Effects of supplemental rumen-degradable protein, rumen undegradable protein, and energy on performance of growing beef steers grazing corn residue

The objective of Exp. 1 was to evaluate the effects of adding urea to modified distillers grains with solubles (MDGS) when supplemented to growing steers and in Exp. 2 to evaluate whole corn as an alternative to dried distillers grains with solubles (DDGS) for growing steers grazing cornstalk residue. In Exp. 1, 120 crossbred beef steers (initial BW = 244 kg, SD = 19 kg) grazed corn residue for 72 d. Treatments were a 2 × 2 factorial arrangement of MDGS inclusion (1.4 or 2.3 kg/d) and urea inclusion (0 or 0.05 kg/d). Residue diet samples were collected to determine CP, in vitro organic matter disap- pearance (IVOMD) and digestible organic matter (DOM). Steers were individually supplemented daily. Data were analyzed using the MIXED procedures of SAS. In Exp. 2, 75 crossbred steer calves (235 kg, SD = 3.5) grazed corn residue for 86 d. Treatments were arranged in a ran- domized complete block design, which included (1) a non- supplemented control, (2) whole corn, (3) whole corn with urea in a molasses carrier (corn/mol/urea), (4) DDGS, and (5) 60% SoyPass + 40% soybean meal (SoyPass/ SBM). Supplements were designed to be isocaloric. Data were analyzed using the GLIMMIX procedures of SAS. Results were considered significant at P ≤ 0.05. No MDGS × urea inter- actions were observed for growth performance (Exp. 1). Steers supplemented with 2.3 kg MDGS had greater ADG than those supplemented with 1.4 kg MDGS (1.05 kg/d vs. 0.83 respectively). Urea level did not affect ending BW or ADG. The IVOMD and DOM of the grazed residue decreased linearly and quadratically, respectively, as the grazing progressed as expected, but CP was not different. In Exp. 2, nonsupplemented calves lost weight (0.08 kg/d) Calves receiving whole corn had greater gains (0.14 kg/d) than the nonsupplemented calves but less than corn/ mol/urea (0.24 kg/d). The DDGS and SoyPass/SBM had greater gains than the other treatments, but were not dif- ferent from each other (0.60 and 0.67 kg/d, respectively). These data suggest adding RDP when supplementing MDGS is unnecessary and that feeding whole corn, even with a source of NPN to supply RDP does not result in similar performance as feeding DDGS or SoyPass/SBM.