Effect of dietary protein variation in terms of net truly digested intestinal protein (DVE) and rumen degraded protein balance (OEB) on the concentrations and excretion of urinary creatinine, purine derivatives and microbial N supply in sheep: comparison with the prediction from the DVE/OEB model

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This study examined (1) the effects of dietary protein variation in terms of net truly digestible intestinal protein (DVE value) and rumen degraded protein balance (OEB value) on the concentrations and daily excretion of urinary products (creatinine, allantoin, combined value for uric acid, hypoxanthine and xanthine, total purine derivatives (PD)) and microbial N supply in sheep; (2) the relationship between the concentration of urinary products on the one hand and daily intakes, protein values (DVE and OEB), urinary excretion of urinary products and microbial N supply on the other hand; (3) the comparison of two different methods in their ability to predict microbial N supply. Sheep were fed a fixed quantity of oaten straw and leucrene plus a daily supplement of either of the following: no legume seeds (CTRL), raw whole lupin seeds (RWLS), roasted WLS (HWLS), raw whole faba bean (RWFB) or roasted WFB (HWFB). All diets were isonitrogenous (15.9% CP) but differed in their DVE and OEB values. In the supplemented diets, about 55% of total protein was supplied by whole lupin seed (WLS) or whole faba bean (WFB) protein. The amount of legume seeds per metabolic liveweight (WLS: 20 g per LW 0.75; WFB: 25 g per LW 0.75) was kept the same throughout the study. Microbial N (MN PD) estimated from the urinary excretion of total PD using the prediction model of Chen and Gomes [Estimation of microbial protein supply to sheep and cattle based on urinary excretion of purine derivatives — an overview of the technical details. Occasional Publication, International Feed Resources Unit, Rowett Research Institute, Aberdeen, UK, 1992], was compared with microbial N (MN FOM) calculated according to the DVE/OEB model. Results showed that the diets had significant effects on the concentrations of urinary creatinine, allantoin, the combined value for uric acid, hypoxanthine and xanthine, and total PD. In comparison with the CTRL, supplementation was higher ( P<0.01) in DVE value (72.9 g per day versus 45.9 g per day) and lower ( P<0.01) in OEB value (24.8 g per day versus 42.2 g per day) and there was an increase ( P<0.01) in the concentrations of creatinine (3.24 mmol/l versus 1.72 mmol/l), allantoin (13.26 mmol/l versus 7.85 mmol/l), PD (16.59 mmol/l versus 9.23 mmol/l). However, there was no effect ( P>0.05) on allantoin to creatinine concentration and PD to creatinine concentration ratios. Diets had no effects ( P>0.05) on urinary excretion of creatinine, allantoin and total PD, which averaged 2.67±1.51, 10.51±3.41 and 13.13±4.04 mmol per day, respectively. Therefore, the MN PD based on total PD was also not affected and amounted to 11.2 g per day, varying from 5.4 to 16.2 g per day. Of the five concentrations of urinary products (creatinine, allantoin, PD, allantoin:creatinine ratio and PD:creatinine ratio) evaluated, none of these could predict daily output of urinary creatinine, allantoin, total PD and microbial N with good precision with the highest R 2<0.25. The correlation between those five concentration parameters in urine and dietary intakes (OM, digestible OM, N, digestible N) and DVE and OEB values and N balance were also poor with the highest R 2<0.40. Comparing two different methods in their ability to predict microbial N, estimates of microbial N from urinary excretion of total PD was more variable than the potential microbial N supply calculated according to the DVE/OEB model, which resulted in a poor correlation between MN PD and MN FOM. Results from this study suggest that the concentrations of those five urinary products had no predictive power on their own and could not predict the microbial N supply with high accuracy, and urinary allantoin to creatinine and PD to creatinine ratios in sheep are fairly constant and may be only marginally affected by large variations in DVE and OEB supply.