Abstract

The stimulation of oestrus in ewes early in the breeding season has been known for a number of years but only recently has there been a full understanding of the changes which occur in the ewe. Recent work indicates that only 17 to 21 days isolation from rams is needed to sensitise ewes to the ram. Within 10 minutes of introducing rams, the luteinising hormone (LH) levels increase in the ewe. This can result in a preovulatory LH surge 27 to 35 hours later and a silent ovulation within 54 to 72 hours in anovular ewes. However, 43 to 59% of ewes have a premature regression of the corpus luteum (CL) and a second silent ovulation occurs 4 to 6 days later. This leads to flocks have peaks of first oestrus around days 18 and 24 after ram introduction. Injection of progesterone at ram introduction prevents premature regression of the CL and first oestrus is synchronised over days 19 to 21 after ram introduction. The percentage of multiple ovulations is higher at the silent ovulation than at the first oestrus or in spontaneously ovulating flock mates. Priming with progestagens for 12 to 16 days can induce oestrus at the first ovulation after ram introduction and still maintain the increased proportion of multiple ovulations. The ram effect is due to pheromones which are present in the ram's wool and wax. The buck is as effective as the ram at stimulating ewes and fatty acid pheromones isolated from the buck are also effective at stimulating ewes.

KF, Thompson, WH Risk, PC Mason, and S Crosbie

Proceedings of the New Zealand Society of Animal Production, Volume 42, , 189-192, 1982