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

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 O2 delivered to the body at every heartbeat (O2P, μl), and by the constant value of 20.47 kJ/l of O2 consumed. Heart rates were recorded continuously over 4 days, using modified heart rate monitors. Individual daily fecal output was measured using Cr2O3 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 O2P was recorded in May. The highest metabolizable energy intake (MEI) (1120 kJ/kg BW0.75 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 BW0.75 per day, respectively), but did not differ significantly from that measured in May (640 kJ/kg BW0.75 per day). The animals were in positive energy balance only during August (energy retention (ER) = 405 kJ/kg BW0.75 per day). Energy balance did not differ between the other seasons: −111 (October), −91 (December) and −13 (May) kJ/kg BW0.75 per day, respectively. HP and ER were highly correlated with MEI (R2 = 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 BW0.75 per day) and maintenance ME requirements (MEm, 545 kJ/kg BW0.75 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.