THE EFFECT OF CONSTANT DELIVERY OF GONADOTROPIN-RELEASING-HORMONE ON FERTILITY OF CATTLE

Four hundred and three (403) postpartum beef cows, synchronized with t he norgestomet and estradiol valerate bovine estrus synchronization pr ocedure, were included in three experiments. In the first experiment, 78 of the 178 cows were administered biodegradable microcapsules conta ining 180 mu g GnRH manufactured to biodegrade (and release GnRH relat ively constantly) in approximately three days. In the second experimen t, 38 of the 90 cows were implanted with osmotic pumps designed to del iver 2.5 mu g of GnRH per hour for six days. The third experiment cons isted of two trials. In the first trial, 21 of the 41 cows were implan ted with biodegradable implants manufactured to delivery 250 mu g of G nRH (relatively constantly) over a four day period. In the second tria l, 46 of the 94 cows were implanted with same implant as in trial 1 ex cept the implants were coated so that there was a slight delay in init iating delivery after implantation. The constant delivery of GnRH duri ng the proestrus period reduced pregnancy rates when GnRH was administ ered via osmotic pumps, microcapsules, and implants. Constant delivery implants with a delay in release after implantation, however, had no effect on pregnancy rates. Tn summary, constant delivery of GnRH, via all of the delivery systems, was not considered a valid method of enha ncing fertility in proestrus cattle.