Safety and Pharmacologic Evaluations of Melengestrol Acetate in Cattle and Other Animals: A Review

SUMMARY Melengestrol acetate (mga), a progestational steroid with unique activity when fed to ruminants, inhibits estrus and ovulation at doses of 0.5 mg. daily. It is currently used to improve rate of gain and feed efficiency in feedlot heifers, and it has been used successfully for estrus synchronization. The capacity of cows to manifest estrus returns following the end of mga feeding, even after long periods of mga feeding. Breeding at estrus within about 10 days of mga feeding often results in some reduction in conception rate, whereas breeding at second estrus results in equal or higher conception rate when compared to contemporary herdmate controls. The feeding of mga after breeding resulted in significant lowering of conception rate when initiated on day 2, but not when initiated on day 4. Feeding mga during pregnancy and early lactation did not result in abnormalities in either cows or calves and did not alter calving time or difficulty. Calf birth weights were decreased, but weaning weights were not different. Subsequent reproductive performance of calves was not affected. When given 1.0 mg. daily during lactation, mga did not alter milk production or constituents. Feeding mga did not markedly decrease the ability of cows to combat uterine bacterial (Escherichia coli) challenge. Studies on safety and pharmacologic action of mga have also been conducted in rats, mice, rabbits, dogs, chickens, sheep, swine, horses, and human beings. The drug has been found to be an extremely safe compound, with unique activity in the ruminant.