Abstract: This study investigated the optimum planting density and harvest date of photoperiod-sensitive (PPS) sorghum-sudangrass hybrids in the rain-fed farming region of the Loess Plateau. Two hybrids, “Big Kahuna BMR” and “Monster”, were planted with 3 different densities (83300, 125000 and 166700 holes·ha^-1) on double ridge soil covered with plastic-film fully mulched. The contents of crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), total digestible nutrient (TDN) and brix (BX) were measured in the plants at different growth stages. Results showed that the growth and development process played a major role in the formation of plant nutritional ingredients. The nutritional difference between the 2 varieties was more significant in the early than late growing stages. The NDF, ADF and BX contents of “Big Kahuna BMR” were all lower than “Monster”, while its TDN contents were higher. CP content declined with increased planting densities. BX contents were more influenced by density in the early than middle or late stages. For the entire growth period, NDF and ADF contents in higher density plantings were significantly lower. In the late growth period (126 to 140 d after seeding), TDN contents progressively decreased as density increased. As density increased, the 2 varieties’ CP contents gradually decreased during the growth process, while BX contents increased. NDF, ADF and TDN contents trended as ascent-descent-ascent from 70 to 140 d after seeding. In the final period of 126 to 140 d, dry matter yield continued to increase but CP and TDN contents did not change significantly (although TDN decreased slightly). These dynamic changes in nutrient content and yield indicate the best harvest date for PPS sorghum-sudangrass. In terms of forage production, the optimum variety for spring sowing with a one-cut system for silage is “Big Kahuna BMR”, with 125000 holes per hectare the preferred density. For both varieties, the optimum harvest period is between 126 to 140 d after seeding (in early or mid-September), when “Big Kahuna BMR” is booting and “Monster” is flowering.

Key words: planting density, growth stage, PPS sorghum-sudangrass hybrids, nutrition value, dynamic change, optimum harvest period