Reproductive performance of lactating dairy cows treated with gonadotrophin-releasing hormone (GnRH) and/or prostaglandin F2a (PGF2a) for synchronization of estrus and ovulation

The objective of this study was to determine the reproductive performance o f lactating daily cows treated with GnRH and/or PGF2a for synchronization o f estrus and ovulation. Between Days 43 and 57 post partum, a total of 374 dairy cows was divided into 4 groups. Cows in Group 1 (n=62) were treated w ith 25 mg, im PGF2a on Days 43 and 57; cows in Group 2 (n=65) were not trea ted at this time; cows in Group 3 (n=118) were treated with 100 ug, im GnRH on Day 50, 25 mg, im PGF2a on Day 57, 100 ug, im GnRH on Day 59, and time- inseminated 16 h later; cows in Group 4 (n=129) were treated with 25 mg, im PGF2a once on Day 57. Cows in Groups 1 and 4 were inseminated at an induce d estrus within 7 d after the last PGF2a treatment, and cows in Group 2 wer e inseminated at a noninduced estrus within a corresponding period of time. Conception rate, estrus detection rate and pregnancy rate were analyzed us ing logistic regression, and controlled for lactation number, body conditio n score and time of year. Days from calving to conception were analyzed usi ng the GLM procedures of SAS, and the model included group, body condition score, lactation number, time of year, and their interactions. Cows in Grou p 3 had a significantly higher pregnancy rate than cows in Groups 1, 2 and 4. Orthogonal contrasts of mean days from calving to conception showed that cows in Group 3 had significantly (P<0.01) less days from calving to conce ption than cows in Group 1 and Group 4. There was a significant effect of t ime of year on pregnancy rate and days from calving to conception, but ther e was no interaction between time of year and these reproductive characteri stics. There was no effect of body condition score and lactation number on the reproductive characteristics evaluated. From the results of this study, it was concluded that better reproductive performance was observed in cows inseminated at a synchronized ovulation than in those inseminated at a syn chronized estrous period. (C) 1998 by Elsevier Science inc.