SERUM CONCENTRATIONS OF TRENBOLONE-17-BETA AND ESTRADIOL-17-BETA AND PERFORMANCE OF HEIFERS TREATED WITH TRENBOLONE ACETATE, MELENGESTROL ACETATE, OR ESTRADIOL-17-BETA

The effects of the growth-promoting steroids estradiol-17 beta (E-2), trenbolone acetate (TEA), and melengestrol acetate (MGA) in heifers on serum concentrations of E-2 and trenbolone-17 beta (TBOH) were examin ed. Feed intake and growth performance were also measured. Serum conce ntrations of E-2 and TBOH were measured on d 0, 1, 3, 5, 7, 13, 21, 28 , 42, 56, 84, 112, and 140 in finishing heifers administered the follo wing treatments: 1) control; 2) MGA, .5 mg per heifer daily; 3) Revalo r-H (140 mg TEA + 14 mg E-2); 4) Revalor-H + MGA; 5) Finaplix-H (200 m g TEA); and 6) Finaplix-H + MGA. Revalor-H implantation (Treatments 3 and 4) increased (P < .05) serum E-2 concentrations; peak concentratio ns (67.5 pg/mL) occurred between d 21 and 56. Feeding MGA (Treatment 4 ) had no effect (P > .05) on this increase in serum E-2 concentrations (63.3 pg/mL). From d 84 until d 140, serum E-2 was greater (P < .05) for the Revalor-H treatment (average of 19 pg/mL) than for the control (7 pg/mL) or Finaplix-H treatments (6.5 pg/mL). Serum E-2 concentrati ons increased numerically two-to threefold from d 56 to 140 in control s fed MGA, compared with controls not fed MGA. There was the expected increase in serum TBOH concentrations after TEA implantation in the Re valor-H and Finaplix-H treatments; concentrations were similar(P > .05 ) for Revalor-H (221 pg/mL) and Finaplix-H (280 pg/mL). After d 56, se rum TBOH concentrations decreased in both treatments to 10 and 20% of these concentrations, respectively. Feeding MGA increased serum TBOH ( P < .05). Dry matter intake by heifers did not differ among treatments . Feeding of MGA improved gains (P = .12) and efficiencies (P < .01) i n nonimplanted heifers and had no effect (P > .4) on gains or efficien cies in Finaplix-implanted heifers.