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interests. Our manuscript doesn’t involve any ethical issues. Authors’ find more contributions WZ, XM conceived the study and participated in its design. YD, HL participated in field and clinical aspects of the study. WZ carried out laboratory work. WS, WZ drafted the manuscript. XM, WS, WZ, XZ, WC edited the manuscript. All authors read and approved the final version of the manuscript.”
“Background Yak (Bos grunniens) and cattle (Bos taurus) separated about 4.4 to 5.3 million years ago . While cattle have a worldwide distribution in most of the low lands, the yak has dominated in high lands especially Sirolimus around the Hindu Kush-Himalayan region and the Qinghai-Tibetan Plateau (QTP), ranging from 3,000 to 5,500 m above sea level. The yak is one of the world’s most remarkable domestic animals, and has been second reported as a typical four season grazing ruminant in the QTP . In order to adapt to the harsh environment with severe cold, less oxygen, strong ultra-violet (UV) radiation, and poor forage resources, yaks have evolved special adaptations in physiology, nutrient metabolism and foraging [3–8]. Recently, Shao et al  anatomically compared the yak tongue with the cattle tongue, and found that the yak tongue was better adapted to the
harsh characteristics of Tibetan pasture. Other recent studies have shown that yaks have an efficient nitrogen metabolism, suggesting an adaptation mechanism to their low-N dietary ingestion under harsh grasslands conditions of the QTP area . Subsequently, using the sulfur-hexafluoride (SF6) tracer technique, Ding et al  measured the enteric methane emissions of yak in the QTP area and showed that yaks produce less methane (per unit of live weight) compared to other ruminants, such as cattle. Greenhouse gases have become a major issue in the world and ruminant livestock are an important source of global enteric methane. Enteric methane gas is produced by microorganisms, called methanogens, in the digestive tract of ruminant livestock during digestion of feed and represents a direct loss of gross energy intake that could more efficiently be used by the animal for increased productivity .