ROASTED SOYBEANS AND AN ESTROGENIC GROWTH PROMOTER AFFECT GROWTH-HORMONE STATUS AND PERFORMANCE OF BEEF STEERS

This study investigated the effects of a roasted soybean (RSB)-supplem ented diet and an estrogen implant (SYN; Synovex-S(R) ear implant, 20 mg estradiol benzoate plus 200 mg progesterone) in young crossbred bee f steers on their performance and plasma growth hormone (GH) response to challenge injections of thyrotropin-releasing hormone (TRH) + GB-re leasing hormone (GHRH). Twenty individually fed steers (body weight 25 5 +/- 5 kg) were assigned to the following treatments: 1) no SYN and f ed a soybean meal-supplemented diet, 2) no SYN and fed the RSB-supplem ented diet, 3) plus SYN and soybean meal, and 4) plus SYN and RSB. Ste ers were fed 1.13 MJ metabolizable energy/kg metabolic body weight dai ly of an 18% protein diet. After a 5-wk growth period, all steers were challenged (intravenous injection) over a 3-wk period with three leve ls of a combination of TRH + GHRH (0.1 + 0.01, 1.0 + 0.1, 2.5 + 0.25 m u g/kg body weight, respectively). After an additional 3 wk, steers we re reimplanted and a second 5-wk growth period was followed by a singl e challenge of the 1.0 + 0.1 TRH + GHRH dose level. Plasma nonesterifi ed fatty acid concentration was greater when steers were fed the RSB c ompared with soybean meal (265 vs. 205 mu mol/L; P < 0.01; SEM = 9.5). Body weight gains for treatments 1, 2, 3 and 4 were 1.35, 1.21, 1.47 and 1.38 kg/d, respectively (RSB, P < 0.10; SYN, P < 0.07; SEM = 0.06) . Gain/dry matter intake (g/kg) means were 184, 167, 197 and 184 (RSB, P < 0.04; SYN, P < 0.07; SEM = 7.5). Feeding roasted soybeans results in depressed area under the GH response curve [907, 555, 827 and 989 mu g/(Lmin) (SYN x RSB, P < 0.03; SEM = 117)] and depressed peak resp onse (37.2, 26.6, 33.5 and 41.1 mu g/L [SYN x RSB, P < 0.05; SEM = 4.5 ]), an effect alleviated by estrogen for young growing steers (Period 1) but not for heavier steers (Period 2).