To study the effects of gonadotropin-releasing hormone (GnRH) and timing of
artificial insemination (Al) on fertility, 328 dairy cows were divided into four
groups. Inseminations at first, second, or third service were done soon after
detected estrus (0 hr) or 12 h later. One-half of the cows in each of the preceding
groups received GnRH (100µg) or saline within 30 sec after AI. Conception at
first service was not improved by GnRH. But conception rates at repeat services
were improved by 21% when cows received GnRH after AI. Time of AI (0 vs 12 hr)
had no effect on conception. Administering GnRH at repeat services should
improve conception rate of lactating dairy cows.
