Comparison of timed insemination versus modified estrus detection protocol in beef heifers

Our objectives were to evaluate a fixed-time AI (FTAI) protocol versus a modified estrus detection with fixed-time AI (MTAI) protocol on estrus synchronization and pregnancy rates. Angus-based, crossbred heifers (n = 971; 346 ± 14 kg) were estrus synchronized via a melengestrol acetate-prostaglandin F2α (PG) protocol. An estrus detection aid (patch) was applied at PG injection. Each heifer was assigned a patch score (reflecting the amount of rub-off coating removed) at AI to reflect estrus status. At 72 h following PG, FTAI heifers received a 2-mL gonadotropin-releasing hormone injection and were inseminated. Estrus was detected in MTAI heifers at 58 and 70 h after PG. Approximately 72 h after PG, MTAI heifers were inseminated in the following order: heifers in estrus at 58 h after PG, heifers in estrus at 70 h after PG, and heifers not appearing in estrus at either observation. Heifers not expressing estrus received gonadotropin-releasing hormone at AI. Pregnancy was determined via transrectal ultrasonography 51 d after AI. In both treatments, heifers exhibiting estrus had greater (P < 0.01) AI conception rates than heifers not expressing estrus (71 and 66 ± 5% for FTAI vs. MTAI, respectively, vs. 47 and 53 ± 9% in nonestrus heifers for FTAI and MTAI, respectively). However, overall AI pregnancy rate (62 ± 5%) and final pregnancy rates were similar between treatments (P ≥ 0.49; 97 ± 3%). Similar AI pregnancy rates were achieved without the added labor of estrus detection.