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Abstract
Kesler, D.J., Faulkner, D.B., Dyson, T.S. and Nash, T.G. 1997. Synchronization of estrus in beef females with the melengestrol acetate/prostaglandin F2α procedure: estrus and efficacy of 72-hour timed AI. J. Appl. Anim. Res., 11: 73–81.
Thirty-eight beef heifers and 48 beef cows suckling calves were fed .5 mg of MGA per day for 14 days and then randomly assigned to receive FGF2α (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 PGF2α treatment were artificially inseminated at 72 h after PGF2α. Females in estrus within first 52 h and 78 h to 6 days after PGF2α, were artificially inseminated based on estrus (am/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 PGF2α 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 PGF2α treatment and pregnancy rates for heifers and cows with 13, 15 and 17 day intervals from MGA to PGF2α 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 Al became pregnant. Of the other 65 females, although only 2 were detected in estrus 52 to 78 h after PGF2α treatment, 25 of the 65 (39%) females with only 72 h Al 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 PGF2α and bred by a combined estrus detection (1, 2, 4, 5 and 6 days after PGF2α treatment) and timed insemination (72 h after PGF2α treatment) procedure.