Effects of seasonal variation of live weight on the breeding activity of Merino ewes

Adult Merino ewes were run continuously in contact with vasectomized rams and maintained at about condition score 2 and 40 kg live weight for 18 months, during which time they showed a clear seasonal pattern of ovarian (ovulation and ovulation rate) and oestrous activity. This pattern was asymmetrical and was composed of a non-breeding season (<70% of ewes cyclic over the period April-May to January-February) into which the ewes drifted slowly to reach minima in mid-December, and a spontaneous breeding season (>70% of ewes cyclic, over the period January-February to April-May), which began rapidly and was characterised by uniform ovulation rate. Two other flocks were run near these ewes, one made to follow the normal annual rhythm in live weight for sheep run at pasture in a Mediterranean-type climate (maximum in spring and minimum in late autumn and early winter) and the other made to follow the reverse of this rhythm. The data from these animals showed that current live weight flux, whether positive or negative, was not associated with the incidence of ovulation or oestrus. However, the annual patterns in live weight were related to the annual patterns in both ovulation and oestrus. The rhythms were apparently 3-6 months out of phase, with the live weight changes preceding those of ovulation and oestrus. By contrast, the ovulation rate in ewes that ovulated was closely associated with the current live weight or plane of nutrition of the ewes, but occasionally it did not appear to respond to current nutritional stimuli. These may have been due to other, more dominant, seasonal influences or a refractory period associated with a previous, prolonged period of sub-maintenance nutrition. After a period of isolation from rams in spring, 60-90% of seasonally anovulatory ewes responded to the reintroduction of rams by ovulating, with a high ovulation rate, within three days. Subsequently, a proportion of the ewes that had been induced to ovulate returned to anoestrus and/or anovulation before the start of the new spontaneous breeding season. Less ewes did so in the group that were on the high plane of nutrition during the preceding autumn-winter (April-June) than in the group maintained at condition score 2 throughout. From these data, we have developed the following hypotheses; (i) the combination of the spontaneous breeding season with the ability of seasonally anovulatory ewes to respond to introduction of rams accounts for the high incidence and uniformity of ovarian and oestrous activity previously reported throughout the commercial breeding season (October-March) of Merino ewes in south Western Australia, (ii) the "ram effect" in spring accounts for the artificially high mean ovulation rates of Merino flocks mated between October and January, while the low ovulation rate of flocks joined after January (their spontaneous breeding season) is probably accounted for by low live weights, (iii) during the ram-induced breeding season (October-January), ewes which failed to conceive and then re-entered anoestrus or anovulation rather than return to service, could account for a large proportion of the ewes that mate but fail to lamb in flocks joined during this period.