WS Effect of delayed insemination of non-estrual beef heifers following a 7-d-CO-Synch plus controlled internal drug release (CIDR) insert timed artificial insemination protocol

Abstract Text:

Synchronizing estrus before AI is an effective way to shorten the calving season, and increasing the number of pregnancies per AI may lead to greater use and acceptance of synchronization protocols among beef producers. Our objective was to determine if pregnancy rates to fixed-time AI (FTAI) would be improved by delaying insemination in heifers not expressing estrus before FTAI in a 7-d CO-Synch + controlled internal drug release (CIDR) estrous synchronization protocol. Four hundred sixty-five yearling beef heifers across three locations of commercial and purebred herds were given 100 μg of GnRH (Cystorelin; Merial) i.m. and a CIDR insert (Zoetis; 1.38 g of progesterone) on d 0. On d 7 CIDR inserts were removed and all heifers received 25 mg of PGF_2α i.m. (Lutalyse; Zoetis) and were fitted with an estrous detection patch (Estrotect; Rockway, Inc.). Heifers were placed in one of three treatment groups based on estrous detection patch color at 48 h after PGF_2α:  1. Estrus-Red 48 h (n = 180) - heifers displayed estrus as indicated by red estrous detection patch and were given GnRH (100 μg i.m. Cystorelin) and inseminated at 48 h after PGF_2α, 2. Non-Estrus-Gray 48 h (n = 137) - heifers did not display estrus by 48 h after PGF_2α and were given GnRH (100 μg i.m. Cystorelin) and inseminated at 48 h after PGF_2α, and 3. Non- Estrus Delayed- Gray 56 h (n = 148) - heifers did not display estrus by 48 h after PGF_2α and were given GnRH (100 μg i.m. Cystorelin) at 48 h and inseminated at 56 h after PGF_2α. Pregnancy data were analyzed using SAS PROC GLIMMIX with treatment as a fixed effect, herd as a random variable, and heifer as the experimental unit. By 48 h after PGF_2α, 38.7% of all heifers were in estrus (as indicated by a red estrous detection patch). Pregnancy rate to AI was greatest for Estrus-Red 48 h heifers (67.8%; P < 0.0001) as compared to heifers in the Non- Estrus- Gray 48 h (39.4%) and Non-Estrus Delayed- Gray 56 h (42.6%) groups. Among heifers not expressing estrus before FTAI, delayed insemination achieved a similar (P = 0.83) percent of pregnancies (Non-Estrus Delayed-Gray 56 h; 42.6%) as compared to Non-Estrus-Gray 48 h heifers (39.4%). Delaying insemination by 8 h in heifers not displaying estrus by 48 h after PGF_2αdid not improve pregnancy rates to AI.

Keywords: beef heifers