Effects of season, lactation and plane of nutrition on the reproductive performance and associated plasma LH and progesterone profiles in hormonally treated ewes.

Two experiments were designed to determine the effects of duration of lactation and plane of nutrition on the reproductive performance of Finn x Dorset Horn ewes mated at a hormonally induced oestrus 9 weeks after lambing. In Exp. 1, ewes were mated on 4 December or 19 March and lactation was terminated at 35 or 15 days before mating or 7 or 35 days after mating. The fertility of ewes lactating at the time of mating was reduced, particularly in ewes mated in March. This was not attributable to differences between lactating and non-lactating ewes for the incidence of oestrus, ovulation rate, litter size or the pattern of LH production. Indirect evidence based on progesterone profiles and returns to service suggest that reproductive failure was due to failure of fertilization and to embryonic death. In Exp. 2 ewes were mated on 1 July or 15 October. Lactation was terminated 15 days before or 7 days after mating and the ewes in each group were subjected to a high or low plane nutrition. Lactation depressed fertility in ewes mated in July but not in those mated in October. While the fertility of non-lactating ewes was independent of nutritional treatment, lactating ewes on a low plane and mated in July had a higher lambing rate than those on a high plane of nutrition. Nutritional effects on fertility appeared to be mediated through changes in milk production and reproductive failures were attributed to failure of fertilization and embryonic death. Seasonal and treatment effects on the incidence of oestrus and on ovulation rates, litter sizes and LH production were small and of little importance.