Variation in conception rates following synchronization of estrus with melengestrol acetate and prostaglandin F2 alpha

Beef cows and heifers (n = 263) at three locations that were exhibiting estrous cycle either were fed .5 mg/d melengestrol acetate (MGA) for 7 d and administered prostaglandin F2 alpha (25 mg, i.m.) on the last day of MGA feeding or were untreated. State of the estrous cycle at the beginning of the experiment was determined based on estrous detection and (or) progesterone concentrations in pretreatment blood samples. Estrous was checked twice daily for 30 d posttreatment. Animals were artificially inseminated approximately 12 h after detection of estrus. A synchronized estrus (less than 7 d posttreatment) was detected in 72% of the treated animals. More animals in the treated group became pregnant during the first 7 d of breeding, but their conception rate was lower than that of animals in the control group (P less than .05). Conception rate (36%) was reduced among treated animals when MGA feeding began late (d 14 to 20) in the estrous cycle. Conversely, the conception rate (66%) of treated animals fed MGA beginning earlier in the cycle was not different from that of control animals (73%; treatment x stage of cycle; P less than .05).