The Effect of Melengestrol Acetate (MGA1) On Ovarian Function and Fertilization in Beef Heifers2

Abstract

Beef heifers were treated with 0.5 or 1.0 mg MGA per head daily for 14 days starting on day 4 or 15 of an estrous cycle. Ovarian morphology, blood progesterone and LH concentrations, cervical mucus index, estrus, ovulation and fertilization after natural mating were compared with similar observations made on a group of control heifers. Both the stage of the estrous cycle at which treatment began and the dose of MGA influenced the cervical mucus index, synchronization of estrus and number of cleaved ova. Although the ovaries of heifers treated with MGA usually contained at least one follicle greater than 10 mm, fewer Graafian follicles (over 3 mm diameter) were observed. Significant amounts of plasma progesterone were found after regression of the corpus luteum in some treated heifers. Serum LH concentrations were also elevated in the heifers that received 0.5 mg MGA from day 15. MGA caused alterations in the occurrence of estrus and in the cervical mucus index. Some treated heifers did not ovulate, and a higher proportion of uncleaved ova were found in treated heifers than in control heifers.