EFFECTS OF ANABOLIC-STEROIDS ON NITROGEN-METABOLISM AND GROWTH OF STEERS FED CORN-SILAGE AND CORN-BASED DIETS SUPPLEMENTED WITH UREA OR COMBINATIONS OF SOYBEAN-MEAL AND FEATHERMEAL

The objectives of these experiments were to examine whether N retention and feedlot performance by steers administered estradiol 17-beta (E(2)) alone (metabolism study) or in combination with trenbolone acetate (TAB; feedlot study) could be enhanced by increasing dietary ruminal escape protein. For the metabolism study, 16 Angus steers (average weight = 308 +/- 5 kg) were used in a randomized block design experiment having a 2 x 4 factorial arrangement of treatments. Main factors were E(2) administration and supplemental CP source. Steers were fed diets based on corn silage and corn (60:30 DM basis) and were supplemented with urea or three combinations of soybean meal (SBM) and feathermeal (Fth). Combinations and SBM and Fth (SBM:Fth) provided 75:25, 50:50, and 25:75 of supplemental CP on a N basis. Estradiol 17-beta increased (P <.05) N retention and decreased (P < .05) plasma urea N concentrations. Total tract N digestion decreased linearly (P <.05) as the proportion of Fth in the diet increased. However, N retention increased linearly(P <.05) with increased Fth. A CP source x E(2) interaction (P <.05) was observed for N retention because retention increased linearly with increasing dietary Fth in cattle receiving E(2) but was unaffected (P >.05) in control steers. A growth study was conducted using 128 crossbred steers (400 +/- 19 kg) that received no implant or a combination of E(2) (Synovex(R)) and TBA (Finaplix-(R)). Steers were fed diets containing 75% high-moisture corn, 15% corn silage, and 10% supplement (DM basis) for 84 d (heavy replicate) or 124 d (light replicate). Supplemental protein was provided by urea or combinations of SBM and Fth. Implantation improved (P < .05) growth rate and efficiency of growth. Feeding higher Fth resulted in linear (P <.10) and quadratic (P <.10) improvements in gain and efficiency. However, no interactions (P >.45) between implant and protein source were observed. Feeding higher ruminal escape protein may enhance the growth of implanted cattle but this relationship is likely impacted by animal growth potential, basal diet fed, and choice of supplemental protein.