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

Summary Concentrations of oestradiol-17 β 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 oestrous cycles were measured by radioimmunoassay (RIA). Both oestradiol-17 β and progesterone were maintained at stable, low levels before the first oestrus. However, oestradiol-17 β 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 β 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 β 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 β 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.