SUPPLEMENTAL PROTEIN-SOURCES FOR STEERS FED CORN-BASED DIETS .2. GROWTH AND ESTIMATED METABOLIZABLE AMINO-ACID SUPPLY

Seventy Simmental-cross steers (average initial weight 301 +/- 24 kg) were individually fed in a 175-d completely randomized design experime nt to evaluate the effects of source and level of protein in the diet on gain and feed efficiency. Steers were allotted to 1 of 10 treatment s (seven steers per treatment) in a 3 x 3 factorial arrangement of tre atments plus a urea-supplemented control diet. Main factors were sourc e of supplemental protein (soybean meal [SBM], a high ruminal escape s oybean meal [SP; SoyPLUS(R)], or a combination of corn gluten meal and blood meal [CB; 50:50 on a nitrogen basis]) and level of each protein source (20, 30, or 40% of total dietary CP). Based on 18-h in situ ru minal incubation, escape N content of the protein sources was 66.0, 82 .5, and 90.8% of total N and metabolizable amino acid (MAA) content wa s 29.1, 33.4, and 67.8 g/100 g of DM for SBM, SP, and CB respectively. The steers were fed 12.5% CP diets based on cracked corn (70%) on d 0 through 70 and were switched to a common 11.5% CP urea-supplemented c racked corn diet (80%) on d 71. The steers were housed in individual c onfinement stalls and had ad libitum access to feed. Replacing urea wi th SBM or SP increased (P < .05) 28- and 70-d ADG and DMI and increase d (P < .05) 28-d efficiency (kg of gain/100 kg of feed). Replacing ure a with CB did not improve (P > .05) 28- or 70-d ADG or DMI but did inc rease (P < .05) 28-d efficiency. The growth rate of steers at 28 and 7 0 d was correlated to a greater degree with ME intake (r(2) = .83 and .85, respectively) rather than MAA supply, suggesting that the MAA sup ply was not first-limiting for growth. The source of supplemental prot ein fed during d 0 through 70 had no effect (P > .05) on 175-d DMI or efficiency; however, feeding SBM increased (P < .05) 175-d ADG compare d with feeding urea, SP, or CB. Increasing supplemental true protein t ended to linearly increase ADG and DMI at 28 and 70 d, but overall, AD G, DMI, and efficiency were not affected (P > .05) by treatment. Repla cing urea with SBM or CB in the first 70 d decreased (P < .05) carcass quality grade, dressing percentage, and longissimus muscle area. Supp lemental protein source and level had no effect(P > .05) on fat thickn ess or yield grade. These results suggest that feeding true protein ca n improve the initial growth rate of steers fed concentrate diets and that this response may be enhanced by feeding ruminally degradable rat her than undegradable protein sources.