MEASURES OF ESTRUS DETECTION AND PREGNANCY IN DAIRY-COWS AFTER ADMINISTRATION OF GONADOTROPIN-RELEASING-HORMONE WITHIN AN ESTRUS SYNCHRONIZATION PROGRAM BASED ON PROSTAGLANDIN-F2-ALPHA

The objective of this study was to assess the use of GnRH in a control led breeding program (7 d prior to the second of two injections of PGF (2 alpha), 14 d apart) as a means to improve the expression and detect ion of estrus, first service conception rate, and overall pregnancy ra te of lactating dairy cows. On 17 farms, 348 cows were assigned random ly to either of two breeding programs prior to first insemination. Cow s in both programs received PGF(2 alpha) approximately 2 wk prior to t he end of a herd-specific voluntary waiting period for breeding. One g roup received GnRH 1 wk later, 7 d prior to the second of two injectio ns of PGF(2 alpha). Control cows received saline and a second injectio n of PGF(2 alpha) at corresponding times. Cows were observed for 7 d a nd were bred by artificial insemination following detection of estrus. There were no differences between programs in estrus detection rate, observed signs of estrus, conception rate, days to first service, or i nterval from calving to conception. The means and standard deviations of the interval from PGF(2 alpha) to estrus were not different between programs. Administration of GnRH 1 wk prior to PGF(2 alpha) did not a lter the expression of estrus or fertility in lactating dairy cows. In this study population, no advantage was found for the addition of GnR H to a controlled breeding program that was based on two administratio ns of PGF(2 alpha) at a 14-d interval.