RESUMO
Our objective was to determine the optimal time to artificially inseminate lactating beef cows (Bos taurus typicus) after using the standard CO-Synch protocol that also included a progesterone-releasing, intravaginal controlled internal drug release (CIDR) insert. Cows (N=605) at three locations were inseminated at four different times after CIDR insert removal and the prostaglandin F(2alpha) administration of the CO-Synch+CIDR protocol: 48, 56, 64, or 72h. Blood samples were collected 9 to 10 d before and on the day of CIDR insertion. Based on elevated (>1 ng/mL) serum progesterone concentrations, 60% of 605 cows had previously ovulated (were cycling) at the initiation of the study, with a range of 39.6% to 67.9% among locations (P<0.05). Age of cow, body condition score, and days postpartum affected (P< or =0.05) cycling status before ovulation was synchronized. Averaged across treatments, pregnancy rate to artificial insemination (AI) at Day 32 was affected (P< or =0.05) by pretreatment cycling status and body condition. Younger cows (< or =3 yr) tended to have greater AI pregnancy rates when inseminated at 56h, whereas older cows had similar pregnancy rates when inseminated at 56h or later (timing of AI by age interaction; P=0.085). Pregnancy loss between Days 32 and 63 was greatest (quadratic effect; P<0.05) when cows were inseminated at 48 and 72h. In summary, insemination times at or after 56h improved AI pregnancy rates when using the CO-Synch+CIDR program. Further work is warranted to examine age effects on timing of AI in the CO-Synch+CIDR program.