Synchronization of estrus in beef females with the melengestrol acetate/prostaglandin f2a procedure: Estrus and efficacy of 72-hour timed ai

Thirty-eight beef heifers and 48 beef cows suckling calves were fed .5 mg of MG A per day for 14 days and then randomly assigned to receive FGF., U (25 mg Lutalyse®) 13, 15 and 17 days after the last day of MGA feeding. Females not in estrus for the first 52 h after PGF_2u treatment were artificially inseminated at 72 h after PGF_2u. Females in estrus within first 52 h and 78 h to 6 days after PGF_2a were artificially inseminated based on estrus (am I pm rule). Pregnancy was determined per rectum 44 to 47 days after the 72 h AI. Only two of the 86 females (2%) were in estrus during the 14 day MGA feeding period. During the interval from the last day of MGA feeding to PGF_2a treatment 16 of the heifers (42%) and 6 of the cows (13%) were in estrus after an average of 5.4 + .4 days after the last day of MGA feeding. Estrus response after PGF_2u treatment and pregnancy rates for heifers and cows with 13, 15 and 17 day intervals from MGA to PGF_2a treatment were similar(P>0.10) and summarized together. Only 5 of the 86 females (6%) were in estrus 1 or 2 days before the 72 h AI while 16 of the 86 females (19%) were in estrus 1 to 3 days after the 72 h AI. Two of the 5 females (40%) in estrus before the 72 h AI became pregnant while 11 of the 16 females (69%) in estrus after the 72 h AI became pregnant. Of the other 65 females, although only 2 were detected in estrus 52 to 78 h after PGF_2u treatment, 25 of the 65 (39%) females with only 72 h AI became pregnant. The combined 6 day synchronized pregnancy rate was 44% (38/86). In summary, acceptable pregnancy rates were obtained in beef heifers and cows synchronized with MGA and PGF_2aL and bred by a combined estrus detection (1, 2, 4, 5 and 6 days after PGF_2a treatment) and timed insemination (72 h after PGF_2u treatment) procedure.