EFFECTS OF PRENATAL ANDROGENIZATION AND POSTNATAL STEROID TREATMENT ON GROWTH-HORMONE, INSULIN-LIKE GROWTH-FACTOR-I AND GROWTH-FACTOR-II, INSULIN, THYROXINE, AND TRIIODOTHYRONINE CONCENTRATIONS IN BEEF HEIFERS

A 2 x 3 factorial arrangement of treatments was used to elucidate the mechanism(s) by which prenatal androgenization improves postnatal rate and efficiency of growth and composition of gain in beef heifers. Fif teen control (C) and 15 prenatally androgenized (PA) Angus x Simmental heifers (prenatal treatment, Pretrt) received no (N), estrogen (E), o r estrogen and testosterone (ET) implants postnatally (postnatal treat ment, Posttrt) to evaluate whether the postpubertal growth response af ter prenatal androgenization could be induced in prepubertal heifers. Blood was collected from the heifers at 6 +/- 1, 9 +/- 1, and 12 +/- 1 mo of age and analyzed for serum concentrations of growth hormone (GH ), IGF-I, IGF-II, insulin, thyroxine (T-4), and triiodothyronine (T-3) . Season of the year had a greater effect on hormone concentrations th an either Pretrt or Posttrt, and there were no Pretrt x Posttrt intera ctions. Prenatal treatment, PA, had no effect on GH; however, Posttrt E and ET increased (P < .001) GH concentrations. Prenatal treatment, P A, increased (P < .05) IGF-I concentrations, and there was a nonsignif icant increase (P = .11) in IGF-I concentrations with Posttrt E and ET . Concentrations of IGF-II were unaffected by Pretrt PA; however, they were lower (P < .01) in the Posttrt E and ET groups. Insulin, T-4, T- 3, BW, and ADG were not affected by Pretrt or Posttrt. Concentrations of GH and IGF-I were increased in heifers that received Pretrt PA and( or) Posttrt E and ET in a manner to support improved growth performanc e; however, BW and ADG were similar. In prepubertal beef heifers, fact ors in addition to increased GH and IGF-I seem to be necessary for imp roved growth performance.