Abstract: The aim of this study was to ascertain the optimal cultivation regime and cutting time of the photoperiod-sensitive sorghum-sudangrass hybrid varieties “Monster” and the brown midrib (BMR) type “Big Kahuna”, when grown on dryland in eastern Gansu. Performance was assessed at 70, 84, 98, 112, 126 and 140 days after planting. Measurements made at each of these 6 growth stages were dry matter yield, total digestible nutrients, and the main nutritional components, including crude protein (CP), neutral detergent fibre (NDF), acid detergent fibre (ADF), total digestible nutrients (TDN), and brix (BX). Four cultivation regimes were compared: plastic-film full mulching on double ridged soil surface (FF-RS), plastic-film full mulching on a flat soil surface (FF-FS), plastic-film semi-mulching on a flat soil surface (FS-FS), and no plastic-film mulching on a flat soil surface (NM-FS, i.e. the traditional planting method). During crop development from 70 to 140 days, CP levels of all cultivation regimes in both varieties fell from 12%-23% to <10% and brix content steadily rose from <4% to 10%-15%, while NDF and ADF gradually rose, apart from a temporary decrease at late head emergence. The BMR genotype, “Big Kahuna”, generally outperformed “Monster” with approximately 15% higher dry matter yield, lower ADF than “Monster” (average across planting regimes 33.13% and 40.39%, respectively), and higher TDN (average 58.14% and 52.24%, respectively). However, the BX content of non-BMR type hybrid, “Monster”, was higher (average 12.41% and 14.59%, respectively). There was no significant effect of cultivation regime on the nutritional contents. However, yield ranked across cultivation regimes: FF-RS (>30 t·ha^-1) =FF-FS>FS-FS>NM-FS (≈20 t·ha^-1), so that TDN yield was enhanced by plastic film mulching. The relative equilibrium point between TDN content and dry matter yield was about 126 days. In conclusion, considering the higher forage production, the BMR type hybrid, “Big Kahuna”, should be used in conjunction with an FF-RS or FF-FS mulching regime and be cut between 120 and 140 days after emergence, for optimal results in this region. These guidelines are best practice for cultivation and utilization for silage forage of the photoperiod-sensitive sorghum-sudangrass hybrids.

Key words: cultivation regime, growth stage, photoperiod-sensitive sorghum-sudangrass hybrids, nutritional value, optimal cutting time