Seasonal heat production and energy balance of grazing yaks on the Qinghai-Tibetan plateau

Abstract 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.