Yaks (Bos grunniens) and yak crossbreds are kept in transhumant systems using different pastures along an altitudinal gradient in the Nepalese Himalayan Mountains. Yaks are known to be well adapted to cope with low oxygen partial pressure, low temperatures and the harsh mountain environment (Wiener et al., 2003), and the resulting energy deficiency. Less information is available on the adaptive capabilities of yak crossbreds. Crossbreeding is a strategy to obtain higher milk yields especially by heterosis. In the Eastern Nepalese Himalayan Mountains, two different local breeds are used for crossbreeding. Bhelang bulls from near Tibet (Bos taunts genotype) are crossed with female yaks, and yak bulls are crossed with female Nepalese hill cattle (Bos indicus). Female crosses of cattle bulls x yak cows are locally called Dimjo chauries, while those which are produced using cows and yak bulls are called Urang chauries (Joshi, 1982). In order to compare the adaptive capacity of these two crossbred typeswith that of yaks, an experiment was conducted at two altitudes in the Taplejung District of Nepal.
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