Labile Nitrogen Reserves in the Ruminant. Metabolic Changes in Growing Cattle Employing a Nitrogen Depletion-Repletion Treatment

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Abstract
A metabolism study was conducted in which eight growing steers were subjected to a nitrogen (N) depletion-repletion regime. Three consecutive time periods were used to standardize, to deplete, and to replete body N. Diets fed in the respective periods provided by analysis 6 and 16% protein equivalent, respectively, from corn gluten or urea. Metabolic parameters were monitored during weeks 3 and 5 of standardization, weeks 1, 3, and 5 of depletion, and weeks 1 through 7 of repletion. Growth-rate trends started to deviate downward from linearity during the last 2 weeks of depletion; a trend that was not reversed until week 3 of repletion. Apparently digested N was defined by regression equations in which truly digested N was 94 and 83% of ingested N and metabolic fecal N totaled 0.64 and 0.48 g N/100 g ingested dry matter for corn gluten and urea N, respectively. The relationship between total urinary N losses and truly digested N was determined in which biological value equaled 60 and endogenous urinary N losses totaled 0.34 g N/kg0.80/day (equivalent to 0.040 g N/kg/day). Retained N decreased from 1.09 g during standardization to 0.16 g/kg0.60/day after 1 week of depletion. A subsequent improvement in retained N to 0.36 g in week 3 was followed by a decrease to 0.19 g/kg0.80/day after week 5 of depletion. In repletion, lowest and highest retained N occurred during weeks 1 and 4. Changes in retained N were produced by changes in urinary urea N excretion. Ammonia N accounted for 20, 21, and 24% of the total urinary N voided during standardization, depletion, and repletion. Except for urinary urea N losses that were greater and inversely related to fecal losses in urea-fed steers, dietary N source did not influence the excretion of ammonia, creatinine, or residual N.