Abstract

Mixed-age Coopworth ewes (280) were allocated to an experiment in which the effects of immunisation against androstenedione, synchronisation with progestagen sponges, and exposure to the ram effect, on oestrus and ovulation were examined. The observations were commenced at 2 different times (mid-January and early February) prior to the commencement of the normal breeding season and continued until all ewes had exhibited oestrus and ovulated. Immunisation against androstenedione increased the ovulation rate (+0.60) at the ram-induced ovulation. However it suppressed the proportion of ewes exhibiting oestrus after sponge withdrawal at both times of ram introduction. Ewes isolated from the ram failed to ovulate and the first spontaneous ovulation was not recorded until 6 weeks after that recorded for ewes exposed to the ram effect. Pre-treatment with sponges was necessary for the ram-induced ovulation to be accompanied by oestrus. The extent of the ram effect was most noticeable with 95% of ewes exposed to the ram ovulating in January.

GJ, Cruickshank, JF Smith, and DG Fraser

Proceedings of the New Zealand Society of Animal Production, Volume 48, , 77-80, 1988