The effect of melengestrol acetate (MGA) on ovarian function and fertilization in beef heifers.

Abstract

Heifers were given .5 or 1 mg/day of melengestrol acetate (MGA) for 14 days to investigate the effect of dose and time of initial treatment on the following parameters: 1) ovarian morphology 2) luteinizing hormone (LH) and progesterone levels in blood 3) cervical mucus index 4) estrus and 5) ovulation and fertilization following mating. Average time for estrus from last MGA feeding was 7.26 days in 1 mg MGA heifers (p less than .01) and 4.75 days in .5 mg MGA-synchronized heifers. 47% of the heifers given MGA on Day 15 failed to become synchronized whereas only 20% of those given MGA on Day 4 failed. Major physiologica l changes occurred in ovaries as a result of MGA treatment; there were f ewer mature follicles and more very large follicles compared with controls. Cervical mucus index coincided with estrus in only 1/3 of the MGA-treated heifers. Serum LH levels were higher (p less than .05) in those heifers first treated on Day 15 rather than on Day 4 and in animals receiving .5 mg MGA. MGA-treated heifers had more uncleaved ova indicating a higher rate of fertilization failure. It is concluded that fertility is reduced in matings at the first synchronized estrus following MGA treatment.