Split-time artificial insemination improves estrous response and pregnancy rates following administration of the MGA-PG protocol to synchronize estrus in beef heifers

This experiment was designed to evaluate pregnancy rates resulting from fixed-time (FTAI) or split-time (STAI) artificial insemination among beef heifers following administration of the 14-day melengestrol acetate (MGA) prostaglandin F2α protocol. Heifers (n = 524) were managed in 10 pens and assigned within pen to balanced treatments based on weight and reproductive tract score (RTS; Scale 1–5). Heifers were fed MGA (0.5 mg∙animal-1∙d-1) in a 1.81 kg grain carrier for 14 days. Prostaglandin F2α (PG; 250 mg cloprostenol sodium) was administered 19 d after MGA withdrawal. Estrus detection aids (Estrotect®) were applied at PG. Estrous status was recorded at 72 h after PG for heifers assigned to FTAI, and 72 and 96 h for heifers assigned to STAI. Estrus was defined as removal of ≥ 50% of the coating from the Estrotect® patch. Heifers assigned to FTAI were inseminated 72 h after PG and GnRH (100 µg gonadorelin acetate) was administered at AI. Heifers were inseminated in the STAI treatment that exhibited estrus by 72 h after PG; however AI was postponed until 96 h for heifers that were non-estrus at 72 h. Only heifers that failed to exhibit estrus by 96 h received GnRH at AI. Inseminations were performed by two technicians using semen from a single sire. Estrous response 72 h after PG did not differ between treatments; however total estrous response was increased (P < 0.001) among heifers assigned to STAI (88%-STAI; 72%-FTAI). Pregnancy rates resulting from AI were greater (P < 0.03) for heifers assigned to STAI compared to FTAI (55% vs 46%, respectively), and were enhanced (P < 0.01) among heifers that exhibited estrus. These data demonstrate potential for improved pregnancy rates in beef heifers when STAI is performed following administration of the MGA-PG protocol.