INFLUENCE OF AN AGONIST OF GONADOTROPIN-RELEASING-HORMONE (BUSERELIN) ON ESTRUS SYNCHRONIZATION AND FERTILITY IN BEEF-COWS

The objective of this experiment was to determine the effect of sequential treatment with buserelin (a GnRH agonist) and cloprostenol (a prostaglandin F2-alpha analog) on estrous response and fertility in beef cattle with different ovarian conditions. On d 0 (1st d of treatment), the control group (n = 52, 10 heifers and 42 cows) and the GnRH group (n = 48, 10 heifers and 38 cows) received 2 mL of saline or 2 mL of Receptal (8-mu-g of buserelin), respectively. On d 6, all cows that had not exhibited spontaneous estrus were given i.m. 500-mu-g of cloprostenol (PGF). Ultrasonography on d 0 and assays of progesterone in blood on d -11, 0, and 6 were used to identify follicular and luteal status of animals. Cattle were observed for estrus from d 0 to 10. Cows showing estrus were bred artificially 12 h after onset of estrus. Over the 10-d period, the number of cows detected in estrus and pregnancy and conception rates were identical for the two groups. However, between d 0 and 6, the proportion of cows exhibiting estrus was lower (P < .01) in the GnRH group than in the control group. Between d 6 and 10, the synchronization rate and precision of estrus were greater (P < .01) in the buserelin-treated group than in the control group. Conception rate and interval from PGF injection to onset of estrus were not different between the two treatment groups. Presence of a large (> 10 mm) follicle on d 0 enhanced synchronization rate and precision of estrus. The estrous response of cyclic cows bearing a large follicle on the ovaries was greater than that of cows without such a structure on the day of buserelin injection. These results suggest that pretreatment with buserelin, 6 d before the administration of PGF, eliminated the need for estrus detection during d 0 to 6 without decreasing pregnancy and conception rates.