DEVELOPMENT OF A PROGESTIN-BASED ESTRUS SYNCHRONIZATION PROGRAM - II - REPRODUCTIVE RESPONSE OF COWS FED MELENGESTROL ACETATE FOR 14 DAYS WITH INJECTIONS OF PROGESTERONE AND PROSTAGLANDIN-F2-ALPHA

We tested the efficacy of an estrus control system designed to provide optimal control of follicular development. In Exp. 1, postpartum cows (n = 133) and yearling heifers (n = 57) were fed either .5 mg.female( -1).d(-1) of melengestrol acetate (MGA) or the carrier for MGA from d -13 to d 0 (d 0 = last day of MGA feeding). All females received 25 mg of PGF(2 alpha) (i.m.) on d -13 and 0. On d -6, cows and heifers fed MGA were administered an i.m. injection of progesterone (200 mg; MGA/P 4), and those fed the corn carrier (2XPGF(2 alpha)) received no proges terone. Beginning on d 1, females were bred by AI from d 1 to at least d 5. During the estrus synchronization period (d 1 to d 5), more (P < .05) postpartum cows were observed in estrus (70.1 vs 42.4%), the tim ing of estrus was more (P < .05) precise, conception rate was similar, and pregnancy rate was higher (P < .05) in the MGA/P4 than in the 2XP GF(2 alpha) treatment. More (P < .05) cows that were anestrous at the beginning of the breeding season were in estrus during the synchroniza tion period in the MGA/P4 (55.8%) than in the 2XPGF(2 alpha) (28.6%) t reatment. In heifers, estrus was synchronized in over 90% of females, and neither conception nor pregnancy rate during the synchronization p eriod differed between treatments. In Exp. 2, postpartum cows (n = 122 ) and heifers (n = 84) received treatments (MGA/P4 or 2XPGF(2 alpha)) as described for Exp. 1 with one exception. In the MGA/P4 treatment, p rogesterone was administered on d -7 rather than d -6. Females were br ed by AI from d 1 to 5. The estrus response and conception rate during the synchronization period did not differ between treatments for eith er cows or heifers. We conclude that the progestin-based estrous synch ronization system used in this study effectively synchronized an estru s of normal fertility in cyclic cows and induced a majority of anestro us cows to reinitiate estrous cycles.