The induction of oestrus and ovulation in seasonally anovular ewes by exposure to rams

Abstract

In anoestrus ewes of many breeds, the introduction of rams will evoke an increase in the tonic secretion of LH, pre-ovulatory surges of LH and FSH, and ovulation, provided the ewes have been preconditioned by a period of complete isolation from the rams. The “ram effect” is apparently mediated by a pheromone which seems to inhibit or reverse the effects of photoperiod on the secretion of LH pulses. In the absence of progesterone “priming” the ewes rarely show oestrus behaviour around the time of the first ovulation. In some of the ewes, the corpus luteum formed at the first ovulation is apparently normal, and the second ovulation and first oestrus is observed about one oestrus cycle later. In the remainder of the ewes, the first corpus luteum regresses prematurely, a second ovulation is seen about 6 days after the first, and the third ovulation and first oestrus are seen after a further 17 days. Progesterone treatment prior to the introduction of rams prevents the occurrence of short cycles, apparently by delaying the LH surge. The combination of the ram effect and progesterone treatments can provide useful synchrony of oestrus for artificial insemination, and also some gains in fecundity. There is considerable potential for using the effect in intensive breeding programmes.