Abstract
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 < .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 (mean = 60) h after ovulation to determine the ovum cleavage rate and the number of accessory sperm in the zona pellucida. The GnRH treatment increased the incidence of twin ovulations but did not increase the number of accessory sperm or improve the proportion of ova that cleaved in either first-service or repeat-breeding cows.
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