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Split-time AI: Delayed insemination of non-estrous beef heifers in timed artificial insemination following the 14-d CIDR-PG protocol
An experiment was designed to test the hypothesis that pregnancy rates in beef heifers after fixed-time artificial insemination (FTAI) may be improved by delaying insemination of those heifers that have not expressed estrus prior to the standard FTAI time. Estrus was synchronized for 931 heifers across three locations using the 14-d CIDR-PG protocol (CIDR insert [1.38 gm progesterone] on d 0 with removal of CIDR on d 14; 25 mg PGF2α 16 d after CIDR 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 expression of estrus was recorded at GnRH on d 33. Treatments were balanced across locations, and heifers within each location were randomly assigned to one of two treatments based on reproductive tract score (RTS) and weight: (1) FTAI (concurrent with GnRH, 66 h after PGF2α) regardless of estrous expression prior or (2) FTAI for heifers having expressed estrus, and delayed AI (20 h after GnRH) for heifers failing to express estrus. A significant effect of treatment was found on AI pregnancy rate, with heifers assigned to treatment 2 achieving a higher AI pregnancy rate than heifers assigned to treatment 1 (54% versus 46%, P = .012). 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 greater for treatment 2 (49% versus 34%, P = .024), while AI pregnancy rates of estrous heifers did not differ by treatment (P = .244). In summary, FTAI pregnancy rates in heifers can be improved through a strategy of “split-time” AI: insemination at the standard time of 66 h after PGF2α for heifers that have expressed estrus and delayed insemination at 20 h after GnRH for heifers that have not expressed estrus prior to the standard AI time in the 14-d CIDR-PG protocol.
Keywords:
artificial insemination
estrus synchronization
beef heifer