Estrual Activity in Lactating Ewes

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Abstract
Fall-lambing ewes were checked for estrus at approximately 12-hr. intervals from time of parturition to the beginning of the breeding season in each of 2 years. The average intervals from lambing to establishment of regular cyclic estrual behavior in the two years were 73.0 and 86.1 days for grade Rambouillets and 38.8 and 66.0 days for purebred Dorsets. Average intervals in 1 year for eight Dorset × Rambouillet and nine Columbia × Rambouillet ewes were 53.4 and 112.0 days, respectively. Two Hampshire × Rambouillet ewes were not observed in estrus by the beginning of the breeding season in May 1959. Approximately 90% of 244 grade Rambouillet ewes were cycling within 120 days after parturition in the 1957 lambing season, as compared to 82% of 262 ewes that lambed in the 1958 season. Approximately 5% of these ewes in 1957 and 9% in 1958 had not exhibited estrus within 150 days after lambing. The correlation coefficient between the day of lambing within the lambing season and the interval to first estrus was highly significant in both years. Age within years was not a significant cause of variation in length of interval from lambing to beginning of estrual activity. The average intervals from lambing to first estrus for non-suckled ewes in the 1957 and 1958 lambing seasons were 57.7 and 54.9 days, respectively. Corresponding figures in each year for ewes that nursed a single lamb were 72.1 and 88.4 days, respectively. Ewes that nursed twin lambs averaged 86.6 days in 1957 and 88.1 days in 1958 before estrual activity occurred after lambing. There was no interruption in cyclic estrual activity from first estrus after lambing to the beginning of the breeding season in mid-May in approximately 39% of Rambouillet ewes that lambed in 1957–58. Similarly, 40.5% of the lambing flock continued regular cyclic estrual activity to the beginning of the breeding season in 1958–59. The average length of the spring anestrus in 1957–58 was approximately 71 days. Ewes in which anestrus began late in the spring season returned to estrus in a shorter period of time than ewes in which anestrus began earlier. Anestrus terminated in most Rambouillet ewes by late May or early June. Sixty-seven Rambouillet ewes were bred to ram lambs in the spring of 1958. All these ewes had lambed between August 15, 1957, and January 28, 1958. The average interval from lambing to first estrus was 67.1 days. The average number of estrual periods prior to conception was 5 (range 2 to 13). Conception occurred an average of 116 days from lambing (range 27 to 276 days). The average length of estrus for fall-lambing ewes was 2.15 checks. The monthly differences in length of estrus were not significant in either year.