Effect of row spacing and cultivar on biomass yield and quality of Sorghum bicolor L. Moench

Effekt von Reihenabstand und Sorte auf den Biomasse-Ertrag und die Qualität von Sorghum bicolor L. Moench

Fig. 1. Monthly average air temperature [°C] and sum of precipitation [mm] for the growing seasons 2008 and 2009 at experimental station Giessen.

Monatliche Durchschnittstemperatur [°C] und Niederschlagssumme [mm] der Vegetationsperioden 2008 und 2009 an der Versuchsstation Gießen.

LAI were measured at three stages of plant development (30, 60 and 90 days after germination) by using a pre-calibrated Sun Scan canopy analysis system (Delta T Company) which measures and analyses incident and transmitted photosynthetic active radiations (PAR) in crop canopies. Before harvest, plant samples of 1 m² from each plot were taken and separated into leaves, stems and panicles. The dry weight of each fraction (leaves, stems and panicles) was detected for the whole plant samples from 1 m². Plant height at the time of harvest was measured by using bricklayer ruler. Immediately after harvest dry matter content of all samples was determined by drying at 105°C for 48 hours. Plant samples intended for NIRS analysis were dried, finely grounded, packaged into dry paper sachets and stored.

NIRS (Near-Infrared Reflectance Spectroscopy) was used to determine ash, lipids (XL), crude protein (XP), sugar (XZ), neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) concentrations. Scanning of the samples was done with a Foss NIR-system scanning monochromator (Model 6500, Silver, Spring, MD) at the range of 780 to 2500 nm. The prediction equation based on a calibration established in our lab with sorghum in 2008. Results from the calibrated samples were used to develop a prediction equation by modified partial least squares regression (Shenk and Westerhaus, 1991). The volatile solids (VS) content was calibrated as weight loss during incineration at 550°C by muffle furnace for the estimation of organic matter in the samples. The ash content was measured as the incineration residues.

An analysis of variance (ANOVA) of the data was conducted by using the PIAF software (planning information analysis program for field trials) and a General Linear Model (GLM) (Table 4; 5). Means were compared based on t-tests by LSD at p < 0.05. Correlation analysis was performed using PASW version 18 software (SPSS INC., Chicago, IL) to determine the relationship among the studied parameters according to Spearman's rho methods (Table 6).

Leaf area index (LAI) of sorghum canopies ranged from 2.1 to 4.1 (30 days after germination) up to 3.9 to 5.5 (60 days after germination) and 4.3 to 5.4 (90 days after germination) (Table 1). In 2008 the LAI differed between cultivars only at 90 days after germination when Bovital reached higher LAI (5.0) than Goliath (4.7). In 2009 the LAI difference between cv. Goliath and Bovital was not significant. Among cultivars in 2009 the lowest LAI was observed for cv. Akklimat at all three measuring dates. Row spacing of 37.5 cm increased LAI in comparison to the other treatments inconsistently as this effect was observed at two measuring dates in 2008 (30 and 60 days after germination) and at one measuring date (90 days after germination) in 2009. Cultivars and row spacing clearly influenced the numbers of tillers/m² in both years. Cv. Bovital and Akklimat produced higher number of tillers/m² compared with other cultivars in 2008 and 2009 respectively. Wider row spacing caused a significant decrease of the number of tillers/m² in both years (Table 2). The DM yield of sorghum ranged from 10.10 t DM/ha (cv. Akklimat 2009) up to 19.72 t DM/ha (cv. Goliath 2008). In both experiments cv. Goliath produced significantly higher DM yields (Table 2). Row spacing had no clear effect on the DM yield in 2008 whereas in 2009 significant effects caused by row spacing were observed. Narrow row spacing of 75 cm (DR) and 37.5 cm exhibited significantly higher DM yield than wider row spacing of 75 cm (Table 2). However regarding DM yield no significant interaction was present between cultivars and row spacing. In 2008 the DM concentration of the harvested biomass was about 25 to 26% which was not influenced either by cultivar or by the row spacing. In 2009 cv. Goliath, Bovital and Akklimat had nearly the same DM percentage but significantly lower value was observed with cv. Aron and Rona 1. Row spacing did not affect the DM content of sorghum biomass and no interaction of cultivar × row spacing was observed in either year. The relative proportion of stems in terms of dry matter which ranged from 54 to 66% had the highest proportion within the whole biomass followed by the proportion of panicles (13–29%) and leaves (16–21%) (Table 2). The biomass of cv. Goliath had the highest proportion of stems (66%) while lowest was observed with cv. Bovital (54%). Row spacing had no significant impact on relative proportion of stem and panicle. However it could be observed that cultivars had clear effect on the relative proportion of the panicles. Early maturing cv. Bovital had significantly higher proportion of panicle whereas late maturing cv. Goliath had lowest among the cultivars. There were main effects of different row spacing and cultivars on the relative proportion of leaves. Cv. Goliath produced considerably higher relative proportion of leaves whereas cv. Rona 1 exhibited the minimum value among cultivars. Row spacing of 37.5 cm led to increase the leaves relative proportion in comparison with 75 cm and double row (DR) 75 cm apart.

Tillering is an important agronomical trait as it has major impact on leaf area development of Sorghum bicolor (Hammer et al., 1987; Lafarge et al., 2002) and on crop water use pattern (van Oosterom et al., 2008). Our study demonstrated that 75 cm row spacing exhibited the lowest number of tillers/m² than narrow row spacing. This is likely the result of greater plant to plant competition in 75 cm row spacing for light. It might be possible that due to better distribution of plants more light was reached the base of plants which resulted in higher number of tillers per plant at narrow row spacing compared with wider row spacing. Among cultivars, cv. Akklimat showed maximum number of tillers per m². Being as hybrids S. sudanense, cv. Akklimat, possesses higher capability for tiller formation than that of S. bicolor hybrids like cv. Goliath, Aron and Rona 1.

We found a clear difference of DM yield among the tested cultivars of sorghum. Tall and late maturing cv. Goliath accumulated more DM in vegetative parts (stems and leaves). It can be suggested that cv. Goliath has higher potential for biomass production than the other tested cultivars. Remarkable changes in DM yield due to different cultivars of forage sorghum have been also reported by other researchers (Habyarimana et al., 2004; Amaducci et al., 2004; Zhao et al., 2009). According to Carmi et al. (2005) for appropriate silage production DM content should not be lower than 26% as it leads to decrease the quality of silage. Cv. Goliath and Bovital reached the optimum DM content for silage production. However, DM contents were below this level in cv. Aron and Rona 1.

Maximum protein concentration was recorded in the present study for cv. Bovital and cv. Akklimat in 2008 and 2009, respectively. We suppose that this effect could be due to their higher capability of tiller formation. Tillers are physiologically younger plant organs with higher activity of protein synthesis than physiologically older leaves and main stems. Additionally the accumulation of carbohydrates and the synthesis of fiber compounds like cellulose are increasing in main stems. These processes might explain the higher protein concentration in cv. Bovital and cv. Akklimat in the current study. Considerable differences within the cultivars of sorghum have been previously reported (Carmi et al., 2006; Miron et al., 2006; Beck et al., 2007).

The results which are observed under our conditions indicate that cultivars had a clear effect on sugar concentration. Cv. Aron and Rona 1 accumulated markedly higher concentration of sugar in comparison with other tested cultivars. Both cultivars belong to S. bicolor which has higher potential for sugar concentration than S. sudanense like cv. Akklimat and Bovital. Significant differences among the sorghum cultivars regarding sugar concentration have been previously documented (Dolciotti et al., 1998).

The NDF concentration in current trials varied from 50 to 56% which is similar with the previous findings in forage sorghum (Miron et al., 2005; Carmi et al., 2005) as well as in maize ranging from 50–52% (Marsalis et al., 2010). A clear difference among cultivars for NDF, ADF and ADL concentration in the present study confirms the findings of other researchers who carried out investigations with forage sorghum hybrids (Carmi et al., 2005; Miron et al., 2006; Beck et al., 2007). We suppose that lower ADF, NDF and ADL concentrations determined for cv. Rona 1 and Aron were caused by higher WSC concentration in stems which reduced the fiber concentration in both cultivars. The concentration of lignin in biomass of sorghum differed among the cultivars evaluated in our experiments. Higher content of lignin was found in cv. Goliath and Akklimat but lower in Bovital, Aron and Rona 1. These differences may explain partially the differences in NDF digestibility between the five cultivars.

Generally it can be supposed that some of the above mentioned differences in chemical composition among the sorghum cultivars can be explained by different capability for tillering and biomass distribution to plant organs (leaves, stems and panicles). Regarding phenotype and plant morphology there is a high diversity within the species of sorghum. For that reason more cultivars should be included in future experiments.

Narrow row spacing increased biomass of sorghum as it reduced the competition between the plants (intra row specific competition). Interactions between row spacing and cultivars were not observed either for biomass or for quality parameters of sorghum. In future investigation on this subject, plants should be separated into main stem and side stems (tillers) which may help to provide some more information about the chemical composition of sorghum. Maize silage is the most important source of biogas production, but further intensification of maize cropping may increase the risk of pest damage and nutrient losses. An attractive alternative to maize is sorghum, which can produce a biomass compositionally similar to that of maize and known to be highly productive with respect to biomass. Therefore it can be supposed that sorghum can be efficiently used for biogas production.