def 0.57fgh

Leaves Leaves Leaves Stems Stems Stems Panicles Panicles Panicles

696a 695a 693ab 653g 668ef 685c 385k 342l 344l 2.12

28.9ij 34.1f 31.6fgh 36.8e 41.3d 46.4c 29.8hi 30.9ghi 25.7k 0.89

0.62efg 0.59h 0.62efg 0.64d 0.62efg 0.61g 0.77a 0.77a 0.76ab 0.01

0.61bcd 0.58fgh 0.62bc 0.56ghi 0.55ij 0.54ijk 0.58fg 0.56hi 0.52k 0.01

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

Means in the same column followed by different superscript letters (a–l) differ significantly at P<0.05. 1 S.E.M., standard error of the means. * Significant at P<0.05 or non-significant (NS) effect of treatments, organs or interaction.

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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 1st cut early heading Low Low Low Medium Medium Medium High High High 2nd cut soft dough Low Low Low Medium Medium Medium High High High S.E.M.1 PD effect at 1st cut PD effect at 2nd cut 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

IL (mm)

DM digestibility

NDF digestibility

Digestible DM yield

Digestible NDF yield

20 100 180 20 100 180 20 100 180

0.70bc 0.68d 0.61h 0.74a 0.72ab 0.61h 0.70cd 0.69cd 0.62gh

0.68bcd 0.65cdef 0.62fgh 0.72a 0.69ab 0.62fgh 0.68bc 0.66bcde 0.62fgh

4.87f 6.72cdef 6.80cdef 6.08def 8.83bc 6.81cdef 5.13ef 7.73cd 7.29cde

2.95g 4.12cdefg 4.60cdef 3.67defg 5.32c 4.55cdefg 3.10efg 4.68cde 4.85cd

20 100 180 20 100 180 20 100 180

0.65e 0.64efg 0.61h 0.65e 0.63efgh 0.63efgh 0.65e 0.64efg 0.63fgh 0.01

0.64defg 0.60hi 0.58ij 0.62fgh 0.57j 0.62fgh 0.63efgh 0.60hi 0.61ghi 0.01

4.75f 7.23cde 12.7a 5.38ef 8.78bc 12.1a 5.51def 9.00bc 10.9ab 0.83 NS NS

3.02fg 4.50cdefg 8.35a 3.23efg 5.10cd 7.86a 3.54defg 5.51bc 7.01ab 0.57 NS NS

*

*

NS

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*

Means in the same column followed by different superscript letters (a–j) differ significantly at P<0.05. 1 S.E.M., standard error of the means. * Significant at P<0.05 or non-significant (NS) effect of treatment or interaction.

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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

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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

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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 physiological 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 accumulation 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).

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Acknowledgement This study was supported by a grant # 362-119 of the Israeli Milk Council. References Amaducci, S., Amaducci, M.T., Enati, R., Venture, G., 2000. Crop yield and quality parameters of four annual fibre crops (hemp, kenaf, maize and sorghum) in the north of Italy. Ind. Crops Prod. 11, 179–186. Association of Official Analytical Chemists, 1980. Official Methods of Analysis, 13th ed. AOAC, Washington, DC. Berenguer, M.J., Faci, J.M., 2001. Sorghum (Sorghum bicolor L. Moench) yield compensation processes under different plant densities and variable water supply. Eur. J. Agron. 15, 43–55. Carmi, A., Umiel, N., Hagiladi, A., Yosef, E., Ben-Ghedalia, D., Miron, J., 2005. Field performance and nutritive value of a new forage sorghum variety Pnina recently developed in Israel. J. Sci. Food Agric. 85, 2567–2573. Casler, M.D., 2000. Breeding forage crops for increased nutritional value. Adv. Agron. 71, 51–107. Castle, M.E., Watson, J.N., 1973. The relationship between the DM content of herbage for silage making and effluents production. J. Br. Grassl. Soc. 28, 135–138. Cherney, Y.H., Cherney, D.J.R., Akin, D.E., Axtell, J.D., 1991. Potential of brown-midrib, low lignin mutants for improving forage quality. Adv. Agron. 46, 157–198. Cusicanqui, J.A., Lauer, J.G., 1999. Plant density and hybrid influence on corn forage yield and quality. Agron. J. 91, 911–915. Defoor, P.J., Cole, N.A., Galyean, M.L., Jones, O.R., 2001. Effect of grain sorghum planting density and processing method on nutrient digestibility and retention by ruminants. J. Anim. Sci. 79, 19–25. Goodchild, A.V., 1997. Effects of rainfall and temperature on the feeding value of barley straw in semi-arid Mediterranean environment. J. Agric. Sci. Camb. 129, 353–366. Lafarge, T.A., Hammer, G.L., 2002. Shoot assimilate partitioning and leaf area ratio, are stable for a wide range of sorghum population densities. Field Crops Res. 77, 137–151. Miron, J., Zuckerman, E., Sadeh, D., Adin, G., Nikbachat, M., Yosef, E., Ben-Ghedalia, D., Carmi, A., Kipnis, T., Solomon, R., 2005. Yield, composition and in vitro digestibility of new forage sorghum varieties and their ensilage characteristics. Anim. Feed Sci. Technol. 120, 17–32. Miron, J., Solomon, R., Adin, G., Nir, U., Nikbachat, M., Yosef, E., Carmi, A., Weinberg, Z.G., Kipnis, T., Zuckerman, E., Ben-Ghedalia, D., 2006. Effects of harvest stage, re-growth and ensilage on the yield, composition and in vitro digestibility of new forage sorghum varieties. J. Sci. Food Agric. 86, 140–147. Reddy, P.R., Snaker, G.R.M., Das, N.D., 1999. Genetic analysis of yield, lodging and maturing of winter season sorghum under irrigated and non irrigated conditions in dryland alfisol. Ind. J. Agric. Sci. 69, 456–457. Rosenthal, W.D., Gerik, T.J., Wade, L.J., 1993. Radiation-use efficiency among grain sorghum cultivars and plant densities. Agron. J. 85, 703–705. Saeed, I.A.M., El-Nadi, A.H., 1998. Forage sorghum yield and water use efficiency under variable irrigation. Irrig. Sci. 18, 67–71. Singh, B.R., Singh, D.P., 1995. Agronomic and physiological responses of sorghum, maize and pearl millet to irrigation. Field Crops Res. 42, 57–67. Staggenborg, S.A., Fjell, D.L., Devlin, D.L., Gordon, W.B., March, B.H., 1999. Grain sorghum response to row spacings and seeding rates in Kansas. J. Prod. Agric. 12, 390–395. Statistical Analysis Systems, 1996. User’s Guide. SAS Inst. Inc., Cary, NC. Sticker, F.C., Pauli, A.W., Laude, H.H., Wilkins, H.D., Mings, J.L., 1961. Row width and plant population studies with grain sorghum at Manhatten, Kansas. Crop. Sci. 1, 297–300. Tabosa, J.N., Andrews, D.J., Tavares-Filho, J.J., Azevedo-Neto, A.D., 1999. Comparison among forage millet and sorghum varieties in semi-arid Pernambuco, Brazil: yield and quality. Int. Sorghum Millet Newslet. 40, 3–6. Tabosa, J.N., Arcoverde, A.S., Araugo, M.A., Santos, J.P., 1986. Preliminary evaluation of forage sorghum and maize lines in semi-arid Pernambuco state, Brazil. Sorghum Newslet. 29, 1. Tilley, J.M., Terry, R.M., 1963. A two stage technique for the in vitro digestion of forage crops. J. Br. Grassl. Soc. 18, 104–111.

132

A. Carmi et al. / Animal Feed Science and Technology 131 (2006) 120–132

Van Soest, P.J., Robertson, J.B., Lewis, B.A., 1991. Methods for dietary fiber, neutral detergent fiber, and non-starch polysaccharides in relation to animal nutrition. J. Dairy Sci. 74, 3583–3597. Widdicombe, W.D., Thelen, K.D., 2002. Row width and plant density effect on corn forage hybrids. Agron. J. 94, 326–330. Wilson, J.R., Ng, T.T., 1975. Influence of water stress on parameters associate with herbage quality of Panicum maximum var. trichoylume. Aust. J. Agric. Res. 26, 127–136. Zerbini, E., Thomas, D., 2003. Opportunities for improvement of nutritive value in sorghum and pearl millet residues in south Asia through genetic enhancement. Field Crop Res. 84, 3–15.