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Yaks ( Bos grunniens ) are adapted to a harsh alpine environment above the altitude of 3000 m, and show strong compensatory growth ability. The objective of this study was to investigate the plasma hormonal status and metabolite levels of growing yak steers when either offered supplements (SP) or not supplemented (NS) during the annual season of severest pasture shortage in late April and early May. The experiment was conducted between 23 April (day 0) and 8 July 2013 (day 76). Twelve 3-year-old yak steers were allocated to one of two treatments, either to receive supplements for 25 days or to remain unsupplemented. The supplement, which was composed of ground maize (75%) and local rapeseed meal (25%), was offered at maintenance level of energy and protein to the group of yaks receiving supplements from 23 April to 18 May (day 25) at evening. All the yaks were released to pasture during the daytime. Rectal temperature was measured fortnightly using an electro thermometer. Heart rate was recorded over four 4-day periods using modified heart rate monitors. Live weights (LW) were weighed weekly in the morning. Afterward, blood samples were drawn before release to the pasture to measure blood glucose (GLU), blood urea nitrogen (BUN), nonesterified fatty acids (NEFA), β-hydroxybutyrate (BHBA), growth hormone (GH), cortisol, the thyroid hormones (triiodothyronine [T 3 ] and thyroxine [T 4 ]), insulin and insulin-like growth factor 1 (IGF-1). The supplemented yaks showed a higher growth rate and compensation ability compared with the unsupplemented yaks. The overall mean heart rates of the unsupplemented yaks were higher than those of the supplemented yaks, and were higher in both treatments in July than in June. There were significant treatment effects on the concentrations of IGF-1, insulin, NEFA and BUN. Whilst supplementation reduced the concentration of IGF-1, it increased the blood concentration of insulin. The differences of blood NEFA between treatments during feed restriction period disappeared in forage growing period. BUN concentrations of SP were significantly lower than NS from day 8 to day 28. Thereafter, BUN concentrations of yaks on the two treatments were similar. The results indicate that supplementary feeding to alleviate nutritional stress in yaks during the period of lowest herbage availability is critical for improving their ability to replace body tissue mobilized over the previous winter months. [ABSTRACT FROM AUTHOR]

A study was conducted to measure the energy balance of free-ranging yak during the four annual seasons in order to elucidate the factors constraining energy utilization by grazing yak. The heat production (HP, kJ/day) of grazing non-lactating female yaks was calculated as the product of heart rate (HR, beats/min) and the amount of O 2 delivered to the body at every heartbeat (O 2 P, μl), and by the constant value of 20.47 kJ/l of O 2 consumed. Heart rates were recorded continuously over 4 days, using modified heart rate monitors. Individual daily fecal output was measured using Cr 2 O 3 as an external marker. Daily herbage dry matter (DM) intake was calculated from fecal output and digestibility of the forage determined in vitro . The greatest herbage mass was measured in August (496 kg DM/ha), and the least in December and May (208 and 226 kg DM/ha). However, the herbage present in both May and August had higher crude protein contents and lower NDF contents than those sampled in October and December. Daily average HR (beats/min) was greater in summer (August) than during the other three seasons (78 vs. 49–52). The greatest O 2 P was recorded in May. The highest metabolizable energy intake (MEI) (1120 kJ/kg BW 0.75 per day) was measured in August when yaks grazed on lush green forage. HP was higher in August than in October and December (715, 548 and 400 kJ/kg BW 0.75 per day, respectively), but did not differ significantly from that measured in May (640 kJ/kg BW 0.75 per day). The animals were in positive energy balance only during August (energy retention (ER) = 405 kJ/kg BW 0.75 per day). Energy balance did not differ between the other seasons: −111 (October), −91 (December) and −13 (May) kJ/kg BW 0.75 per day, respectively. HP and ER were highly correlated with MEI ( R 2 = 0.73 and 0.88, respectively). The formulas calculated through the regression of HP and ER on MEI were used to estimate fasting heat production (FHP = 341 kJ/kg BW 0.75 per day) and maintenance ME requirements (ME m , 545 kJ/kg BW 0.75 per day) of the free grazing yaks. The results showed that free-ranging yaks expended much more energy to resist harsh environmental and sward conditions compared with confined yak or cattle and grazing cattle in low land area. [ABSTRACT FROM AUTHOR]