Measures of Estrus Detection and Pregnancy in Dairy Cows After Administration of Gonadotropin-Releasing Hormone Within an Estrus Synchronization Program Based on Prostaglandin F₂

¹ Department of Population Medicine, Ontario Veterinary College, University of Guelph, ON, Canada N1G 2W1
² Grenville-Dundas Veterinary Clinic, Kemptville, ON K0G 1J0
³ Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, PEI, Canada C1A 4P3

The objective of this study was to assess the use of GnRH in a controlled breeding program (7 d prior to the second of two injections of PGF₂, 14 d apart) as a means to improve the expression and detection of estrus, first service conception rate, and overall pregnancy rate of lactating dairy cows. On 17 farms, 348 cows were assigned randomly to either of two breeding programs prior to first insemination. Cows in both programs received PGF₂ approximately 2 wk prior to the end of a herd-specific voluntary waiting period for breeding. One group received GnRH 1 wk later, 7 d prior to the second of two injections of PGF₂. Control cows received saline and a second injection of PGF₂ at corresponding times. Cows were observed for 7 d and 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 interval from calving to conception. The means and standard deviations of the interval from PGF₂ to estrus were not different between programs. Administration of GnRH 1 wk prior to PGF₂ did not alter the expression of estrus or fertility in lactating dairy cows. In this study population, no advantage was found for the addition of GnRH to a controlled breeding program that was based on two administrations of PGF_2, at a 14-d interval.

Key Words: gonadotropin-releasing hormone • prostaglandin F₂ • estrus synchronization • controlled breeding

Submitted on February 6, 1997
Accepted on September 12, 1997