Abstract
The objective of this study was to determine the effect of GnRH (100 mg i.m.) treatment 5 and 15
days after timed insemination (TAI) on pregnancy rate and pregnancy loss in lactating dairy cows
subjected to synchronization of ovulation. The study included 831 lactating dairy cows subjected to a
Presynch–Ovsynch protocol for first service. On the day of TAI (Day 0), cows were randomly
assigned to one of four experimental groups. Cows in Group 1 (n = 214) were treated with GnRH on
Day 5; cows in Group 2 (n = 209) were treated with GnRH on Day 15; cows in Group 3 (n = 212) were
treated with GnRH on both Day 5 and Day 15; cows in Group 4 (n = 196) were not treated. Pregnancy
rate was evaluated at Day 27 and Day 45 after TAI. The interestrus interval and the proportion of cows