Gene H. Deutscher

Extending Interval from Seventeen to Nineteen Days in the Melengestrol Acetate - Prostaglandin Estrous Synchronization Program for Heifers1

Two experiments were conducted to determine whether extending the interval between removal of melengestrol acetate (MGA) from feed and injection of prostaglandin F2α (PGF) from 17 to 19 d would affect synchronization of estrus, conception, and pregnancy rates of beef heifers. In both experiments, heifers were fed MGA for 14 d, and PGF was given at either 17 or 19 d after cessation of MGA feeding. Heifers were observed for estrus and artificially inseminated for 5 d after PGF injection. In Exp. 1, 240 yearling heifers were randomly assigned to either a 17- or a 19-d treatment group according to estrous status and day of the estrous cycle. In Exp. 2, 1409 yearling heifers on a cooperating ranch were randomly assigned to the same two treatment groups without knowledge of estrous status. The PGF injection at 19 d (Exp. 1) caused a higher (P<0.05) percentage of heifers to exhibit estrus by 72 h after the injection compared with heifers receiving the injection at 17 d. A greater percentage (P<0.01) of heifers in the 19-d group were in the late luteal phase of the estrous cycle at the time of PGF injection compared with the heifers in the 17-d group, and pregnancy rates were higher for the heifers in the late luteal phase. In Exp. 2, heifers injected with PGF at 19 d after MGA had a greater (P<0.05) percentage in estrus (10%) during the 5-d breeding period, and had higher (P<0.05) pregnancy rates in 5 d (7.6%) and 50 d of breeding (5.5%), compared with heifers injected with PGF 17 d after withdrawal of MGA. These results indicate that the PGF injection given at 19 d after removal of MGA from the diet increases synchronized estrous response and results in higher pregnancy rates in heifers compared with the 17-d injection treatment.

Melengestrol Acetate (MGA) and Prostaglandin for Estrous Induction and Synchronization in Peripuberal Beef Heifers1

Abstract One hundred twenty crossbred yearling heifers were used to compare a combination of MGA and prostaglandin F 2 α (PGF 2 α) as an estrous synchronization program to a conventional Al program. Heifers were stratified by estrous status (prepuberal or postpuberal) and assigned to receive MGA for 14 d followed by an injection of PGF 2 α 16 d post-MGA feeding (TRT) or to an untreated control group (CON). Puberty was determined by the detection of behavioral estrus or the presence of a palpable corpus luteum arid an increased level of serum progesterone. All heifers were observed for behavioral estrus and bred by Al during a 30-d breeding season. A higher percentage ( P P >.10) for the two groups. A higher percentage ( P 2 α injection (60%) compared with the CON group (5%) during the same time period. A higher percentage ( P 2 α was effective in estrous synchronization.