Breeding Performance in Ewes after Synchronizing Estrus by Feeding 6-Methyl-17-Acetoxyprogesterone

Abstract

Thirty-five dark-faced, crossbred Western ewes were fed 50 mg. of 6-methyl-17-acetoxyprogesterone (MAP) daily for 14 days. Two ewes came into estrus on the fifth day of treatment, and there was evidence that another ewe had ovulated during treatment without showing estrus. Twenty-six ewes (74.3%) came into estrus within 4 days and 29 ewes (82.8%) within 8 days post-treatment (av., 3.14 days). Ovulation rate in 10 ewes slaughtered for fertility data was 1.27 and 70% of the ova were cleaved. Of 19 ewes retained for lambing data, 9 returned to estrus in 15 to 17 days (av., 16.4 days) and 8 (42.1%) lambed to breeding at the first post-treatment estrus. Forty ewes were used in a factorial design in which four groups of 10 ewes each received 60 or 90 mg. of MAP for 15 or 18 days. None of these ewes came into estrus during treatment. Thirty-eight (95.0%) of these ewes came in heat 2 to 5 days (av., 3.2 days) post-treatment. The ovulation rate for 19 ewes slaughtered was 1.42, and 63% of the ova were cleaved. Sixteen of 19 ewes (84.2%) retained for lambing data lambed to breeding at the first post-treatment estrus. In the two trials 89.3% of the ewes returned to heat within 8 days (av., 3.16 days) after the end of treatment. Sixty-three percent lambed after breeding at the first post-treatment estrus, and 84.2% after breeding at the second post-treatment estrus. The variance in onset of post-treatment estrus was significantly lower than that for pre-treatment estrus (1.53 vs. 20.22). In Trial II the interval from end of treatment to estrus was significantly longer for the 90-mg. group (2.7 vs. 3.6 days) and for the 18-day group (2.9 vs. 3.6 days).