Fertility following fixed-time AI or insemination at observed estrus in Ovsynch and Heatsynch programs in lactating dairy cows

The objective of this study was to compare the conception rate for fixed-timed artificial insemination (FTAI) and observed heat artificial insemination (HAI) prior to the scheduled FTAI in Ovsynch and Heatsynch synchronization protocols. In Experiment 1, lactating dairy cows (n = 535) received two set-up injections of 25 mg prostaglandin F-2 alpha (PGF(2 alpha)) i.m., 14 days apart starting at 36 3 days in milk (DIM). Cows were blocked by parity and were randomly allocated to either Ovsynch or Heatsynch groups. All cows received 100 mu g of GnRH i.m. 14 days after the second set-up injection of PGF(2 alpha), followed by a third injection of 25 mg PGF(2 alpha) i.m., 7 days later. In the Ovsynch group, HAI cows (n = 29) were bred on standing estrus after the third PGF(2 alpha) before the scheduled second GnRH, whereas FTAI cows (n = 218) that were not observed in estrus, received a second injection of 100 mu g of GnRH i.m., 48 It after the third PGF(2 alpha) and received TAI 8 h after the second GnRH. In the Heatsynch group, all cows (n = 288) received 0.5 mg of estradiol cypionate (ECP) 24 It after third PGF(2 alpha) and HAI cows (n = 172) were bred on standing estrus and FTAI cows (n = 116) that were not observed in estrus, received TAI 72 It after the third PGF(2 alpha). In Experiment 2, repeat breeder cows (n = 186) were randomly assigned to either Ovsynch or Heatsynch groups. The FTAI and HAI cows were inseminated similar to Experiment 1. All cows were observed for estrus three times daily. The associations with the conception rate were modeled with logistic regression separately for Experiments 1 and 2. Of all the variables included in the model in Experiment 1, type of AI (HAI versus FTAI, P = 0.0003) and parity (primiparous versus multiparous, P = 0.05) influenced the first service conception rate. Over-all conception rate and first service conception rate for HAI cows were higher compared to FTAI cows (33.8% versus 21.3%, and 35.3% versus 21.0%; P = 0.001). In the Heatsynch group, cows that received HAI had significantly higher over-all conception rate and first service conception rate compared to FTAI (35.2% versus 17.3% and 36.0% versus 15.5%; P = 0.0001). The conception rates in repeat breeder cows for HAI and FTAI (30.1% versus 22.3%) were not different (P > 0.1). In conclusion, it was recommended to include AI at observed estrus and fixed-time AI for cows not observed in estrus in order to improve the conception rate in synchronization protocols. (C) 2004 Elsevier Inc. All rights reserved.