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Reproductive Performance following Long-Term Treatment with Melengestrol Acetate^{1, 2,}

Texas Tech University, Lubbock

Abstract

In each of two trials, three of four groups of beef heifers received oral MGA at about 0.4 mg per head daily. Oral MGA was provided for short periods (21 days) or for long periods (63 or 84 days). The heifers used were under typical feedlot conditions for fattening. There were no differences in percentages of total group detected in estrus in 32 days (82 to 91% range). Groups fed MGA for 21 or 84 days were inseminated at each estrus beginning on the day of last feeding. Most inseminations in these MGA groups were at day 3 or greater after last MGA, but very few pregnancies resulted from inseminations made during the first 7 days of the breeding period. In the group fed MGA for 63 days and inseminated at each estrus beginning 21 days after last MGA, significantly more pregnancies (P<.05) occurred than in the other three groups. Therefore, it was concluded that there was no detrimental influence of long-term MGA on reproductive performance of feedlot heifers as measured by either ability to demonstrate estrus or to conceive. There was no evidence of either multiple ovulations or multiple fetuses.

¹ This investigation was supported in part by a grant of funds and drugs from The Upjohn Company, Kalamazoo, Michigan.

² Contribution No. 29, International Center for Arid and Semi-Arid Land studies.

³ Department of Animal Science, Texas Tech University.

⁴ Reproduction and Physiology, Agricultural Division, The Upjohn Company. ® Registered trademark, The Upjohn Company.