Methane emissions from yak ( Bos grunniens) steers grazing or kept indoors and fed diets with varying forage:concentrate ratio during the cold season on the Qinghai-Tibetan Plateau

Abstract

For national greenhouse gas inventories, exact data on all major domesticated livestock systems are required. Such data is completely lacking for yak ( Bos grunniens) and, as this ruminant livestock species seems to have digestion and metabolism which differs from cattle, making data extrapolations from cattle are potentially inaccurate. In the present study methane (CH 4) emissions from yak were estimated under grazing and indoor feeding conditions in the cold season on the Qinghai-Tibetan Plateau at >3200 m above sea level. Twelve 3 year old growing yak steers (175 ± 10.7 kg) were subjected to measurements of CH 4 output using the sulfur hexafluoride tracer gas technique, nutrient intake and apparent digestibilities. The first group of three yaks grazed on native winter alpine meadows, while the indoor groups were fed either oat hay or total mixed diets of oat hay and concentrate in ratios of 0.6:0.4 and 0.4:0.6, respectively, on a dry matter (DM) basis. The experimental period lasted for 6 weeks, with the first 5 weeks for adaptation and the last for measurement. The oat hay contained slightly more crude protein (CP) and less fiber compared to the winter pasture grass, while inclusion of concentrate increased the CP intake of the yaks considerably. Yaks fed indoors gained body weight (BW) during the experimental period (about 200 g/day and slightly higher with concentrate), while grazing yaks lost 550 g/day of BW. Daily CH 4 emissions of the grazing yaks were 81.4 g/day or 1.68 g/kg BW 0.75 at an estimated daily grass DM intake of 3.78 kg. Absolute levels of CH 4 emissions were slightly higher in the oat hay fed yaks. Compared to the oat hay treatment, CH 4 production declined by 0.3 and 0.4 with forage:concentrate ratios of 0.6:0.4 and 0.4:0.6, respectively. Overall, the average proportion of gross energy lost as CH 4 energy (kJ/MJ) was 54 ± 15.2. These values are the ‘CH 4 emission factors’ (according to ‘Tier 1’ of IPCC, 2006) of about 28 kg/year for grazing non-lactating yaks weighing 175 kg. When adjusted to metabolic BW, yaks were at the lower range of CH 4 emission compared to cattle, while they were average when CH 4 was related to DM and gross energy intake. Implementing proper grazing management practices to improve winter diet quality and quantity, and thus animal productivity, can be expected to substantially mitigate CH 4 emissions per unit of meat and milk produced by yak.