Plasma and Ruminal Constituents and Performance of Steers Fed Different Nitrogen Sources and Levels of Sulfur

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Abstract
Sixty Holstein steers averaging 253 kg in body weight were utilized in a 2 × 3 factorial arrangement of treatments to study the effects of nitrogen source and level of added elemental sulfur on growth rate, feed efficiency, plasma amino acid patterns, plasma urea nitrogen levels and ruminal ammonia concentrations. The steers were offered ad libitum corn-cottonseed hull diets supplemented with soybean meal (SBM) or urea that were calculated to be isonitrogenous (10.35% C.P.). Diets in which urea was utilized as a nitrogen supplement had 33.7% of total dietary nitrogen as urea. Within each nitrogen source, sulfur was included in the diet to equal 0, .15 and .30% added sulfur which provided N:S ratios of 15.4, 6.4 and 4.0, respectively, in the SBM supplemented diets, and 14.2, 6.9 and 4.3, respectively, in the urea supplemented diets. Daily gains of the SBM supplemented steers averaged 1.61 kg vs 1.51 kg on the urea supplemented diets during the 140-day trial (P<.01). Supplementing sulfur did not improve rate of gain or efficiency of feed conversion. Changes in plasma amino acid concentrations with respect to diet were apparent, especially trends for higher valine, lysine and threonine in SBM-supplemented steers and higher glycine in urea-supplemented steers. Plasma urea nitrogen was elevated in all groups of steers by day 84 of the trial and remained elevated within nitrogen source at the end of the 140-day study. Although this difference was not significant, it could indicate an accumulation due to a somewhat decreased requirement for protein as the steers approached maturity. Ruminal ammonia was higher (P<.01) in urea than in SBM fed steers when sampled on day 84 of the 140-day trial. Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science