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.

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Reproductive patterns of the yak. II. Progesterone and oestradiol-17 beta levels in plasma and milk just before the breeding season; also during normal and short oestrous cycles.

Concentrations of oestradiol-17 beta and progesterone in plasma and milk of the yak before the first oestrus at the beginning of the breeding season and during normal and short oestrus cycles were measured by radioimmunoassay (RIA). Both oestradiol-17 beta and progesterone were maintained at stable, low levels before the first oestrus. However, oestradiol-17 beta showed a peak, similar to that on the day of oestrus (P > 0.05), about 6 days before the first oestrus and a small progesterone peak was seen 2 days before it. There were three peaks of oestradiol-17 beta in plasma and milk on the day of oestrus [24.57 (SD 11.35) pg ml-1 and 97.84 (SD 26.02) pg ml-1 respectively], and on days 5 and 14 of the cycle. Progesterone levels in plasma and milk were low during oestrus but peaks were seen on day 15 [5.23 (SD 1.02) ng ml-1 and 20.44 (SD 3.64) ng ml-1 respectively]. The patterns of oestradiol-17 beta and progesterone during the short cycle were similar to those during the normal cycle (P > 0.05), but the values were lower. The levels of oestradiol-17 beta and progesterone in milk were about 3 or 4 times higher than those in plasma before the first oestrus, and about 4 or 5 times higher during the normal cycle.

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Reproductive patterns of the yak. III. Levels of progesterone and oestradiol-17 beta during pregnancy and the periparturient period.

Changes in plasma progesterone and oestradiol-17 beta in yaks during pregnancy and the periparturient period of yaks and in milk during the first month of pregnancy were analysed. The profiles of progesterone were similar for pregnant and non-pregnant yaks within 14 days following oestrus (P > 0.05), but the concentration was significantly higher on day 19 (P < 0.05) and in pregnant yaks tended to increase gradually thereafter. The plasma progesterone decreased rapidly on day 120 (P < 0.05), then increased to reach its maximum of 15.5 (SD 3.74) ng ml-1 on day 210; declined again 20 days before parturition and resumed the same levels at parturition as during mating (P > 0.05). The oestradiol-17 beta levels in plasma and milk increased gradually from day 23 after conception, decreased abruptly on day 60 (P < 0.05), then increased to reach their maximum at parturition of 551.4 (SD 70.86) pg ml-1. The oestradiol decreased again after parturition to the same level as during mating (P > 0.05).

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