Effects of harvest stage and re-growth on yield, composition, ensilage and in vitro digestibility of new forage sorghum varieties

The effect of growth stage and re-growth on the nutritional and ensilage characteristics of two new sorghum hybrids, BMR-101 and Silobuster, and one commercial variety, FS-5, was examined in this study. Varieties were sampled during the summer at the early heading (EH) stage and were harvested at the soft dough (SD) stage. Additional irrigation enabled autumn re-growth and a second harvest. Plants of FS-5 and BMR-101 were resistant to lodging at EH. However, BMR-101 and Silobuster suffered from high lodging at the SD stage of the summer harvest. Dry matter (DM) content of FS-5 and BMR-101 at EH was below 250 g kg^-1. DM yields of the varieties were similar at the summer harvest and higher than their respective re-growth cuts. Ensilage DM losses were moderate and similar across varieties. Hemicellulose of SD plants was partly solubilised and most of the water-soluble carbohydrate fermented, yielding lactic acid, ethanol and volatile fatty acids (VFA), and a pH < 4. In vitro DM digestibility of varieties was similar in summer silages, but lower in the respective re-growth silages of FS-5 and BMR-101, reflecting the higher content of neutral detergent fibre and lignin in the re-growth silages. The summer plus re-growth cumulative yields of digestible DM of the respective FS-5, Silobuster and BMR-101 silages were 14.7, 16.6 and 14.5 t ha^-1. The commercial variety, FS-5, may have some advantage over BMR-101 and Silobuster owing to its relative resistance to lodging in addition to its high yield and good ensilage properties.