Resynchrony of postpartum dairy cows previously treated for anestrus.

Cows that are diagnosed as anestrus (defined as >3 weeks postpartum, not detected in estrus by 1 week before commencement of seasonal breeding and without a palpable corpus luteum) have a lower probability of being inseminated, a lower conception rate to insemination and hence a lower probability of pregnancy within the breeding period. A current progesterone (P) and estradiol benzoate (EB) treatment for anestrus results in an 85% insemination rate, but only 57% of cows not conceiving to that first insemination are subsequently detected in estrus 14-28 days later. Resynchrony after an initial anestrous treatment has been used to increase the probability that cows not conceiving to first insemination will subsequently be detected in estrus. Anestrous cows (n=971) were initially treated with an intravaginal P-releasing device for 6 days and given 1 mg EB 1 day after device removal (day of EB treatment=Day 0). Cows detected in estrus between Days 0 and 3 were assigned randomly to be treated with reinsertion of a used P-releasing device for 8 days (commencing on Day 15), with 0.5 mg EB at reinsertion and again 1 day after removal (EB-RS), treatment as for EB-RS group but with substitution of 250 microg GnRH for EB at device reinsertion (GnRH-RS), or left as untreated controls (no-RS). Resynchrony treatments increased the proportion of non-pregnant cows detected in estrus from Days 14 to 28 compared to no-RS (79.1, 69.8, and 55.1% for EB-RS, GnRH-RS, and no-RS, respectively; P<0.05). Fewer cows were pregnant by Days 28 and 56 following GnRH-RS than EB-RS and no-RS. The final pregnancy rate was higher following EB-RS treatment than no-RS or GnRH-RS treatment (95.0, 88.3, and 88.6%; P<0.05). In conclusion, EB-RS enhanced reproductive performance of anestrous cows compared to no-RS.