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Comparison of timed insemination vs. modified estrus detection protocol in beef heifers
Angus-based, crossbred heifers (n = 972, 346 kg ± 14 kg ) were assigned to either a fixed-time AI (FTAI) protocol or modified estrus detection with fixed-time AI (MTAI) to evaluate synchronization, conception, and pregnancy rates. During the pre-breeding development period, heifers were fed to achieve a target of 60 ± 5% mature BW at breeding. Heifers were synchronized via melengestrol acetate-prostaglandin F2α (MGA-PG) protocol and received an estrus detection aid (patch) at PG administration. A patch score was recorded for each heifer at AI to reflect what percentage of rub-off coating had been removed. Heifers in the FTAI treatment received 2 mL GnRH injection and were AI 72 ± 2 h following PG. Heifers in MTAI treatment were observed for estrus at 58 ± 2 and 70 ± 2 h after PG. Approximately 72 ± 2 h after PGF2α, heifers in MTAI were AI in the following order: heifers in estrus at 58 h post-PG, heifers in estrus at 70 h post-PG, and heifers not expressing estrus at either estrus observation. Heifers not expressing estrus received GnRH at AI. Pregnancy was determined via transrectal ultrasonography. Heifers exhibiting estrus had greater (P < 0.01; 71 and 66 ± 5% for FTAI vs. MTAI, respectively) AI conception rates than heifers not expressing estrus in both FTAI and MTAI treatments vs. 47 and 53 ± 9 % AI conception rates in non-estrus heifers for FTAI and MTAI, respectively. However, overall AI conception rate (62 ± 5%, P = 0.49) and final pregnancy rates were similar (P = 0.98; 96 and 97 ± 3% for FTAI vs. MTAI, respectively). Similar AI conception rates were achieved without estrus detection.
Keywords: beef heifers, estrus synchronization, timed artificial insemination