SYNCHRONIZATION OF ESTRUS IN BEEF FEMALES WITH THE MELENGESTROL ACETATE PROSTAGLANDIN-F2-ALPHA PROCEDURE - ESTRUS AND EFFICACY OF 72-HOUR TIMED AI

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 F GF(2 alpha) (25 mg Lutalyse(R)) 13, 15 and 17 days after the last day of MGA feeding. Females not in estrus for the first 52 h after PGF(2 a lpha) treatment were artificially inseminated at 72 h after PGF(2 alph a.) Females in estrus within first 52 h and 78 h to 6 days after PGF(2 alpha) were artificially inseminated based on estrus (am/pm rule). Pr egnancy was determined per rectum 44 to 47 days after the 72 h AI. Onl y two of the 86 females (2%) were in estrus during the 14 day MGA feed ing period. During the interval from the last day of MGA feeding to PG F(2 alpha) treatment 16 of the heifers (42%) and 6 of the cows (13%) w ere in estrus after an average of 5.4 +/- .4 days after the last day o f MGA feeding. Estrus response after PGF(2 alpha) treatment and pregna ncy rates for heifers and cows with 13, 15 and 17 day intervals from M GA to PGF(2 alpha) treatment were similar (P>0.10) and summarized toge ther. Only 5 of 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 af ter 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 th e 72 h AI became pregnant. Of the other 65 females, although only 2 we re detected in estrus 52 to 78 h after PGF(2 alpha) treatment, 25 of t he 65 (39%) females with only 72 h AI became pregnant. The combined 6 day synchronized pregnancy rate was 44% (38/86). In summary, acceptabl e pregnancy rates were obtained in beef heifers and cows synchronized with MGA and PGF(2 alpha) and bred by a combined estrus detection (1, 2, 4, 5 and 6 days after PGF(2 alpha) treatment) and timed inseminatio n (72 h after PGF(2 alpha) treatment) procedure.