GONADOTROPIN-RELEASING-HORMONE ENHANCES THE CALVING RATE OF BEEF FEMALES ADMINISTERED NORGESTOMET AND ALFAPROSTOL FOR ESTRUS SYNCHRONIZATION

One hundred fifty beef heifers and 403 beef cows suckling calves were administered norgestomet implants (8 d) and alfaprostol, a PGF(2 alpha ) analogue, approximately 28 h before implant removal. Thirty hours af ter implant removal, females were administered either GnRH via injecti on, GnRH via implantation, or no GnRH. The dosage of GnRH was 250 mu g , and implants prolong the induced LH surge. Ovulation response, incid ence of short cycles, and calving rate were analyzed as a 2 x 2 x 3 co mpletely randomized factorial design with female (heifers and cows), e strous cycles (with or without), and GnRH as the main effects. There w ere no interactions (P >.10), and because heifers and cows had respons es that did not differ (P >.25), they were summarized together. Female s with estrous cycles had a higher (P <.05) ovulation response, fewer (P <.01) short luteal phases, and a higher (P <.01) calving rate than females without estrous cycles. Gonadotropin-releasing hormone treatme nt increased the ovulation response(P <.01) and the calving rate (P <. 05), and these responses were not affected (P >.10) by the method of G nRH administration. Based on these data, the increased ovulation respo nse to GnRH may account for 29% of the increase in calving rate observ ed in the GnRH-treated females. In summary, in norgestomet- and alfapr ostol-synchronized females, GnRH enhanced calving rate regardless of h ow it was administered. This increase was due to more than an increase d ovulation rate.