diagnosed not pregnant based on expression of estrus and insemination before pregnancy diagnosis
on Day 27 were determined. The results of this study are: (1) GnRH treatment on Day 5 or Day 15 did
not increase pregnancy rate, or reduce pregnancy loss between Day 27 and Day 55 after TAI; (2) cows
treated with GnRH on both Day 5 and Day 15 had a lower (P < 0.01) proportion of cows diagnosed
not pregnant based on expression of estrus before ultrasonography on Day 27 (26.5%) compared to
control cows (52.9%), and these cows had an extended (P = 0.05) interestrus interval (23.4 days vs.
21.5 days); and (3) GnRH treatment on both Day 5 and Day 15 after TAI reduced pregnancy rate on
Day 27 (36.8% vs. 44.4% for control cows; P < 0.03) and Day 55 (28.3% vs. 36.2% for control cows;
P < 0.01). Therefore, strategies to stimulate CL function using multiple doses of GnRH during the
luteal phase need to consider potential negative effects.
# 2004 Elsevier Inc. All rights reserved.
Keywords: Dairy cows; Timed insemination; Pregnancy loss; GnRH