Microbial Protein Synthesis with Low Quality Roughage Rations: Isonitrogenous Substitution of Urea for Soybean Meal

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Abstract
Microbial protein production and nutrient digestibility were measured using 275 kg steers, fitted with permanent rumen and abomasal cannulas, and fed low quality roughage with soybean meal (SBM) or SBM plus urea supplement. The 5 kg daily feed consisted of ground weathered range grass (nitrogen .5%) and nitrogen supplement. Urea replaced 0, 25, 50 or 75% of the supplemental SBM nitrogen. Ground milo was added with urea to make all rations isocaloric and isonitrogenous. Steers were fed hourly with an automatic feeding system to maintain a constant flow of digesta. Nitrogen retention and digestibility of dry matter (DM), organic matter (OM), and nitrogen decreased (P<.01) as urea replaced more SBM in the ration. Microbial protein production was relatively constant among rations regardless of source of nitrogen. However, the amount of total nitrogen and feed protein reaching the abomasum decreased (P<.01) as urea replaced more SBM in the supplement. Microbial protein synthesis per 100 g dry matter digested in the rumen was 9.9, 10.4, 10.9 and 11.6 g for rations containing 0, 25, 50 and 75% of the supplemental nitrogen from urea, respectively. Protem synthesis appeared to be limited by available energy and rumen turnover time rather than by nitrogen availability. Urea apparently was well utilized for the synthesis of microbial protein when fed in combination with low quality roughage at hourly intervals, but was inferior to SBM for supporting digestion of DM and OM and retention of nitrogen. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.