COMPARISON OF PREGNANCY RATES OF REPEAT-BREEDER DAIRY-COWS GIVEN GONADOTROPIN-RELEASING-HORMONE AT OR PRIOR TO THE TIME OF INSEMINATION

A total of 585 repeat-breeder dairy cows was used to study the effect of GnRH treatment, either at or prior to insemination, on the pregnanc y rate. The cows were divided into 6 treatment groups. Cows in Group 1 (n = 142 were observed in estrus, and 11 +/- 0.42 hours (mean +/- SEM later they were given 100 ug, i.m. gonadotropin releasing hormone (Gn RH) and were inseminated. Cows in Group 2 (n = 139 ) were observed in estrus and were inseminated 11.4 +/- 0.43 hours later. Cows in Group 3 (n = 33) were monitored for estrus with an activated heatmount detect or but were not observed in estrus; they were inseminated 1.5 +/- 0.87 hours later and were given 100 ug, i.m. GnRH. Cows in Group 4 (n = 35 ) were not observed in estrus, but they did activate the heatmount de tector and were inseminated 2.2 +/- 0.87 hours later. Cows in Group 5 (n = 107 ) were observed in estrus, given 100 ug, i.m. GnRH 2.0 +/- 0. 40 hours later, and were inseminated 9 +/- 0.60 hours after GnRH treat ment. Cows in Group 6 (n = 129 ) were observed in estrus and were inse minated 10 +/- 0.50 hours later. Pregnancy rates were analyzed by Chi- square. Interactions between pregnancy rate, treatment and time of ins emination were evaluated using ANOVA and LSM (P < 0.05 ). There was no effect on pregnancy rate when GnRH was given at or prior to inseminat ion. Cows inseminated on the basis of observed estrus had a higher pre gnancy rate (P < 0.05 ) than cows inseminated on the observation of an activated heatmount detector. From the results of this study, it is c oncluded that treatment with GnRH at or prior to insemination did not improve the pregnancy rate of repeat-breeder dairy cows.