Reproductive patterns of the yak. I. Reproductive phenomena of the female yak.

The reproductive phenomena of 62 multiparous yaks from two grassland areas were investigated. The lengths of oestrus and the oestrus cycle were 12-36 h and 19-21 days respectively. Ovulation occurred 12-36 h after the end of oestrus. The oestrus and conception rates were 85.5% and 94.3%. The duration of pregnancy ranged from 248 to 258 days. The signs of oestrus, pregnancy and parturition, and the changes of the uterus and ovaries during oestrus and pregnancy are described.