Some Factors Affecting the Postpartum Interval in Sheep

The effect of genotype on the occurrence of ovulation and estrus during the interval from lambing to ovulation accompanied by estrus (postpartum interval) was measured in White-Face and Black-Face ewes that lambed during the breeding season in 1986 and 1987. A group of St. Croix ewes was added to the study in 1987. The influence of lactation on the events of the postpartum interval was measured by comparing intact and mastectomized White-Face and Black-Face ewes. The effect of prolactin on postpartum interval was measured in these two genotypes and in a group of bromocriptine-treated Black-Face ewes that lambed in 1986. 1. The interval between parturition and first ovulation was not influenced by genotype or lactation but differed between years (P\u3c.05). 2. The number of ovarian cycles (estimated by progesterone levels) and the interval between parturition and ovulation accompanied by estrus was not different among treatment groups, but the bromocriptine-treated ewes had a shorter postpartum interval (P\u3c.05). 3. The interval from lambing to conception, conception rate and number of estrous cycles to conception (not measured in St. Croix ewes) did not differ among treatments or years. The results of this experiment suggest that the events of the postpartum interval and time from lambing to conception are not influenced by genotype or lactation and that the earlier occurrence of estrus does not reduce the time of conception. The results of hormone profiles suggest that the low LH levels present during early postpartum (around day 10) are sufficient to induce ovulation and estrus. Prolactin secretion on the day of lambing is independent of the presence of the mammary gland and decreases within a week in nonlactating ewes. Concentration of prolactin apparently does not contribute to the length of the postpartum interval