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Determination of the Minimal Effective Dose of 6a-Methyl-17a-Acetoxyprogesterone for Control of the Estrual Cycle of Cattle

Veterinary Division, The Upjohn Company, Kalamazoo, Michigan

Abstract

The minimal effective dose of MAP for inhibition of ovulation was studied in 74 dairy and 96 beef heifers. Individual twice-daily feeding of dairy heifers beginning on the 15th day of the estrual cycle indicated that doses above 135 mg. daily were effective. Lower doses were partially or completely ineffective. Group feeding of doses of 150, 180, 210, and 400 mg. daily to dairy heifers was accompanied by complete inhibition of estrus and ovulation during treatment. In beef heifers group feeding of doses of 120 to 180 mg. daily inhibited and synchronized ovulation in 90 (94%) of 96 heifers. Of the dairy and beef heifers which were observed in estrus, 93% were in estrus on the second, third, or fourth days after last feeding.

Post-partum beef cows were synchronized by initiation of treatment prior to the occurrenceof the first post-partum ovulation. Treatment caused a significant reduction in the variability, but not in the average interval from calving to first post-treatment ovulation.

The average conception rate of all treated animals bred within 7 days of last feeding was 51% but varied markedly from trial to trial. The conception rates of treated animals at second service and of untreated animals were 76% and 74%, respectively. No apparent effect of MAP treatment was noted on the average gestation length or birth weight of the calf. Discussion concerning the mode of action suggests that inhibition of ovulating hormone release occurs but that follicular development is allowed to proceed.

¹ The author wishes to acknowledge the assistance of J. I. Northam in statistical evaluation of the data.

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