IMPROVED SYNCHRONY, CONCEPTION RATE, AND FECUNDITY IN POSTPARTUM SUCKLED BEEF-COWS FED MELENGESTROL ACETATE PRIOR TO PROSTAGLANDIN-F2-ALPHA

Occurrence of estrus, conception rate, and fecundity were compared bet ween postpartum suckled beef cows fed or not fed melengestrol acetate (MGA) before synchronizing estrus with prostaglandin F-2 alpha (PG). O ne hundred thirty-three purebred Angus cows were stratified by age and days postpartum to one of two treatments. All cows received an inject ion containing 25 mg of PG; however, one group (MGA + PG) was fed .5 m g of MGA . cow(-1). d(-1) for 14 d then received PG 17 d after the las t feeding of MGA. The other group (PG) received only PG. Cows in both treatments that failed to exhibit estrus within 6 d after PG received a second injection of PG 11 d after the first injection was administer ed. Cows were checked for estrus from the first PG injection until 6 d after the second PG injection. Cows were inseminated with semen from a 3/4 Brahman bull 12 h after observed estrus. Cows were exposed to An gus bulls after the AI period. Conception date was determined by palpa tion per rectum and verified by calving date and calf phenotype. Diffe rences in response variables were analyzed by chi-square. A greater pr oportion of cows (P < .04) that were pretreated with MGA exhibited est rus after the first injection of PG (76%) than of cows that received o nly PG (60%). Total response did not differ between treatments after b oth injections of PG. Conception rate among cows that received a first service was higher (P < .007) for the MGA + PG (88%) group than for t he PG (67%) group, although the total number of cows that received a f irst service was not different between groups. Twinning rate was signi ficantly higher (P < .05) in cows that received MGA (15% vs 0%). Pregn ancy rate was higher (P < .01) in cows fed MGA (82%) than in cows that were not fed MGA (63%). Feeding MGA before PG resulted in more cows e xhibiting estrus after the first PG injection and increased conception and calving rate among cows in estrus within 6 d after PG.