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

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Abstract
Two experiments were designed to determine the effects of duration of lactation and plane of nutrition of the reproductive performance of Finn .times. Dorset Horn ewes mated at a hormonally [30 mg Cronolone, 400 mg progesterone] induced estrus 9 wk after lambing. In the first experiment, ewes were mated on Dec 4 or March 19 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 estrus, ovulation rate, litter size or the pattern of luteinizing hormone [LH] production. Indirect evidence based on progesterone profiles and returns to service suggests that reproductive failure was due to fertilization failure and embryonic death. In Exp. 2, ewes were mated on July 1 or Oct. 15. 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 of nutrition. Lactation depressed fertility in ewes mated in July but not in those mated in Oct. 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 fertilization failure and embryonic death. Seasonal and treatment effects on the incidence of estrus and on ovulation rates, litter sizes and LH production were small and of little importance.

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