Does litter size affect the response of postpartum nursing ewes to hormonal estrous induction during spring?

Abstract

The objective of this study was to determine if the litter size of nursing ewes affects the response to hormonal treatments to induce estrus and ovulation during the non-breeding season. For this, 124 Corriedale ewes were used, 56 ewes that had lambed during spring and remaining nursing a single lamb (GS, n = 30) or twin lambs (GT, n = 26). Another group of ewes that did not lamb was included as a control group (GC, n = 68). Estrus was induced 38 days after lambing with intravaginal sponges impregned with medroxyprogesterone acetate, that remained 7 days in situ, plus the administration of 350 IU of eCG at withdrawal. Once detected in estrus, the ewes were cervically inseminated, and 8 and 40 days later the presence of corpus luteum (CL) and pregnancy was determined by transrectal ultrasonography respectively. The percentage of ewes that came into estrus did not differ between groups. More GC than GT ewes had CL (P = 0.02), with GS not differing from the other groups. The conception and pregnancy rates were greater in GC than in GS and GT ewes (P ≤ 0.03). The litter size had no effect on the response to estrous induction; however, nursing impaired the pregnancy outcome, probably due to the failure in the ovulatory process or the early regression of the CL.