Immunization of ewes to oestrogens: Effects on the oestrous cycle and parturition

Abstract

Four treatments, designed to increase plasma concentrations of LH, were assessed for efficacy at inducing oestrus and ovulation in anoestrous progestagen-pretreated Merino ewes that were 20–37 days post partum during the ovulatory season. The treatments involved the introduction of rams to ewes that had been isolated from rams, GnRH administration, and immunisation against androstenedione or oestrone. Ram introduction resulted in a higher (P<0.001) proportion of ewes showing oestrus and ovulating within 7 days (1619 ewes, flock R-+) than in ewes having continuous contact with rams (420 ewes, flock R++). Treatment with GnRH (125 ng/h s.c. for 7 days) inhibited the ram effect on the number of ewes showing oestrus, but not on the number of ewes ovulating. GnRH treatment, and immunisation against androstenedione or against oestrone did not induce oestrus or ovulation. Immunisation against oestrone inhibited the occurrence of ovulation in ewes in both flocks (R++, R-+). Pregnancy rates were lower (P<0.001) in ewes mated less than 40 days post partum than in ewes mated 41–70 days post partum (17.2% vs. 57.3%). The number of services/pregnancy were lower (P<0.05) in the R-+ flock than in the R++ flock (1.3±0.1 vs. 1.6±0.1), but pregnancy rates over a 32-day joining period were similar (63.7%, 59.7%). The study showed that ram introduction can be an effective method of advancing and synchronising oestrus and ovulation and of advancing the onset of pregnancy in progestagen-pretreated anoestrous post-partum ewes.