Nutritive value for ruminants of different fresh and ensiled sorghum (Sorghum bicolor (L.) Moench) varieties harvested at varying maturity stages

The occurrence of the Western corn rootworm and increasing frequency of summer droughts have reduced forage maize yields and quality in Central Europe in recent years. Therefore, sorghum (Sorghum bicolor (L.) Moench) has been increasingly used as alternative forage in ruminant feeding, although information on the nutritive value of whole crop sorghum silage (WCSS) under Central European climate conditions is scarce. This research project aimed to study differences between sorghum varieties and harvest dates regarding dry matter (DM) yield, chemical composition of fresh panicle, fresh stover, fresh whole plant and WCSS as well as whole tract digestibility and energy content of WCSS. For this purpose, six sorghum varieties, classified by their predominant use (biomass, silage, grain), were cultivated and harvested at three maturity stages (late milk stage, dough stage, full maturity). Furthermore, WCSS quality was compared with feed value of fresh and ensiled forage maize. Dry matter yield was higher (p < 0.05) in biomass sorghum (24.9 t/ha) compared to other sorghum varieties (12.4 to 16.7 t/ha). However, panicle proportion was lowest in biomass sorghum (10.5%), highest in grain sorghum (59.1%) and intermediate in silage sorghum. Fibre content declined and starch content increased with rising panicle proportion in fresh panicle, fresh whole plant and WCSS. Therefore, whole tract organic matter digestibility (66.8%) and metabolisable energy (ME) content (9.49 MJ/kg DM) were highest in grain sorghum silage. Especially low ME content was found in the biomass sorghum silage (7.43 MJ/kg DM). Dry matter yield of sorghum increased between late milk and dough stage of maturity, while it remained constant afterwards. Furthermore, ME content of WCSS rose tendentially (0.05 < p < 0.10) between late milk and dough stage of maturity and dropped again between dough stage and full maturity. Highest nutritive value of WCSS is achieved if grain sorghum varieties are used and both nutritive value and DM yield are optimised if harvest is carried out at dough stage of maturity. However, compared to forage maize, grain sorghum varieties have disadvantages both in DM yield and in nutritive value under common Central European climate conditions.