Evaluation of nitrogen metabolites as indices of nitrogen utilization in sheep given frozen and dry mature herbages

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Abstract
1. Twenty herbages at different stages of maturity containing from 0·77 to 5·23 g nitrogen/100 g organic matter were harvested, and green herbages were rapidly frozen and stored at - 1·5°. Each herbage was offered at levels approximating to ad lib. to a group of three sheep, individually penned. Intake, digestion and retention of N were studied during a 10 d collec-tion period after a 9 d preliminary period.2. General regressions relating apparently digested N (NA, g/d), urinary N excretion (UN, g/d) and retention of N (NR, g/d) to N intake (NI, g/d) were established: NA= 0·88NI-3·25 (±1·14; SEb= ±O·013); UN = 2·48+0·35 NI+O·0049 NI2(±3·09; SEb1, =±0·156; SEb2= ±0·003); NR = 0·25 NI- 1·92 (±2·40; SEb= ·o·029). These equations accounted for 99% of the variability in N apparently digested, 86% of the variability in urinary N excreted, and 63 % of the variability in N retained.3. Concentrations of ammonia and trichloroacetic-acid-precipitable N in the reticulo-ruminal digesta and of urea and a-amino-N in plasma were determined immediately before feeding, and at I and 4 h after feeding. Relationships between concentrations of metabolites in the reticulo-ruminal digesta and in plasma, and N intake, N apparently digested, urinary N output and retained N were examined. Estimates of urinary N loss and of retained N based upon metabolite concentrations at individual sampling times were characterized by high residual variabilities. Error terms were in some instances reduced significantly when both thebasal (before feeding) metabolite concentrations and the increments in metabolite concentra-tions after feeding were included in the one equation. The best relationships, based on plasma urea concentrations, accounted for only 74 % of variability in urinary N output.4. In multiple regressions a significant portion of the variability in urinary N output and in retained N which was not accounted for by N intake could be accounted for by inclusion of metabolite values.5. The concentration of urea in plasma was the most effective basis for prediction of N utilization. However, no prediction equation accounted for more than 90% of the total variabilityin N output, and no equation accounted for more than 71 % of the total variability in N retention. The feasibility of developing techniques for examination of efficiency of N utilization by a particular class of animal, based upon digesta and plasma metabolite con-centrations is discussed.

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