Response of a new and a commonly grown forage sorghum cultivar to limited irrigation and planting density

Agricultural Water Management 117 (2013) 62–69

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Response of a new and a commonly grown forage sorghum cultivar to limited irrigation and planting density E. Jahanzad a,∗ , M. Jorat b , H. Moghadam b , A. Sadeghpour a , M.-R. Chaichi b , M. Dashtaki b a b

Department of Plant, Soil, and Insect Sciences, University of Massachusetts, Amherst, MA, 01003-9294, USA Department of Agronomy and Plant Breeding, College of Agriculture & Natural Resources, Karaj, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 28 April 2012 Accepted 5 November 2012 Available online 6 December 2012 Keywords: Fodder production Forage quality Limited available water Planting density Sorghum

a b s t r a c t Increasing scarcity of water for irrigation is one of the major challenges for forage producers in all arid and semi-arid regions. Selecting drought tolerant forage species such as sorghum (Sorghum spp. L.) over corn, as a common forage crop, could be a viable option to cope with the limited available water for irrigation and increasing the productivity in such climates. A two-year experiment was conducted during the 2009 and 2010 growing seasons to determine if limited irrigation regimes and different plant densities may produce high-yielding forage sorghum with acceptable forage quality. The experiment was arranged in a three-replicated split-plot factorial design with three irrigation regimes including optimum irrigation (when evaporation reached 70 mm, using evaporation pan class “A”), moderate drought stress (100 mm), and severe drought stress (130 mm) as main plots. A common sorghum cultivar (Speedfeed) and a newly released cultivar (Pegah) were factorially combined with three plant densities (150,000, 200,000, and 250,000 plants ha−1 ), as sub-plots. Results of this study indicated that forage dry matter and forage quality parameters were significantly influenced by irrigation regimes, plant densities, and cultivars. Increasing water stress from optimum irrigation (Ir70 ) to moderate (Ir100 ) and low irrigation (Ir130 ) resulted in 20 and 34% less forage dry matter yield. Protein yield was also lower when applying moderate and severe drought stress than with the optimum irrigation regime, whereas some forage quality parameters including crude protein, dry matter digestibility, water soluble carbohydrates, dry matter intake, relative feed value, and net energy for lactation improved when limited irrigation was imposed. Highest protein yield (1688 kg ha−1 ) was obtained from the combination of optimum irrigation regime and lowest plant density, whereas forage produced in moderate stress and low plant density was richer in relative feed value. Speedfeed outyielded Pegah cultivar and produced higher protein yield. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Forage production has recently decreased in some arid and semi arid countries due to intensive grazing and consecutive dryness. Increasing scarcity of water for irrigation, particularly in arid and semi-arid environments where agricultural systems are dependent upon supplemental irrigation, is becoming the most

Abbreviations: Ir70 , Ir100 , and Ir130 , optimum, moderate, and low irrigation levels (when evaporation was 70, 100 and 130 mm from the surface of evaporation pan, respectively); D15 , D20 , and D25 , plant densities of 150,000 200,000, and 250,000 plants ha−1 , respectively; ADF, acid detergent fiber; CF, crude fiber; CP, crude protein; DMD, dry matter digestibility; DMI, dry matter intake; NDF, neutral detergent fiber; IVDMD, in vitro dry matter digestibility; NE1 , net energy for lactation; RFV, relative feed value; TDN, total digestible nutrients; WSC, water soluble carbohydrates; OM, organic matter; EC, electrical conductivity; SP, saturation point; TNV, total neutralizing value. ∗ Corresponding author. Tel.: +1 413 557 8641; fax: +1 413 545 0260. E-mail address: [email protected] (E. Jahanzad). 0378-3774/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.agwat.2012.11.001

important problem for producing forage (Rostamza et al., 2011a, 2011b; Marsalis et al., 2010). Alternative forage sources which have flexibility in extreme conditions must be utilized to cope with declining water availability (Marsalis and Bean, 2010; Tabosa et al., 1999). Sorghum is becoming an increasingly important forage crop in many arid and semi-arid regions of the world because of its advantages over other forage crops (Zerbini and Thomas, 2003). In Iran, it is cultivated in over 30,000 ha, mainly in the southern provinces of the country (Bahrani and Deghani Ghenateghestani, 2004) because of its advantages over corn in warm and dry climates (Berenguer and Faci, 2001; Singh and Singh, 1995). These advantages include high water productivity (Marsalis et al., 2010), low nitrogen demand (Barbanti et al., 2006), and high salt and drought tolerance (Saberi et al., 2011; Tabosa et al., 1999). Fast re-growth after cutting makes sorghum a reliable and profitable summer forage crop for feed production. Low contents of hydrocyanic acid (HCN) in forage sorghum tissues make feeding at early stages quite safe compared to grain sorghum (Chatterjee and Das, 1989). Improving forage nutritive value is considered to be a primary goal

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Table 1 Selected properties of the top soil (0–30 cm) at the experimental site. Year

N (%)

CaCO3 (%TNV)

P (mg kg−1 )

K (mg kg−1 )

2009 2010

0.11 0.14

11 10

14 11

237 251

Year

OM (%)

EC (ds/m)

pH

SP (%)

2009 2010

0.12 0.14

3.41 1.91

7.8 8.0

Zn (mg kg−1 )

Cu (mg kg−1 )

1.9 2.1

44.5 43.6

1.8 1.6

Mn (mg kg−1 )

Fe (mg kg−1 )

12.7 18.6

6.4 7.2

Clay (%)

Silt (%)

Sand (%)

Soil texture

31 30

36 39

33 31

Clay loam Clay loam

SP: saturation point; TNV: total neutralizing value; OM: organic matter.

for productive ruminants (Carmi et al., 2006). Irrigation level and plant density are two important factors contributing to sorghum forage production in regard to quality and chemical composition (Widdicombe and Thelen, 2002; Defoor et al., 2001; Cusicanqui and Lauer, 1999; Rosenthal et al., 1993). Apparently, sorghum can respond to additional irrigation by stem elongation and increased yield (Saeed and El-Nadi, 1998; Singh and Singh, 1995). However, Amaducci et al. (2000) declared that well watered plants accumulated more lignin compared to water deficit plants, likewise, additional irrigation reduced sorghum digestibility. Increasing plant density might result in limited water availability due to increased intra-species competition for water sources which leads to yield reduction (Berenguer and Faci, 2001). Similar reports suggested decreasing sorghum dry matter yield (DMY) but an increase in dry matter digestibility when irrigation is limited (Amaducci et al., 2000; Caravetta et al., 1990). Carmi et al. (2006) claimed that sufficient irrigation increased plant height and DMY in the first and second cuts and enhanced content of neutral detergent fiber (NDF) and lignin. They also reported that in most cases, surplus irrigation decreased dry matter (DM) content and in vitro DM digestibility (IVDMD). Caravetta et al. (1990) demonstrated lower DMY but higher forage digestibility with lower densities. Furthermore, they revealed that reduction of light penetration into the canopy, as a result of increased plant density, suppressed tillering in sorghum. The optimum plant density varies depending on genotypes and environmental factors such as soil fertility, moisture supply, and planting date (Cusicanqui and Lauer, 1999). Increasing plant density has been shown to be effective in building up forage dry matter while reducing crude protein content (Bahrani and Deghani Ghenateghestani, 2004). There is little research concerning the integrated effects of irrigation and plant density on yield and forage quality of sorghum. Speedfeed is currently the most common type of forage sorghum planted in Iran, however, a newly released variety, Pegah, is considered as a well established forage sorghum cultivar which eliminates the probable problems concerning a single sorghum planting in the country. The objective of this study was to determine the best plant density of two forage sorghum varieties (Pegah and Speedfeed) under normal and limited irrigation. Also, forage yield and quality of the two sorghum varieties were evaluated as influenced by irrigation treatments and plant densities.

2. Materials and methods 2.1. Experimental site A two-year field experiment was conducted at the Experimental Farm of University of Tehran (35◦ 48 N, 50◦ 57 W, altitude 1312.5 m) during the 2009 and 2010 growing seasons. The area has an arid to semi-arid climate with 38-year average annual precipitation of 251 mm, annual average temperature of 13.5 ◦ C, annual average soil temperature of 14.5 ◦ C, and total annual class “A” pan evaporation of 2184 mm. Experiments were carried out on a clay-loam soil in which the average field capacity of root zone was 23%. Prior to seeding, soil samples from each plot were taken from the top 30 cm of soil to test its background nutritional level. Selected chemical properties of soil are presented in Table 1. The average monthly precipitation and temperature obtained from the Karaj Synoptic Meteorology Station, located at the experimental farm, are presented in Table 2. Furthermore, weekly average rainfall, minimum temperature, maximum temperature, and evaporation are presented in Table 3. 2.2. Experimental design and cultural practices The experiment was arranged as a Randomized Complete Block Design (RCBD). Treatments were arranged in a three-replicated split plot-factorial design with three irrigation regimes including optimum irrigation (Ir70 ) (when evaporation was 70 mm from the surface of evaporation pan class “A”), moderate (Ir100 ), and low irrigation (Ir130 ) as main plots, three plant densities (150,000, 200,000, and 250,000 plants ha−1 ) as subplots factorially combined with two sorghum cultivars namely Speedfeed and Pegah, the latter being a recently released cultivar (LFS56 × Early Orange). The experimental site was plowed by moldboard plow, harrowed and divided into three blocks, each contained eighteen plots. Each subplot was 2 m wide and 5 m long and consisted of four rows of sorghum which were 0.5 m wide and 5 m long. A gap of two meters was considered between adjacent main plots and the distance between replications was 3 m. Within-row spacings were 7, 5, and 4 cm for plant densities of 150,000, 200,000, and 250,000 plants ha−1 , respectively. Prior to planting, plots were leveled to equalize the distribution of water in each plot. Plots

Table 2 Monthly rainfall and average temperature during the growing seasons in 2009 and 2010 and their 30 year average. Month

April May June July August September October Total

Mean temperature (◦ C)

Rainfall (mm) 2009

2010

30-Year average

2009

2010

30-Year average

46.7 43.2 10.3 0 0 10.1 0.6

54.0 47.0 0.4 0 0 0 0.5

34.7 20.8 2.3 3.1 1.4 0.6 13.7

10.9 17.2 23.2 27.9 26.6 23.2 18.9

12.1 17.6 25.8 29.0 27.3 24.3 19.5

14.4 19.0 24.4 26.9 26.7 22.8 16.8

110.9

101.9

76.6

–

–

–

64

E. Jahanzad et al. / Agricultural Water Management 117 (2013) 62–69

Table 3 Average rainfall, minimum and maximum temperature, and evaporation on a standard week basis in the 2009 and 2010 growing seasons. Week

Year 2009

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

2010 ◦

◦

Rain fall (mm)

Min. temp. ( C)

Max. temp. ( C)

Evaporation (mm)

Rain fall (mm)

Min. temp. (◦ C)

Max. temp. (◦ C)

Evaporation (mm)

4.6 0.1 0 0 0 0 0 0 0 0 0 0 0 0 10.1 0.6

15.8 15.4 17.7 17.4 18.5 25.1 19.1 17.1 18.6 16.4 17.8 15.1 16.5 16.4 14.8 12.2

31.0 31.9 33.7 34.7 34.6 39.0 38.4 36.1 34.5 35.1 36.3 33.0 31.6 30.6 27.8 25.5

57.7 77.1 83.4 84.1 78.8 115.0 90.9 75.9 81.8 72.2 69.6 53.7 56.5 62.2 61.0 49.2

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 –

20.0 19.6 19.0 21.5 24.8 19.2 19.4 18.0 21.1 20.8 21.0 18.2 19.0 16.8 13.4 –

35.9 35.6 35.0 38.4 39.0 36.4 35.4 32.1 34.5 34.6 35.3 32.9 34.4 30.8 27.1 –

75.4 80.4 86.9 89.6 98.0 88.9 66.7 83.8 82.5 104.3 76.6 51.7 59.3 58.9 55.0 –

were seeded on the 8th and 10th of June in 2009 and 2010, respectively, at a row spacing of 50 cm. After planting, plots were irrigated equally to facilitate seed germination. Thinning was applied when plants were at 4 leaf-stage to reach the considered plant population at each plant density, where deemed necessary. Plots were weeded two times, once 15 days after planting and the second time when plants were 30 cm in height. Afterwards, weeds were suppressed with sorghum canopy. Major weed species were common Purslane (Portulaca oleracea L.) and Johnson grass (Sorghum halepense L.). Irrigation regimes were applied when plants were completely established and were at tillering stage with 4–5 leaves on their main stem, by average. Plots were irrigated anytime evaporation reached the considered amount for each irrigation level (70, 100, and 130 mm evaporation from the surface of the evaporation pan). A high output PVC irrigation water meter was used to measure the amount of water applied to each plot. Numbers of irrigations were 16, 11, and 8 for Ir70 , Ir100 , and Ir130 , respectively. Each plot received an approximate amount of 1.25 m3 of water in each irrigation and the end of each plot was blocked to control the volume of water. Each combination of plant density and cultivar received a total amount of 20 m3 of water at Ir70 irrigation regime while it was 13.75 and 10 m3 at Ir100 and Ir130 , respectively. The total amount of irrigation for Ir70 , Ir100 , and Ir130 were 120, 82.5 and 60 m3 during each growing season. Intervals of irrigations were roughly 7, 10 and 15 days for Ir70 , Ir100 , and Ir130 irrigation levels, respectively. Dates of irrigation for each irrigation regime are presented in Table 4.

matter digestibility (DMD), dry matter intake (DMI), total digestible nutrients (TDN), and net energy for lactation (NEl ) were estimated according to the following equations (Lithourgidis et al., 2006; Horrocks and Vallentine, 1999) in which all values are given in % on dry matter basis: RFV = DMD × DMI × 0.775 DMD = 88.9 − (0.779 × ADF) DMI =

120 NDF

TDN = −1.291 × ADF + 101.35 NEl = [1.044 − (0.0119 × ADF)] × 2.205 Yields of sorghum from the first and second cuts for each year were combined to calculate total dry matter. Data were analyzed using the analysis of variance (ANOVA) and general linear model (GLM) procedures of SAS (SAS Institute, 2003). Effects were considered significant at P-values ≤0.05 in the F-test. Duncan multiple range test was conducted for comparison of means. Since ANOVA indicated that there was no interaction between treatment and growing season, the values are reported as means of both growing seasons. 3. Results and discussion

2.3. Measurements and data analysis 3.1. Forage dry matter In the first year of the experiment (2009), the first and second forage cut took place on July 27th and September 24th, while it was July 18th and September 20th for the second year (2010), respectively. Plant height was measured and leaves were separated from the stem to calculate the leaf/stem ratio. After recording the fresh weight of sorghum in each cut, samples were dried in a forcedair oven at 70 ◦ C for 72 h. For forage quality analysis, samples were taken and dried separately. Dried samples were ground through the 0.2 mm screen of a cyclone mill and scanned using a near-infrared reflectance spectroscopy (NIRS, Informatics Perten 8600 Feed Analyzer) with 6–20 wavelengths ranging from 500 to 2400 nm. Forage quality parameters measured included crude protein (CP), water soluble carbohydrate (WSC), acid detergent fiber (ADF, on dry matter basis), crude fiber (CF) neutral detergent fiber (NDF, on dry matter basis), and ash. Total relative feed value (RFV), dry

Forage dry matter of sorghum was highly affected by different irrigation regimes (Table 5). Shorter interval irrigation (Ir70 ) yielded 25 and 34% higher forage dry matter compared to the longer Ir100 and Ir130 interval irrigations, respectively (Table 5). It is assumed that when soil water content is not enough to facilitate nutrient uptake by roots, plants face difficulty in absorbing essential elements such as nitrogen and phosphorus for their growth and development leading to yield reduction. Moreover, reduced transpiration deriving from dry soil might disrupt nutrient uptake by roots and ion transportation from roots to shoots (Kramer and Boyer, 1995). Increased dry matter yield of forage associated with higher irrigation have been reported in several studies (Vasilakoglou et al., 2011; Marsalis et al., 2009; Berenguer and Faci, 2001). Sorghum cultivars also differed significantly in terms

E. Jahanzad et al. / Agricultural Water Management 117 (2013) 62–69

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Table 4 Date of irrigation for each irrigation regime during the 2009 and 2010 growing seasons. 2009

2010

Ir70

Ir100

Ir130

Ir70

Ir100

Ir130

June 15th June 21st June 27th July 2nd July 9th July 14th July 18th July 23rd July 29th Aug 5th Aug 11th Aug 18th Aug 26th Sep 4th Sep 12th Sep 22nd

June 20th June 28th July 8th July 15th July 21st July 29th Aug 8th Aug 17th Aug 27th Sep 9th Sep 22nd – – – – –

June 21st July 1st July 13th July 21st Aug 2nd Aug 14th Aug 28th Sep 13th – – – – – – – –

June 15th June 21st June 27th July 3rd July 8th July 13th July 19th Aug 1st Aug 7th Aug 12th Aug 19th Aug 24th Aug 30th Sep 7th Sep 17th –

June 18th June 26th July 5th July 12th July 20th July 29th Aug 8th Aug 16th Aug 23rd Sep 3rd Sep 16th – – – – –

June 21st July 2nd July 12th July 22nd Aug 4th Aug 15th Aug 24th Sep 9th – – – – – – – –

of forage dry matter. Speedfeed responded more strongly to less irrigation levels compared with Pegah, which led to a relatively higher yield (Table 5). According to Staggenborg et al. (1999) when soil water content is adequate, increasing plant density results in yield increase. It is possible that lack of any effect of plant density on yield in this experiment is due to the high range of plant density used, which may be above the influential level considered for sorghum when it is grown for forage yield (Staggenborg et al., 1999). Irrigation regimes and cultivars both had a highly significant effect on total leaf weight of sorghum (Table 5). The highest leaf weight was recorded when plants were irrigated every 7 days (Ir70 ). As the intervals between irrigations increased in Ir100 and Ir130 irrigation regimes, leaf weight of sorghum was exacerbated by water deficit, resulting in a linear reduction in dry matter yield (Table 5). Such results are similar to those reported in millet, sorghum, and corn (Nagaz et al., 2009; Blum, 2005; Panda et al., 2004). Speedfeed was superior to Pegah both in terms of leaf and stem weights (Table 5). Stem weight of sorghum was significantly influenced by irrigation levels and by plant cultivars. The trend observed in stem weight, where shorter irrigation interval yielded highest stem dry matter, was similar to that of leaf weight (Table 5). Leaf/stem ratio of grass stands is an important factor affecting diet selection, quality and forage intake (Smart, 1998). Overall, the differences between leaf/stem ratios at different irrigation levels were significant. The greatest leaf/stem ratio of 0.72 was recorded for Ir70 irrigation level whereas this value was lower for Ir100 and Ir130 irrigation levels (Table 5). This result could be explained by reduction of leaf surface area to reduce transpiration in plant as a response to drought stress in moderate and severe limited irrigation regimes. This probably increases evaporation from the soil surface by decreasing leaf

shading. These results are in agreement with those reported by Carraw (1996). Leaf/stem ratio, which reflects the changes in the stem IVDMD during maturation, was less affected by lignin content, perhaps due to the accumulation of digestible soluble carbohydrates (Carmi et al., 2006). Sorghum cultivars differed significantly in terms of leaf/stem ratio. The leaf/stem ratio value of Pegah was higher than that of Speedfeed (Table 5). According to field observations, Speedfeed had smaller and lighter leaves whereas Pegah had less but larger leaves when compared to Speedfeed, which justifies the higher leaf/stem ratio in Pegah cultivar. 3.2. Forage quality Crude protein content which is one of the most important factors in forage quality, (Assefa and Ledin, 2001; Caballero et al., 1995) was significantly (P < 0.01) affected by irrigation levels and plant densities as shown in Table 6. Also, forage crude protein content was influenced by the interaction of irrigation regime and plant density. In this experiment, less irrigation led to a progressive rise in CP content (Table 6). It has been reported in several studies that when water deficit intensifies, forage CP content improves as a result of nitrogen accumulation (Haberle et al., 2008; Pessarakli et al., 2005; Abreu et al., 2004). CP concentration followed a declining trend as plant density increased, lowest CP content was obtained from the highest plant density (D25 ) while forage produced at the lowest plant density (D15 ) was richer in terms of CP content (Table 6). It seems that plants at lower plant density with less intra-specific competition accumulate more nitrogen, resulting in higher crude protein content. Such results correspond to the findings of Bahrani and Deghani Ghenateghestani (2004) who reported that increasing plant density led to lower CP content.

Table 5 Effect of irrigation level and cultivar on forage dry matter, leaf weight, stem weight, and leaf/stem ratio of forage sorghum. Forage dry matter (kg ha−1 )

Treatments Irrigation level

Ir70 Ir100 Ir130

L.S. Cultivar L.S.

P S

Leaf weight (kg ha−1 )

Stem weight (kg ha−1 )

12,267 a 9820 b 8009 c

5091 a 3867 b 3355 c

7251 a 5950 b 4693 c

**

**

**

8611 b 11,453 a

3640 b 4568 a

5029 b 6900 a

**

**

**

Means in the same column followed by different letters differ significantly at P < 0.05. Ir70 , Ir100 , and Ir130 represent high, moderate, and low irrigation levels. S and P represent Speedfeed and Pegah cultivars, respectively. L.S.: level of significance. ** P < 0.01.

Leaf/stem ratio 0.72 a 0.70 a 0.64 b **

0.73 a 0.66 b **

66

E. Jahanzad et al. / Agricultural Water Management 117 (2013) 62–69

Table 6 Effects of irrigation level, plant density, and cultivar on chemical composition and protein yield of forage sorghum. ASH (g kg−1 )

Forage quality Parameters Ir70 Ir100 Ir130

Irrigation level

DMD (g kg−1 )

60 c 65 b 70 a

623 c 639 b 648 a

**

L.S. D15 D20 D25

Plant density

P S

Forage quality parameters Ir70 Ir100 Ir130

Irrigation level L.S.

D15 D20 D25

Plant density L.S.

P S

Cultivars

NDF (g kg−1 )

CF (g kg−1 )

128 b 117 c 137 a

126 c 138 b 147 a

255 ab 265 a 243 b

569 a 565 b 561 c

419 c 431 b 443 a

**

*

**

144 a 139 b 129 c

238 b 260 a 264 a

556 c 568 b 571 a

**

**

**

139 135 ns

249 260 ns

565 565 ns

118 c 127 b 137 a

**

66 64 ns

L.S.

ADF (g kg−1 )

**

645 a 638 b 628 c

**

Cultivar

CP (g kg−1 )

**

67 a 66 a 61 b

L.S.

WSC (g kg−1 )

L.S.

**

644 a 630 b

122 a 132 b

**

**

**

425 c 431 b 438 a **

429 433 *

TDN (g kg−1 )

DMI (g kg−1 )

RFV (%)

NE1 (%)

Protein yield (kg ha−1 )

227 ab 241 a 211 b

21.11 c 21.25 b 21.38 a

113 b 113 b 116 a

1.64 b 1.64 b 1.66 a

1549 a 1352 b 1179 c

**

**

**

**

**

205 b 234 a 240 a

21.59 a 21.13 b 21.02 c

118 a 113 b 111 c

1316 a 1404 a 1431 b

**

**

**

1.69 a 1.65 b 1.60 c ns

219 233 ns

21.26 21.24 ns

114 114 ns

1.65 1.64 ns

1384 1265 ns

**

Means in the same column followed by letters differ significantly at P < 0.05. Ir70 , Ir100 , and Ir130 represent high, moderate, and low irrigation levels. D15 , D20 , and D25 represent low, moderate, and high plant density, respectively. S and P represent Speedfeed and Pegah cultivars, respectively. L.S.: level of significance. * P < 0.05. ** P < 0.01. Non-significant (ns) effect of interaction.

Table 7 Effects of irrigation level and plant density interaction on chemical composition and protein yield of forage sorghum. Irrigation level

Plant density

ASH (g kg−1 )

DMD (g kg−1 )

WSC (g kg−1 )

ADF (g kg−1 )

NDF (g kg−1 )

CF (g kg−1 )

Ir70

D15 D20 D25

63 61 56

631 626 613

119 128 137

137 c 127 d 115 e

240 256 268

559 e 571 bc 576 a

413 416 427

Ir100

D15 D20 D25

67 66 60

649 640 629

104 118 128

143 b 140 bc 130 d

244 282 270

555 f 568 cd 572 b

423 433 438

Ir130

D15 D20 D25

70 71 68

656 647 642

129 135 146

151 a 149 a 142 b

229 242 256

554 f 565 d 565 d

438 442 449

ns

ns

ns

L.S.

CP (g kg−1 )

**

**

ns

ns

Irrigation level

Plant density

TDN (g kg−1 )

DMI (g kg−1 )

RFV (%)

NE1 (%)

Ir70

D15 D20 D25

208 228 244

21.48 b 21.02 e 20.83 f

117 112 109

1.68 1.63 1.59

1688 a 1568 b 1390 c

Ir100

D15 D20 D25

214 262 246

21.63 a 21.12 de 21.00 e

118 113 110

1.68 1.63 1.63

1384 c 1388 c 1285 d

Ir130

D15 D20 D25

194 211 229

21.67 a 21.27 c 21.22 cd

119 115 113

1.70 1.67 1.62

1220 de 1196 e 1120 f

ns

ns

L.S.

ns

*

Means in the same column followed by different letters differ significantly at P < 0.05. Ir70 , Ir100 , and Ir130 represent high, moderate, and low irrigation levels. D15 , D20 , and D25 represent low, moderate, and high plant density, respectively. L.S.: level of significance. * P < 0.05. ** P < 0.01. Non-significant (ns) effect of interaction.

Protein yield (kg ha−1 )

**

E. Jahanzad et al. / Agricultural Water Management 117 (2013) 62–69

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Table 8 Effect of irrigation level and cultivar interaction on chemical composition and protein yield of forage sorghum. ASH (g kg−1 )

WSC (g kg−1 )

CP (gkg−1 )

ADF (gkg−1 )

NDF (g kg−1 )

CF (g kg−1 )

632 b 615 c

124 b 132 ab

128 125

252 125

568 a 568 a

421 c 417 c

65 b 64 b

651 a 628 b

105 c 128 ab

140 135

250 280

565 ab 565 ab

429 b 434 b

70 a 70 a

651 a 646 a

136 a 137 a

148 146

244 241

561 b 561 b

437 b 449 a

**

**

**

ns

ns

ns

ns

Irrigation level

Cultivar

Ir70

P S

62 c 58 d

Ir100

P S

Ir130

P S

L.S.

DMD (g kg−1 )

Irrigation level

Cultivar

TDN (g kg−1 )

DMI (g kg−1 )

RFV (%)

NE1 (%)

Protein yield (kg ha−1 )

Ir70

P S

224 230

21.10 21.12

113 113

1.63 cd 1.64 bcd

1501 ab 1596 a

Ir100

P S

221 260

21.27 21.23

114 113

1.65 bc 1.62 d

1318 c 1387 bc

Ir130

P S

213 210

21.40 21.37

116 115

1.67 a 1.66 ab

1128 d 1229 cd

ns

ns

ns

L.S.

*

**

Means in the same column followed by different letters differ significantly at P < 0.05. Ir70 , Ir100 , and Ir130 represent high, moderate, and low irrigation levels. S and P represent Speedfeed and Pegah cultivars, respectively. L.S.: level of significance. * P < 0.05. ** P < 0.01. Non-significant (ns) effect of interaction.

Cusicanqui and Lauer (1999) also found a negative correlation between CP and high plant density. Among interaction effects, the highest CP content was observed at Ir130 and D15 (Table 7). The total amount of protein achieved in a forage crop in livestock enterprises is as important as CP concentration in forage as a forage quality factor (Carmi et al., 2006). Protein yield combines the total biomass produced and forage CP content. Unlike CP, protein yield declined as intervals of irrigation increased (Table 6) which could be explained by higher forage dry matter yield at Ir70 compared with Ir100 and Ir130 . In contrast to forage dry matter, protein yield was influenced by plant densities which could be due to differences in forage protein content. Protein yields of 1431, 1404, and 1316 kg ha−1 were obtained from D25 , D20 , and D15 plant densities, respectively. Significant interactions between irrigation levels and plant densities (Table 7) and irrigation levels and cultivars (Table 8) were also observed in this study. The combination of Ir70 and D15 was advantageous over other combinations in terms of protein yield (Table 7). Although Pegah was the superior cultivar in terms of CP content, protein yield produced by Speedfeed was higher than by Pegah in Ir70 irrigation level due to higher dry matter production (Table 8). Acid detergent fiber (cellulose and lignin) and NDF (hemicelluloses, cellulose, and lignin) are considered to be two important characteristics of forage quality (Assefa and Ledin, 2001; Caballero et al., 1995). High-quality forages have low concentrations of both NDF and ADF and high DMD (Paterson et al., 1994). Since NDF is negatively correlated with DMI, high concentrations of NDF results in lower DMI and inferior forage quality (Horrocks and Vallentine, 1999). Overall, irrigation treatments had a significant effect on ADF and NDF concentrations of forage. Such effects, however, were less consistent for ADF. The highest ADF concentration of DM was recorded for the moderate irrigation level (Ir100 ), while NDF had its highest content at the optimum irrigation level (Ir70 ) (Table 6). The lowest NDF and ADF content were observed when irrigation interval increased to Ir130 irrigation level (Table 6). Increased lignin content associated with higher irrigation was previously observed for sorghum (Amaducci et al., 2000) and for other forage crops (Goodchild, 1997). Besides, Carmi et al. (2006) indicated that NDF

and ADF correlate positively with shorter interval irrigation and build up as intervals of irrigation decreases. In this study, ADF and NDF were significantly affected by plant density changes. Both ADF and NDF followed an incremental trend as plant densities increased from 150,000 plants ha−1 to a high of 250,000 plants ha−1 (Table 6). Likewise, a significant interaction between irrigation levels and plant densities was observed when considering NDF content (Table 7). Highest NDF was recorded when plants were sown at the highest plant density (D25 ) and irrigated every 7 days (Ir70 ) (Table 7). These results are in accordance with those reported in maize (Widdicombe and Thelen, 2002). Forage DMD was significantly affected by irrigation levels, this was true both for plant densities and for sorghum cultivars. Opposite to ADF and NDF concentrations, forage DMD improved as the amount of irrigation water declined (Table 6). Dry matter digestibility reduction at Ir70 could be attributed to its negative correlation with ADF and NDF accumulation at the same irrigation level. Negative correlation between DMD and hemicelluloses has already been reported in several studies (Hatfield, 1993; Francisco et al., 2009; Theander and Westerlund, 1986). Also, Hatfield (1993) reported that grasses are higher in hemicelluloses and lower in lignin than legumes, which may affect digestibility. In contrast, Wilson and Ng (1975) observed that a better water status in maturing plants alleviated the extent of digestibility decrease in senescing leaves and stems. The highest DMD was recorded for D15 plant density while increasing it from D15 to D25 resulted in a progressively less DMD (Table 6) which is in agreement with the results reported by Jafari et al. (2003) and Widdicombe and Thelen (2002). In this study, Pegah had more DMD content than Speedfeed (Table 6). A significant interaction was also observed between irrigation regimes and sorghum cultivars, the combination of Ir100 or Ir130 and the Pegah cultivar showed to have the highest DMD content (Table 8), however, the differences between Ir130 and Ir100 were less consistent (Table 8). Water soluble carbohydrate (WSC) was significantly affected by irrigation levels, plant densities, and cultivars. Nevertheless, in response to irrigation levels, WSC followed a different trend than other forage quality parameters did. Among irrigation regimes,

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E. Jahanzad et al. / Agricultural Water Management 117 (2013) 62–69

Table 9 Effect of plant density and cultivar interaction on chemical composition and protein yield of forage sorghum. Cultivar

Plant density

ASH (g kg−1 )

DMD (g kg−1 )

WSC (g kg−1 )

CP (g kg−1 )

ADF (g kg−1 )

NDF (g kg−1 )

CF (g kg−1 )

P

D15 D20 D25

68 a 67 a 63 c

651 645 637

111 c 120 b 135 a

145 140 132

238 250 259

556 568 571

421 430 436

S

D15 D20 D25

65 b 65 b 60 d

639 631 619

124 b 134 a 139 a

142 138 126

238 270 270

556 568 571

429 431 439

ns

ns

ns

ns

*

L.S.

**

ns

Cultivar

Plant density

TDN (g kg−1 )

DMI (g kg−1 )

RFV (%)

NE1 (%)

Protein yield (kg ha−1 )

P

D15 D20 D25

205 220 232

21.59 21.14 21.03

118 a 114 b 111 c

1.68 ab 1.65 bc 1.62 d

1227 1593 1126

S

D15 D20 D25

205 247 247

21.60 21.12 21.00

118 a 113 b 110 d

1.69 a 1.64 cd 1.59 e

1634 1174 1404

ns

ns

**

ns

L.S.

**

Means in the same column followed by different letters differ significantly at P < 0.05. D15 , D20 , and D25 represent low, moderate, and high plant density, respectively. S and P represent Speedfeed and Pegah cultivars, respectively. L.S: Level of Significance. * P < 0.05. ** P < 0.01. Non-significant (ns) effect of interaction.

Ir130 had the highest concentration of WSC while the lowest content of WSC was observed in Ir100 (Table 6). On the other hand, there was an increase in WSC content as plant densities increased (Table 6). With regard to cultivars, greater WSC content was observed in Speedfeed compared with Pegah (Table 6). Sorghum cultivars and irrigation levels interacted significantly to affect WSC (Table 8), as well as plant density (Table 9). Forage ash content was influenced by irrigation regimes and there was an increase of ash content as irrigation intervals increased (Table 6). Planting density also had a significant effect on ash content (Table 6). Results showed no statistical difference between low and moderate plant densities (D15 and D20 ), however, high plant density (D25 ) had less ash content when compared with those of low and moderate plant densities (D15 and D20 ) (Table 6). The interaction between irrigation levels and sorghum cultivars was highly significant. Both Speedfeed and Pegah cultivars had the greatest ash content at Ir130 whereas, the lowest ash content was observed for both cultivars at Ir70 (Table 8). In this study, plots having a better water status had less CF than plots with water deficit (Table 6). In addition, less dense plots (D15 ) had less CF than densely planted plots (Table 6). Sorghum cultivars also differed significantly in terms of CF concentration (Table 6). Moreover, irrigation levels and cultivars interacted significantly (Table 8). Total digestible nutrient (TDN) refers to nutrients that are available for livestock and are related to the ADF concentration of forage (Lithourgidis et al., 2006). As ADF increases, there is a decline in TDN content which limits an animal’s ability to utilize the nutrients that are present in the forage (Carmi et al., 2006). Ir100 had the highest TDN among irrigation levels while among planting densities the highest TDN content was obtained from D25 , followed by D20 and D15 (Table 6). Cultivars did not differ in TDN content, statistically. NDF is used to predict DMI and is negatively correlated with DMI, which means that when NDF is high forage quality and DMI are low (Horrocks and Vallentine, 1999). As the intervals of irrigation increased, DMI increased as well and the greatest DMI of 21.38 g kg−1 of body weight was obtained from Ir130 followed by Ir100 and Ir70 (Table 6). A similar trend to that of irrigation levels was observed in plant densities and DMI values declined as plant densities increased (Table 6). Also, interaction effects between

irrigation levels and plant densities were significant in this study (Table 7). Relative feed value (RFV) is an index which is used to predict intake and energy value of forage which is derived from DMD and DMI (Lithourgidis et al., 2006). According to Horrocks and Vallentine (1999), forage with RFV value >151 is considered prime. Irrigation levels and plant densities both affected RFV significantly, however, RFV did not change dramatically as water deficit intensified (Table 6). Likewise, plant density and cultivars interacted significantly (Table 9). The highest RFV value of 116 and 118% were obtained from Ir130 and D15 , respectively. Pegah and Speedfeed both had the same highest RFV (118%) at D15 (Table 9). This shows that regardless of Pegah advantage in terms of forage quality, Speedfeed can produce high quality forage at D15 plant density level. Net energy for lactation (NE1 ) response to irrigation levels was similar to that of RFV, the better the status of water in plants, the higher NE1 . Net energy for lactation (NE1 ) values of 1.66, 1.64, and 1.64 were obtained from irrigation levels of Ir130 , Ir100 , and Ir70 , respectively.

4. Conclusion The results of this study revealed that water deficit suppresses forage production and both cultivars had their highest forage production at the Ir70 irrigation level. Speedfeed showed to have a higher potential for forage production at different irrigation levels and plant densities than Pegah. On the other hand, Pegah produced more desirable forage in terms of some forage quality parameters. Under such experimental conditions, irrigation levels seemed to be a more influential factor compared to plant density regarding to most forage quality and quantity parameters. However, interactions between irrigation levels, plant densities, and cultivars were observed for most agronomic and forage quality parameters studied. The combination of high irrigation level (Ir70 ) and low plant density (D15 ) produced the highest protein yield, whereas the forage produced with moderate irrigation level (Ir100 ) and low plant density (D15 ) was richer in terms of relative feed value. Overall, Speedfeed cultivar may be superior to Pegah due to greater forage dry matter production and higher protein yield.

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