In Qinghai-Tibet plateau of China, forage supply from grassland, regardless of the different grassland types, is greatest in August and lowest in the beginning of May. Seasonal forage supply is quite imbalance. There is a severe shortage of CP supply in winter and spring, especially in spring season. Food intake of yaks reached the greatest level in spring, followed by winter, except for that of one-year-old yaks, whose intake reached the greatest level in winter, followed by summer. In any age groups of yaks, feed intake was the lowest in autumn. Gross energy (GE), CP and DM digestibility varied greatly among different seasons, with the greatest in summer, followed by autumn, and the lowest in winter. The growth curve of yaks in Qinghai-Tibet plateau follows a zigzag growing pattern, which is corresponded to the seasonal imbalance of nutrient supply from grassland. Grazing yaks showed highly efficient compensatory growth relative to feedlot yaks. About 30% of body weight (BW) loss in the cold season was gut fill loss, and 18% to 26% of BW gain in warm season was gut fill gain. The proportion of energy mobilized in winter was greater than in spring. About 50.8% of total body energy accumulated in the second warm season was mobilized in the second cold season. Water and CP were the major components of the chemical composition of EBW loss in yaks in winter; however, the proportion of water increased in spring. Crude protein can be degraded rapidly in the rumen of yaks. Ammonia concentration reaches the greatest level only 1.5 hr after feeding. The ammonia concentration in the rumen of yaks varies with forage maturity and grassland types. Fully matured forages can result in less ruminal ammonia in the grazing yaks than the young forages do. Effective crude protein degradability in the rumen is always lower in yaks than in Holstein cows. With the supplementation of urea, rumen N degradability, rumen microbial protein (MCP) production and the efficiency of microbial protein synthesis increased dramatically in yaks. Peptide is possibly the primary form of amino acid absorption, and that the stomach area (including rumen) probably is the major site of peptide absorption.Carrying capacity of yaks in winter-spring grassland calculated on degradable crude protein (DCP) basis is 15.95 capita/km2 in Sunan county, Gansu Province, which is 85.1% of the carrying capacity calculated on dietary DM basis, and only 52.2% of that calculated on ME basis. However, carrying capacity of yaks in summer-autumn grassland calculated on DCP basis is 169.98 capita/km2, almost the same as that calculated on ME basis, some 2.64 times of that calculated on DM basis. Nitrogen is the first priority for supplement feeding choice, and maybe there is no need for supplementation of nitrogen or energy on summer-autumn grassland. During the first cold season, grazing yaks should be supplemented with DCP at 67 g/d and MEat 4.7 MJ/d for normal growth. During the second cold season, yaks should be supplemented with 58 g/d DCP and 5 MJ/d ME. Generally, yaks should be supplemented with 60 g DCP and 4.8 MJ ME In cold season. Nutrient supply from the summer-autumn grassland is sufficient for compensatory growth and needs no supplementation.
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