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Plasma Ovarian Hormone Levels and Fertility in Beef Heifers Treated with Melengestrol Acetate (MGA¹)^,2

Clemson University³, Clemson, S. C. 29631

Abstract

Melengestrol acetate (MGA), a synthetic progestin, was fed to 25 beef heifers at a rate of 0.5 mg per animal per day for 14 days beginning on Day 15 of the estrous cycle. An untreated group of 25 heifers was included in the experiment. Estrus was synchronized in 56% of the treated heifers. Whereas the fertilization rate was 100% in the untreated group it was 50% in the treated group. Pregnancy rate was 67% in the untreated group compared to 40% in the treated group.

During the period of MGA-feeding the estrogen level remained below the level measured just prior to estrus. The levels during proestrus and after mating were similar to those obtained in the untreated group. There was a difference in the estrogen pattern during proestrus in heifers becoming pregnant compared with those that did not. A high level of plasma estrogen was confined to fewer days before estrus in the cows becoming pregnant.

Plasma progesterone fell to 1 ng/ml or less within 7 days after MGA treatment began in the heifers showing a synchronized estrus and within 9 days in heifers showing an unsynchronized estrus. In the untreated group the progesterone trough (the period from the day on which progesterone was less than 1 ng/ml until estrus) lasted 2 days. In the treated group the progesterone trough lasted 13 days. After mating there was no difference in levels between the two groups.

¹ MGA — Registered Trademark of the Upjohn Company for melengesterol acetate.

² Published with the approval of the Director of the South Carolina Agricultural Experiment Station as Technical Contribution No. 1055.

³ Departments of Food Science, Animal Science and Dairy Science.