The aim of this study was to evaluate five methods for the resynchronization of estrus in lactating dairy cows. One hundred and seventy-three Holstein cows were assigned at random to five treatments: Ovsynch-24, IPD + Ovsynch-24, Ovsynch-31, Heatsynch, and IPD + Heatsynch. The start of the resynchronization protocols, and all subsequent procedures, was timed in relation to the pre-enrolment AI (PAI) and ranged over 7 days for each treatment. The pregnancy status of all the animals was evaluated by ultrasound examination on day 31±3 PAI. In all treatments, the resynchronized AI (RAI) was at an observed estrus or at a fixed time after the completion of the resynchronization procedures. Cowswere observed for estrus thrice daily and those observed in estrus were inseminated according to the AM–PM rule. Those cows diagnosed as not pregnant and not observed in estrus were inseminated at a fixed time. The average proportion of cows with an active CL was 31.2% (54/173) on day 14 PAI, 22.0% (38/173) on day 21 PAI, and 18.5% (32/173) on day 24 PAI (NS). The incidence of luteolysis averaged 12.7% (22/173) from day 21 to 24, and 28.9% (50/173) from day 14 to 24. The differences between individual treatments were not significant but there was a significantly greater incidence of luteolysis (P < 0.0009) from day 14 to 21 for the IPD treatments combined (30.3%; 24/63) than for the non-IPD treatments combined (26.6%; 17/110). Conception rates at day 31 PAI ranged from 16.6% to 42.1% (NS), and at day 61 PAI ranged from 16.6% to 37.1% (NS). Also conception rates at day 31 and 61 RAI ranged from 5% to 25% (NS). Pregnancy losses between days 31 and 61 PAI varied from 0 to 25.0% (NS) between treatments for the pre-enrolment AI but were 0% for all treatments for the resynchronized AI. It was concluded that using IPD, PGF2, estradiol and GnRH in different resynchronization methods had no effects on conception rate in this study. Also conception rate was not significantly different when resynchronization programs began before ultrasonographic examination for pregnancy diagnosis. Using IPD did not reduce luteolysis during resynchronization programs and estrus detection rates were similar among treatment groups.

Keywords