Evaluation of a melengestrol acetate and prostaglandin F2 alpha system for the synchronization of estrus in beef heifers

This study was conducted to determine the efficacy of feeding melengestrol acetate (MGA) for 14 days and administering prostaglandin F2 alpha (PGF) 17 days after MGA to synchronize or induce estrus in yearling beef heifers. The study involved 56 Angus (n = 19), Hereford (n = 15) and Simmental (n = 22) heifers that were assigned by breed and pubertal status to either MGA+PGF or to control groups. Heifers in the synchronized group were fed 0.5 mg MGA per head per day for 14 days from a grain carrier and were injected with 25 mg, i.m. PGF 17 days after the last daily feeding of MGA. Control heifers were fed from a grain carrier without MGA and were not treated with PGF. Heifers were classified as pubertal when concentrations of progesterone in the serum exceeded 1 ng/ml in 1 of 2 samples collected prior to the initiation of treatments. Blood samples were collected 7 days before and on the day that treatment with MGA or carrier began and 7 days before and on the day that PGF was administered. Progesterone concentrations in the serum were elevated (> 1 ng/ml) in 61% (17/28) of the MGA+PGF-treated heifers and in 61% (17/28) of the control heifers prior to feeding MGA. However, concentrations of progesterone in the serum at the time PGF was administered differed (P<0.05) between MGA + PGF and control groups. Concentrations of progesterone in the serum exceeded 1 ng/ml in 100% (28/28) of the MGA+PGF-treated heifers and in 71% (20/28) of control heifers at the time PGF was administered (P < 0.05). All heifers were inseminated 12 hours after the first detected estrus. Twenty-two of 28 (79%) of the MGA+PGF-treated heifers exhibited estrus within 6 days after PGF compared with 9 of 28 (32%) of control heifers (P<0.05). The conception rate at first service did not differ between MGA+PGF and control groups (64% and 67%, respectively). Synchronized pregnancy rates were higher (P < 0.05) for MGA + PGF-treated heifers than for control heifers (14/28, 50% vs 6/28, 21%). Increased concentrations of progesterone in serum at the time PGF was administered and higher pregnancy rates during the synchronized period among MGA + PGF-treated heifers demonstrate the efficacy of this treatment for use in estrus synchronization. Moreover, this treatment may have a potential effect on inducing puberty in breeding age heifers.