Evaluating progestin-based protocols to control estrous cycles of beef heifers prior to timed artificial insemination
Authors:   , Knickmeyer Emma Rose 
Published: September 2024

The recent development of split-time artificial insemination as a breeding strategy improved reproductive outcomes following treatment with the 14d CIDR[copyright]-PG protocol; however, STAI has not been evaluated with other protocols designed to synchronize estrus in beef heifers. Two experiments (Chapter 2) were designed to evaluate estrous response and pregnancy rates resulting from fixed-time (FTAI) or split-time (STAI) artificial insemination among beef heifers. Experiment 1 (Chapter 2) evaluated FTAI and STAI following administration of the melengestrol acetate (MGA[copyright]) prostaglandin F[super script 2a] protocol. Experiment 2 (Chapter 2) evaluated FTAI and STAI following administration of the 7d CO-Synch + controlled internal drug release (CIDR[copyright]) estrus synchronization protocol. Heifers (n = 524) in Experiment 1 (Chapter 2) 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[copyright] (0.5 mg x animal[superscript -1]d[superscript -1]) in a 1.8 kg grain carrier for 14 d. Prostaglandin F[subscript 2a] (PG; 250 [mu]g im cloprostenol sodium) was administered 19 d after MGA[copyright] withdrawal. Estrus detection aids (Estrotect[copyright]) were applied at PG. Estrous status was recorded at 72 h after PG for heifers assigned to FTAI, and 72 and 96 h after PG for heifers assigned to STAI. Estrus was defined as removal of [greater than or equal to] 50 percent of the coating from the Estrotect[copyright] patch. Heifers assigned to FTAI were inseminated 72 h after PG and gonadotropin-releasing hormone (GnRH; 100 [mu]g im gonadorelin acetate) was administered at AI. Heifers in the STAI treatment that exhibited estrus by 72 h after PG were inseminated at 72 h; however, AI was postponed until 96 h for heifers that were non-estrous at 72 h. Only heifers that failed to exhibit estrus by 96 h received GnRH at AI. 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 percent, STAI; 72 percent, FTAI). Pregnancy rates resulting from AI were greater (P < 0.04) for heifers assigned to STAI compared to FTAI (55 percent vs 46 percent, respectively), and were enhanced (P < 0.05) among heifers that exhibited estrus. Heifers (n=456) in Experiment 2 (Chapter 2) were managed at one location in three pens and assigned within pen to one of two balanced treatments based on weight and reproductive tract score (RTS; Scale 1-5). All heifers were subject to the 7d CO-Synch + controlled internal drug release (CIDR[copyright]) estrus synchronization protocol and managed in two synchronization groups. Gonadotropin-releasing hormone (100 [mu]g im gonadorelin acetate) was administered and an EAZI-Breed CIDR[copyright] [1.38g progesterone (P[subscript 4]); Zoetis, Madison, NJ] inserted in heifers at the start of the protocol. The CIDR[copyright] inserts were removed after 7 d. Prostaglandin F[subscript 2a] (PG; 250 [mu]g im cloprostenol sodium) was administered and estrus detection aids (Estrotect[copyright]) were applied concurrent with CIDR[copyright] removal. Estrous status was recorded at 54 h after PG for heifers assigned to FTAI, and 54 and 78 h after PG for heifers assigned to STAI. Estrous was defined as removal of [greater than or equal to]50 percent of the grey coating from the Estrotect[copyright] patch. Heifers assigned to FTAI were inseminated 54 h after PG and GnRH (100 [mu]g im gonadorelin acetate) was administered at AI. Heifers in the STAI treatment were inseminated when considered estrous by 54 h after PG; however, AI was postponed until 78 h for heifers that were non-estrous at 54 h. Only heifers that failed to exhibit estrus by 78 h received GnRH at AI. Estrous response 54 h after PG did not differ between treatments; however, total estrous response was increased (P < 0.001) among heifers assigned to STAI (74 percent, STAI; 47 percent, FTAI). Pregnancy rates resulting from AI were not different for heifers assigned to STAI compared to FTAI (48 percent and 46 percent, respectively; P=0.6), and were enhanced (P < 0.05) among heifers in the STAI treatment that exhibited estrus. In summary of Experiment 1 (Chapter 2), STAI has potential to improve estrous response and pregnancy rate to AI following the MGA [copyright]PG protocol. When compared to FTAI in Experiment 2 (Chapter 2), STAI enhanced estrous response following the 7d CO-Synch + CIDR[copyright],however, this did not result in an increase in pregnancy rate to AI.