Split-time artificial insemination in beef cattle: III. Comparing fixed-time artificial insemination to split-time...

This experiment was designed to compare pregnancy rates in postpartum beef cows following split-time (STAI) or fixed-time (FTAI) artificial insemination. Estrus was synchronized for 671 cows at seven locations following administration of the 7-d CO-Synch + CIDR protocol (100 μg GnRH + CIDR insert [1.38 g progesterone] on d 0; 25 mg prostaglandin F_2α [PG] at CIDR removal on d 7). Cows were assigned to treatments that were balanced across locations based on age, body condition score, and days postpartum at the time treatments were initiated. All cows in treatment 1 (n = 333; FTAI) were inseminated at 66 h after PG and GnRH was administered concurrent with insemination regardless of estrus expression. For cows in treatment 2 (n = 338; STAI), inseminations were performed at 66 or 90 h after PG, and estrous status was recorded at these times. Cows in the STAI treatment that exhibited estrus by 66 h were inseminated at that time and did not receive GnRH, whereas AI was delayed 24 h until 90 h after PG for cows that failed to exhibit estrus by 66 h. Gonadotropin-releasing hormone (100 μg) was administered concurrent with AI at 90 h only to cows failing to exhibit estrus. Estrus expression that occurred during the 24 h delay period among cows assigned to the STAI treatment increased the total proportion of cows that expressed estrus prior to insemination (1 = 60%; 2 = 86%; P < 0.001). Pregnancy rates for cows inseminated at 66 h that exhibited estrus did not differ between treatments (1 = 58%; 2 = 58%; P = 0.93); however, pregnancy rates among non-estrous cows at 66 h were improved (1 = 35%; 2 = 51%; P = 0.01) among cows assigned to the STAI treatment when insemination was postponed by 24 h. Consequently, total AI pregnancy rate tended to be higher for cows that received STAI (1 = 49%; 2 = 56%; P = 0.06). In summary, following administration of the 7-d CO-Synch + CIDR protocol, total estrous response increased and pregnancy rates resulting from AI tended to be higher among cows assigned to STAI versus FTAI treatments.