ESTRUS SYNCHRONIZATION IN CATTLE USING ESTRADIOL, MELENGESTROL ACETATE AND PGF

In Experiment 1, all cattle were fed MGA (0.5 mg/head/d) for 7 d (desi gnated Days 0 to 6) and given PGF on Day 6. One-half were administered estradiol valerate (EV; 5 mg, im) on Day 0. At Location 1, a higher p roportion (P < 0.005) of EV-treated heifers were detected in estrus an d bred by AI between Days 7 and 13 than control heifers not receiving EV (27 of 33 versus 15 of 32), but the number of pregnancies (12 vs 10 ) was not significantly different. Eighty-three of 104 EV-treated and 89 of 106 control cows were inseminated, resulting in 50 and 45 pregna ncies, respectively (not significant). At Location 2, cattle were simi larly treated and exposed to bulls on Days 7 to 49. Fall pregnancy rat e was higher (P < 0.015) for EV-treated than control heifers (44 of 48 vs 33 of 46), but was not significantly different for cows (22 of 26 vs 19 of 23). In Experiment 2, estradiol 17 beta (E17 beta; 5 mg, im) and progesterone (100 mg, im) were administered on Day 0 (instead of E V). In a third group (designated the PGF group), cattle were bred on D ays 0 to 6, and PGF was administered on Day 6 to those not yet bred. F or 213 cows, the percentage pregnant to a synchronized estrus was grea ter in the PGF group (72%) than in either the control group treated wi th MGA (49%; P=0.005) or the group receiving MGA and E17 beta (54%; P < 0.025). Fall pregnancy rates were 91, 89, and 96% for the 213 cows ( not significant) and 89, 93, and 98% for 131 heifers (not significant) in the PGF, MGA and E17 beta groups, respectively. In cattle without a functional CL, the average diameter of the largest follicle at Day 6 was 1 to 2 mm smaller in the E17 beta +MGA group than in the MGA grou p (difference significant only in cows at Location 1). Combined for bo th locations, the synchronized pregnancy rate in heifers without a fun ctional CL on Day 6 was higher (P < 0.05) in the E17 beta CMGA group t han in the MGA group (11 of 21, 52% versus 4 of 20, 20%). Estrogen tre atment caused regression of ovarian follicles with emergence of a new follicular wave. Including estrogen in an estrus synchronization progr am utilizing MGA and PGF significantly increased fall pregnancy rate i n heifers (at 1 location) and the synchronized pregnancy rate of heife rs without a functional CL at the time of PGF treatment (combined for both locations).