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Effects of Date of Breeding and an Orally Administered Synthetic Progestogen on Reproductive Performance of Crossbred Ewes¹

University of Kentucky, Lexington

Abstract

Two hundred and ten Northwestern dark-faced crossbred ewes were used in a study to determine the effects of an oral progestogen (MAP) early in the breeding season on estrous synchronization, conception rate, and lambing performance. MAP was fed for 14 days before August 1, 14, and 28 breeding dates. Feeding MAP for 14 days prior to turning in rams effectively synchronized estrus, but reduced the average days to conception only at the first (August 1) breeding date. Lambing rate and lambing percentage both tended to be higher for the MAP-treated ewes in the August 1 breeding group, but they were lower in the August 14 and August 28 breeding groups than those of control ewes. Over-all lambing rate for the treated ewes was 1.73 with 92.3% of the ewes lambing after a 51-day breeding period. For control ewes the lambing rate was 1.82 with 89.5% of the ewes lambing. Lambing rate and lambing percentage were not significantly affected by treatment and both increased at successive breeding dates. At the August 1 breeding date, lambing rate was 1.71 and lambing percent was 145 compared with 1.81 and 173, respectively, at the August 28 breeding date.

¹ The investigation reported in this paper (67-5-73) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the director.

² Repromix containing 6.82 gm. of 6-methyl-17-acetoxyprogesterone per kg. was supplied through the courtesy of the Upjohn Company, Kalamazoo, Michigan.