| Three experiments were conducted to evaluate the effects of prenatal androgenization on serum concentrations of hormones that regulate growth and reproductive processes in cattle. In Experiment 1, the efficacy of Synovex$spcircler$H as an agent for prenatal androgenization was evaluated in 15 pregnant beef heifers. Fifteen additional heifers were used as controls. During the 84-day blood collection period, treatment with Synovex$spcircler$H resulted in greater (P $<$.001) serum concentrations of testosterone, similar serum estradiol concentrations, and lower (P $<$.05) serum concentrations of progesterone in treated heifers as compared with control heifers. In addition, treatment of pregnant heifers with Synovex$spcircler$H decreased (P $<$.05) calving rates and calf birth weights in those heifers. A 2 x 3 factorial arrangement of treatments was used in Experiment 2 to elucidate the mechanism(s) by which prenatal exposure to exogenous androgens improve postnatal rate and efficiency of growth and composition of gain in beef heifers. Fifteen control (C) and 15 prenatally androgenized (PA) heifers received no (N), estrogen (E), or estrogen and testosterone (ET) implants postnatally to evaluate whether the postpubertal growth response observed with prenatal androgenization could be induced in prepubertal heifers. Prenatal androgenization resulted in greater (P $<$.05) serum IGF-I concentrations. Postnatal treatment with E and ET resulted in greater (P $<$.001) serum GH concentrations and lower (P $<$.01) serum IGF-II concentrations. Serum concentrations of insulin, T$sb4$, and T$sb3$, as well as heifer weights and ADG were not affected by prenatal androgenization or postnatal steroid treatment. In Experiment 3, twenty-two PA and 21 C primiparous beef heifers in a single-calf heifer system trial were studied to evaluate the effects of prenatal androgenization on parturition and periparturient, reproductive, and metabolic hormone concentrations. Serum concentrations of progesterone, estradiol, testosterone, prostaglandin F$sb2alpha$, and prolactin, as well as calf birth weights and calving ease scores, were similar for C and PA heifers. Postpartum cyclicity was also similar, with 6 of 22 PA (27.3%) and 3 of 21 (14.3%) heifers cycling by 70 days postpartum, based on weekly serum progesterone concentrations. During lactation, serum concentrations of insulin, T$sb4$, and T$sb3$ were similar, while prolactin concentrations were greater (P $<$.05) in C than PA heifers. |
