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.