Effect of gonadotropin-releasing hormone on estrus, ovulation, and ovum cleavage rates of dairy cows

In a 2 X 2 factorial experiment, 110 Holstein cows (55 first-breeding and 55 repeat-breeding), free of genital abnormalities, were injected i.m. at the time of insemination with either saline solution or 100 micrograms of GnRH. Blood samples were drawn from the tail vein or artery of 22 cows (10 first- and 12 repeat-breeding) immediately before GnRH injection and again 1 and 2 h later to determine whether the GnRH induced the release of LH. The GnRH caused LH release in 18 of 22 cows, with a greater (P 0.05) mean concentration of LH than that in saline-treated cows at 1 h (2.3 vs 7.0 ng/mL) and 2 h (2.5 vs 6.0 ng/mL) after injection. Length of estrus and time of ovulation were calculated from estrus checks and ovarian palpations per rectum at 8-h intervals. The GnRH injections produced no change in duration of estrus (19.2 h) or time of ovulation postestrus (9.5 h). The cows were slaughtered 41 to 90 (average