Factors Affecting the Utilization of Nitrogen from Different Sources by Lambs1

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Abstract
A 3 × 2 × 2 × 2 factorial experiment compared soybean meal, urea and biuret as nitrogen sources in lamb finishing diets, and the effect of steam treatment of corn, dehydrated alfalfa meal, diethylstilbestrol implants and their relationships on utilization of nitrogen sources. Steam treatment of corn did not increase rumen microbial activity as measured by concentrations and molar ratios of rumen VFA. Increased nitrogen retention and lower blood urea nitrogen indicate that the utilization of nitrogen from cracked corn may be affected by steaming. Steam treatment of corn resulted in 17% faster gains when soybean meal was the primary nitrogen source, but gains were 10% slower when urea was the primary nitrogen source. Steam-treated corn significantly increased carcass grade and dressing percent and lowered carcass shrink. Partial substitution of dehydrated alfalfa meal for dehydrated ground corncobs and lowering of the primary nitrogen sources markedly increased dry matter digestibility and nitrogen retention. This was associated with significantly higher concentrations of ruminal VFA. Lower blood urea nitrogen in conjunction with improved nitrogen retention indicates improved biological value of absorbed nitrogen. Improved dry matter digestibility and nitrogen utilization were reflected in 75% faster gains on 33% less feed per pound of gain and significantly improved carcass grade and yield. Response of lambs to dehydrated alfalfa meal was greatest for most criteria when urea was the primary nitrogen source. Diethylstilbestrol implants improved lamb gains and carcass grade. This appeared to be due to increased growth and protein deposition as indicated by larger loin-eye area, increased nitrogen retention and similar external fat measurement when compared with those of control lambs. The effect of DES apparently takes place at the tissue level, since rumen microbial activity and dietary dry matter and nitrogen digestibility coefficients were unaffected. DES markedly reduced the adaptation response time to urea nitrogen but it had little, if any, effect on the adaptation response time of lambs receiving biuret, indicating that the mechanisms of adaptation to the two nitrogen sources are different. The combined effects of dehydrated alfalfa meal and DES on the response of lambs were greatest when urea was the primary nitrogen source. The potential of formulating a low-cost protein supplement made up of dehydrated alfalfa meal and urea to be used in conjunction with DES is indicated. The data suggest that dehydrated alfalfa meal increases ruminal microbial activity, thus improving utilization of the soluble dietary nitrogen while DES improves the utilization of absorbed nitrogen at the tissue level.