The Effect of Constant Delivery of Gonadotropin Releasing Hormone on Fertility of Cattle

Abstract

Four hundred and three (403) postpartum beef cows, synchronized with the norgestomet and estradiol valerate bovine estrus synchronization procedure, were included in three experiments. In the first experiment, 78 of the 178 cows were administered biodegradable microcapsules containing 180 μg GnRH manufactured to biodegrade (and release GnRH relatively constantly) in approximately three days. In the second experiment, 38 of the 90 cows were implanted with osmotic pumps designed to deliver 2.5 μg of GnRH per hour for six days. The third experiment consisted of two trials. In the first trial, 21 of the 41 cows were implanted with biodegradable implants manufactured to delivery 250 μg of GnRH (relatively constantly) over a four day period. In the second trial, 46 of the 94 cows were implanted with same implant as in trial 1 except the implants were coated so that there was a slight delay in initiating delivery after implantation. The constant delivery of GnRH during the proestrus period reduced pregnancy rates when GnRH was administered via osmotic pumps, microcapsules, and implants. Constant delivery implants with a delay in release after implantation, however, had no effect on pregnancy rates. In summary, constant delivery of GnRH, via all of the delivery systems, was not considered a valid method of enhancing fertility in proestrus cattle.