Postpartum Mating Performance of Ewes Involved in a Twice-Yearly Lambing Program

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Abstract
A TOTAL of 182 ewes of Dorset, Dorset × Rambouillet and Ram bouillet breeding were utilized between 1964 and 1968 to observe mating and lambing performance on a twice-yearly schedule. One-third of the ewes of each breed group were bred first during the spring of 1964, one-third during the fall of 1964 and the last third during the spring of 1965. All ewes were bred first at about 12 months of age. The bi-yearly breeding seasons started April 20 and October 20 and each lasted for 60 days. All ewes were exposed to fertile rams during these periods except that ewes were kept from rams for 10 days following lambing. There were 537 ewe seasons recorded during the fall and 591 during the spring. Thirty-five percent of ewes that could have lambed during the fall had lambs as compared to 84% during the spring. Of 1,049 lambs born, 254 were born during the fall and 795 were born during the spring. Eliminating each ewe's first lambing season there were 898 lambs born which involved rebreeding. Of these 82% were born during the spring indicating the degree to which the ewes reverted to spring lambing under the schedule imposed. There were 1.86 lambs born per ewe year which included each ewe's first lambing at about 18 months of age. As a measure of the “extra” lambs resulting from mating at 6-month intervals, 326 (36%) of 898 lambs born excluding the first lambing season were conceived during a season that the dam lambed. Seventy-two percent of fall-born lambs were from ewes that had lambed the previous spring, whereas, only 28% of spring-born lambs were from ewes that lambed the previous fall. Of ewes that lambed during the fall, 71% conceived at an average interval of 44 days as compared to 23% conceiving after an average interval of 66 days for the spring-lambing ewes. Relatively more Dorset and Dorset × Rambouillet ewes lambed during the fall and relatively less Dorset ewes lambed during the spring. Lambing rate was highest for crossbred ewes but not significantly higher than for Dorsets. Rambouillet ewes produced the least multiple births but reared a higher percent of lambs born. Dorset ewes showed fewer matings and significantly less conceptions than the other breed groups during the fall. During the spring, however, Dorsets and crossbreds mated more readily and had a higher percent of ewes conceiving than did the Rambouillet ewes. The data permitted observations on the mating and conception of lambing ewes that reared or did not rear their lambs. More ewes that lost their lambs tended to show estrus. During the fall there was essentially no difference in the percent ewes conceiving as contrasted to the spring when ewes not rearing their lambs had a higher conception rate than ewes that reared lambs. Intervals from lambing to first estrus and to conception were similar during the fall but the lactating ewes had longer intervals to first estrus and conception drring the spring. Observations were also made on the mating and conception of open ewes during the two seasons. There were no significant associations between the mating performance of the dry ewes and their postpartum mating performance the previous season. Copyright © 1972. American Society of Animal Science . Copyright 1972 by American Society of Animal Science.

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