Delayed insemination of non-estrous heifers and cows when using conventional semen in timed artificial insemination

Two experiments were designed to test the hypothesis that pregnancy rates after fixed-time artificial insemination (FTAI) in beef heifers and cows may be improved by delaying insemination of females that have not expressed estrus before FTAI. In Exp. 1, estrus was synchronized for 931 heifers across 3 locations using the 14-d CIDR-PG protocol (controlled internal drug-release [CIDR] insert [1.38 gm progesterone] on d 0 with removal of CIDR insert on d 14; 25 mg PGF2α 16 d after CIDR insert removal on d 30; and 100 μg GnRH on d 33, 66 h after PGF2α). Estrous detection aids (Estrotect) were applied at PGF2α on d 30, and estrous expression was recorded at GnRH on d 33. Heifers within each location were randomly assigned to 1 of 2 treatments based on weight and reproductive tract score (RTS): 1) FTAI (concurrent with GnRH, 66 h after PGF2α) regardless of estrous expression or 2) FTAI for heifers expressing estrus and delayed AI (20 h after GnRH) for heifers failing to express estrus. Heifers assigned to treatment 2 achieved a higher AI pregnancy rate than heifers assigned to treatment 1 (54 versus 46%; P = 0.01). The observed increase in AI pregnancy rate is attributed to the delayed AI of non-estrous heifers in treatment 2, as AI pregnancy rates for non-estrous heifers were significantly higher for treatment 2 (49 versus 34%; P = 0.02), while AI pregnancy rates of estrous heifers did not differ by treatment (P = 0.24). In Exp. 2, estrus was synchronized for 951 mature, suckled cows across 9 locations using the 7-d CO-Synch + CIDR protocol (100 μg GnRH + CIDR insert [1.38 gm progesterone] on d 0; 25 mg PGF2α at CIDR insert removal on d 7; and 100 μg GnRH on d 10, 66 h after CIDR insert removal). Estrus detection aids (Estrotect) were applied at PGF2α and CIDR insert removal on d 7, and estrous expression was recorded at GnRH on d 10. Cows within each location were assigned to 1 of 2 treatments based on age, days postpartum, and BCS: 1) FTAI (concurrent with GnRH, 66 h after PGF2α) regardless of estrous expression or 2) FTAI for cows expressing estrus and delayed AI (20 h after GnRH) for cows failing to express estrus. No significant effect of treatment was found on AI pregnancy rate (P = 0.76). In summary, FTAI pregnancy rates in heifers can be improved through a strategy of "split-time" AI. However, a statistically significant increase was not observed in the pregnancy rates of mature suckled cows when using a similar strategy.