Introduction

High performance of farm animals, especially dairy cows, depends on the availability of adequate amounts of quality fodder and in developing countries, inadequacy of quality forage is the critical limitation to profitable animal production (^{Sarwar et al. 2002}). Among the many options for overcoming the shortage of forage, the introduction of high-yielding crop varieties ranks highly (^{Bilal et al. 2001}). However, in many developing countries, because of the ever-growing need for food for humans, only limited cultivated land can be allocated to produce fodder for livestock. ^{Douglas (1980}) recommended annual summer crops such as forage sorghum hybrids (Sorghum spp.) for use as alternative forage crops in drier areas in order to bridge the feed shortage gap.

Sugar Graze, a sweet sorghum x sweet sorghum hybrid, is a popular forage source among the livestock farmers of Sri Lanka and Jumbo Plus, a sweet sorghum x Sudan grass hybrid, is still in the initial stages of introduction. Sugar Graze is a late-flowering cultivar with high yields, a crude protein (CP) concentration of 1218% and a high sugar content that boosts feed quality and palatability, resulting in minimal feed wastage. In addition, the crop is resistant to a wide range of diseases. Mature Sugar Graze promotes good weight gains and provides adequate energy for livestock ^{(Pacific Seeds 2009)}. Jumbo Plus, a forage sorghum hybrid cultivar, has excellent re-growth potential and high productivity and is adapted to both dryland and irrigated situations. It has similar CP concentration to Sugar Graze with 56-64% dry matter (DM) digestibility when the plant is 55-60 days old or at 5-10% flowering stage and can be used for grazing, silage making and rotational cropping ^{(Forage Sorghum Guide 2015)}.

These crops have the potential to compete favorably with maize silage in terms of yield and nutritive value (^{Ketterings et al. 2005}) and may be an appropriate alternative to maize for utilizing irrigation water in drought-prone areas. The shortage of ground water is the primary limitation to cultivating grass in the dry zone. As such, it is essential to select a drought-tolerant grass/fodder species, and Sri Lankan farmers cultivate fodder sorghum. In an initial study 7 cuttings were achieved from a single planting yielding 24 kg of fresh fodder/m² from a single cutting with plant spacing of 45 x 15 cm (^{Sivayoganathan 2016}). While research on sorghum cultivars in Pakistan has shown marked differences between cultivars in green fodder yield and morphology under 30 cm row spacing (^{Bakhsh et al. 2015}), similar data on forage sorghum hybrid cultivars in the dry zone of Sri Lanka are limited. There is a need to assess the growth of these cultivars and how varying the plant spacing affects both yield and quality of forage so that the growing demand for forage by livestock can be met.

The present study was designed to determine the crop morphology, growth parameters and forage yield of Sugar Graze and Jumbo Plus under irrigation in the dry zone of Sri Lanka under 3 different plant spacings (30 x 15, 30 x 45 and 30 x 60 cm).

Materials and Methods

This experiment was carried out at the livestock farm, Department of Animal Science, Faculty of Agriculture, University of Jaffna, Ariviyal Nagar, Kilinochchi (Figure 1), from August 2015 to January 2016. These months fall into Sri Lanka's maha season, i.e. the period September -February which experiences rainfall through the Northeast monsoon.

Figure 1 The experimental site, Kilinochchi District, in the dry zone of Sri Lanka. 

The remaining period of the year is dry with the driest period being June to August. During the study period (August 2015-January 2016), rainfall in individual months varied greatly (Figure 2) and due to the extremely low water retention capacity irrigation had to be applied in some months.

Figure 2 Monthly rainfall of Kilinochchi District from January 2015 to February 2016. Source: ^{Department of Census and Statistics of Sri Lanka (2015)}; ^{Kilinochchi District (2016)}. 

According to Vavuniya meteorological data, the average monthly temperature in the region is 28.4 °C (range 25.6-30.0 °C), while maximum and minimum averages are 35.0 and 21.3 °C, respectively ^{(Jaffna and Kilinochchi Water Supply and Sanitation Project 2010)}.

Soils of the area are red-yellow latosols (Haplustox), which are the most intensively cultivated soils of Jaffna Peninsula and have very low inherent fertility. Extremely poor water retention properties mean that dryland cropping is inappropriate, while conventional flood irrigation is impractical owing to very rapid infiltration and soil drying.

The experiment was laid out in a completely randomized design (CRD) in a factorial arrangement of 3 plant spacings (30 x 15, 30 x 45 and 30 x 60 cm) and 2 cultivars (Sugar Graze and Jumbo Plus) with 3 replications. Sowing was on 5 August 2015.

Within rows spacing was kept constant at 30 cm and the spacing between rows was varied. Seeds were sown at the rate of 2 seeds per hill and seedlings were thinned to a single plant per hill 2 weeks after sowing resulting in plant populations of approximately 222,000, 74,000 and 55,000 plants/ha for inter-row spacings of 15, 45 and 60 cm, respectively. Cattle manure was applied at planting at the rate of 100 kg/ha (N: 1.2-1.9%, P: 0.2-0.5%, K: 0.51.1%) and inorganic fertilizers were applied 1 week after establishment of plants at the rate of 50 kg urea, 25 kg triple superphosphate and 12.5 kg muriate of potash/ha. During the dry spell of the study period, irrigation was done at weekly intervals. Plots were manually weeded at 30 days after planting to reduce competition from weeds.

Five plants were randomly selected from each plot for recording of leaf length and width, leaf area [leaf area factor (0.72 for forage sorghum) x length x width] (^{Arkel 1978}), number of leaves per plant, stem girth, root length from the base of the plant to tip of the selected average lengthier rootlet, internodal elongation and plant height at weekly intervals. At 60 days after planting, on 3 October 2015, the crops were cut 15 cm from ground level and allowed to ratoon. Two ratoon cuts were made at 60-day intervals, on 1 December 2015 and 29 January 2016. Immediately after harvesting, fresh forage was weighed using a spring balance. Samples (approximately 2 kg) of the harvested forage from each experimental plot were selected and air-dried for 24 hours, followed by oven-drying at 75 °C for 72 h to constant weight for dry matter yield determination. Data were subjected to Analysis of Variance and mean separation was done with ^{Duncan's multiple range test (P<0.05) (Duncan 1955}) with SPSS (Statistical Package for Social Science) version 16.0 for Windows.

Results

Initial harvest

For the initial harvest the 2 cultivars generally responded differently to variation in plant spacing in terms of plant morphology (Table 1).

Table 1 Effects of row spacing (30 x 15, 30 x 45, 30 x 60 cm) on growth parameters and yields of Sugar Graze and Jumbo Plus 60 days after planting (initial harvest). 

Each value is a mean ± SD for 3 replicates.

Within rows and cultivars, means without a common letter differ (P<0.05).

While leaf length, leaf width and leaf area were unaffected by plant spacing for Jumbo Plus, the narrow spacing (30 x 15 cm) produced narrower leaves with smaller area than the medium (30 x 45 cm) and wide (30 x 60 cm) spacings for Sugar Graze. Similarly, the narrow and wide spacings for Jumbo Plus produced more leaves per plant than the medium spacing, while the medium spacing for Sugar Graze produced more leaves than the narrow spacing. In general, the thickest stems were produced at the wide spacing and the thinnest at the narrow spacing.

A similar trend occurred with root length with longer root lengths generally being associated with wide plant spacing and shorter root lengths with narrow spacing. Plant height was unaffected by spacing for Jumbo Plus, while the wide spacing for Sugar Graze produced the tallest plants.

Overall, Sugar Graze displayed slightly longer, wider leaves with much greater area than Jumbo Plus but produced fewer leaves, though differences were not significant. This cultivar also produced longer roots than Jumbo Plus but internodal elongation was greater for Jumbo Plus, resulting in taller plants.

Dry matter (DM) yields were inversely related to plant spacing with yield decreasing progressively as plant spacing increased, although differences were not always significant. Sugar Graze produced higher DM yields than Jumbo Plus (Table 1).

First ratoon crop

In the first ratoon crop some parameters, viz. leaf length, number of leaves and plant height, were not influenced by spacing in either cultivar, while leaf width, leaf area, internodal elongation and stem girth varied inconsistently with row spacing in the 2 cultivars (Table 2). Varietal differences also were noted among the morphological parameters of the first ratoon crop, where generally leaf length and width, leaf area, number of leaves per plant and stem girth were greater for Sugar Graze, while Jumbo Plus showed higher values for internodal elongation and plant height.

Table 2 Effects of row spacing (30 x 15, 30 x 45, 30 x 60 cm) on growth parameters and yields of first ratoon crop of Sugar Graze and Jumbo Plus. 

Each value is a mean ± SD for 3 replicates.

Within rows and cultivars, means without a common letter differ (P<0.05).

Dry matter yields followed similar trends in both cultivars with declining yields as plant spacing increased, but differences were not always significant (Table 2). Varietal differences in DM yield were small.

Second ratoon crop

As for the first ratoon crop, row spacing had no significant effect on leaf width, leaf area and stem girth in either cultivar, while inconsistent responses occurred for the remaining morphological parameters (Table 3). There were consistent effects of row spacing on DM yields in both cultivars with yields declining as plant spacing increased, but differences were significant (P<0.05) only for Jumbo Plus. Dry matter yields for Jumbo Plus at the medium and wide spacings declined dramatically to about half those for Sugar Graze. Overall, increases in row spacing resulted in greater percentage yield decreases in Jumbo Plus than in Sugar Graze. Yields for both ratoon crops were generally about 25-35% of those obtained at the initial harvest.

Table 3 Effects of row spacing (30 x 15, 30 x 45, 30 x 60 cm) on growth parameters and yields of second ratoon crop of Sugar Graze and Jumbo Plus. 

Each value is a mean ± SD for 3 replicates.

Within rows and cultivars, means without a common letter differ (P<0.05).

Discussion

Leaf length and width

Leaf development has been described extensively for fodders, as growth is mostly reflected in large increases in leaf length as plants grow to maturity, accompanied by relatively small increases in width and thickness (^{Skinner and Nelson 1994}). Large leaf lengths are also important for the survival of individual plants within a sward (^{Barre et al. 2015}). Leaf length and width values observed for both cultivars during the present study were slightly greater than the values recorded by ^{Singh et al. (2014}) for leaf length (45-70 cm) and width (4-7 cm) of sorghum hybrids. Leaf length of Sugar Graze was similar to the 95 ± 2.0 cm reported by ^{Pahuja et al. (2014}) in India, for the first cut at 50% flowering and a spacing of 15 x 45 cm, whereas leaf width was slightly higher than that recorded by the same authors (6 ± 0.58 cm).

Leaf area

The results of the present study demonstrate that leaf area increases as plant spacing increases as shown by ^{Lamana (2007}). This could be due to less competition among plants for space and soil nutrients as the plant population per unit area decreased. Therefore, the lower population density which resulted from the wider plant spacing gives better conditions for more accumulation of photosynthetic products, better growth and expansion of foliage, which was in turn expressed in greater DM yields. The range of values for leaf area (440-615 cm²) for Sugar Graze and Jumbo Plus in the present study were in agreement with the values reported by ^{Nabi et al. (2006}) for advanced lines of forage sorghum cultivars. The higher mean leaf area in Sugar Graze (595 ± 62.3 cm²) than in Jumbo Plus (460 ± 58.9 cm²) may be due to differences in genetic makeup of the cultivars. ^{Musa et al. (1993}), ^{Naeem et al. (2002}), ^{Mahmud et al. (2003}) and ^{Chohan et al. (2003}; ²⁰⁰⁶) also observed variation in leaf area among various cultivars and varieties of forage sorghum.

Number of leaves per plant

The general absence of any consistent effect of row spacing on leaf number per plant is in agreement with the findings of ^{Liu et al. (2004}), who observed for maize that it did not affect leaf number. In contrast ^{Lamana (2007}) reported that wider plant spacing in maize had a positive effect on number of leaves. The values recorded for number of leaves per plant for both cultivars in the present study were consistent with those of ^{Monteiro et al. (2012}), who reported that number of leaves in forage sorghum is generally between 14 and 17. ^{Chohan et al. (2003}) and ^{Naeem et al. (2002}) also reported variation among different cultivars of sorghum for number of leaves per plant.

Stem girth

Stem girth recorded in the present study was similar to that reported by ^{Pahuja et al. (2014}) in India for stem girth of Sugar Graze (5.9 ± 0.21 cm at 50% flowering stage and 15 x 45 cm spacing). While ^{Yosef et al. (2009}) and ^{Ayub et al. (1999}) found significant variation in stem diameter among different cultivars of sorghum, cultivar differences in our study were not statistically significant (P>0.05).

Root length

The trend for root length to increase as row spacing increased would reflect greater competition between plants at the narrower spacings. Despite the shorter root length per plant at the narrow spacing, the much greater plant populations at this spacing would have resulted in substantially greater root length per unit area than at the wider spacings. As a result plants at the narrow spacing would have had better opportunity to utilize soil water and nutrients than at wider spacing, resulting in higher DM yields.

Internodal elongation and plant height

Plant height as a growth parameter is a result of elongation of the stem internodes, which is influenced by the environment as suggested by ^{Weston (1967}). In the current study taller plants were observed with wider spacing, which contrasts with reports in the literature that narrower spacing will give taller plants as a result of competition for sunlight (^{Lamana 2007}). However, the absence of any effect of plant spacing on plant height of Jumbo Plus supports the finding of ^{Roy and Biswas (1992}) that plant height at maturity was not affected by plant spacing. The significant differences in plant height between the 2 cultivars may be due to genotypic variation, as differences in internodal elongation between varieties can lead to differences in height as reported by ^{Evans (1975}) and Weston (1967). ^{Nabi et al. (2006}) and ^{Silungwe (2011}), who worked with different forage sorghum cultivars, also reported plant heights (203-230 cm) lower than those in the present study (227-298 cm), as did ^{Pahuja et al. (2014}) for Sugar Graze (189 ± 1.9 cm) in India.

Dry matter yield

Plant spacing has a marked impact on the efficiency of use of land, light, water and nutrients. By optimizing plant spacing, highest yield potential can be achieved from the smallest possible area (^{Oseni and Fawusi 1986}). The direct relationship between DM yield and plant population agrees with the findings of ^{Fisher and Wilson (1975}), who reported greater DM yield with higher plant populations than with lower plant populations. ^{Wolf et al. (1993}) and ^{Graybill et al. (1991}) also reported that DM yield of forage maize responded positively to plant density. This relationship would be affected by the availability of soil moisture, and the application of irrigation on a regular basis in this study would have ensured that all row spacings/plant populations had adequate water. Dry matter yield recorded for Sugar Graze in the current study seemed to be less affected by differences in row spacing than Jumbo Plus, which appeared not to be related to root length as there were no significant differences in root length between the cultivars. ^{Epasinghe et al. (2012}) reported DM yields of Sugar Graze in Sri Lanka of 5,230 kg/ha at 60 days after planting at 45 x 15 cm spacing and these lower yields might be attributed to the differences in the spacing, soil fertility and environmental conditions. By contrast ^{Nabi et al. (2006}) recorded yields of 10,400-13,100 kg DM/ha for advanced lines of forage sorghums and ^{Silungwe (2011}) recorded 13,262 kg DM/ha at 15 cm row spacing 78 days after sowing for Sugar Graze.

Forage yield is a function of growth parameters, viz. plant population, plant height, leaf:stem ratio, leaf area, and leaf area index (^{Lamana 2007}). The differences in DM yield between the 2 cultivars could be attributed to the fact that Sugar Graze exceeded Jumbo Plus in the growth parameters leaf length and width, leaf area and root length. ^{Watson (1947}) has shown that variation in total dry weight of plants is more dependent on variation in leaf area. Light interception capacity of the leaf is amplified with the increase in leaf area often leading to increase in photosynthesis and DM yield. Therefore, higher DM yield recorded for Sugar Graze might be attributed to its higher leaf area than Jumbo Plus.

First and second ratoon crops

The most consistent findings with the ratoon crops were that there were fewer leaves per plant, leaves were smaller, height was less and DM yields were lower than for the initial harvest. However, DM yield remained a factor of plant spacing with higher yields at narrower row spacing, indicating that plants were still accessing moisture and nutrients from the soil in sufficient quantities to maintain acceptable growth levels. The reduced yields are possibly a function of nutrient supply in the soil being depleted by the initial crop and a change in seasonal conditions over time. There were no significant differences between Sugar Graze and Jumbo Plus in DM yields for the first ratoon crop, in contrast with the generally higher yields for Sugar Graze in the initial crop and second ratoon crop. Despite having smaller leaves and thinner stems than Sugar Graze, the greater height of Jumbo Plus ensured that yields in the 2 cultivars were similar. The success of the second crop is often a function of how early the main crop was planted and harvested, which determines the seasonal conditions under which the first and second ratoon crops must grow. However, normally ratoon crops of sorghum are expected to yield from 25 to 35% of the main crop (^{Livingston and Coffman 1996}), and our yields fall within this range. Significant differences in DM yield between main and ratoon crops have been reported by ^{Saberi and Aishah (2014}). when assessing yield responses of forage ratoon sorghum under varying salinity levels and irrigation frequencies.

Our findings suggest that both Sugar Graze and Jumbo Plus will grow satisfactorily under irrigation in this environment. While DM yields from the first harvest were excellent. yields from the ratoon crops were significantly lower despite the application of irrigation. A plant spacing of 30 x 15 cm produced the highest yields but results under rain-fed conditions would not necessarily be the same. Further studies to determine the performance in the low-rainfall (yala) season are necessary to determine desirable spacings under such dry conditions. Chemical analyses of forage and digestion studies would provide valuable information on the relative merits of these two cultivars for livestock feeding.