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 microbialN supply in sheep: comparison with the prediction from the DVE/OEB model

This study examined (1) the effects of dietary protein variation in terms o f net truly digestible intestinal protein (DVE value) and rumen degraded pr otein balance (OEB value) on the concentrations and daily excretion of urin ary products (creatinine, allantoin, combined value for uric acid, hypoxant hine 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 ex cretion of urinary products and microbial N supply on the other hand; (3) t he comparison of two different methods in their ability to predict microbia l N supply. Sheep were fed a fixed quantity of oaten straw and leucrene plu s a daily supplement of either of the following: no legume seeds (CTRL), ra w whole lupin seeds (RWLS), roasted WLS (HWLS), raw whole faba bean (RWFB) or roasted WFB (HWFB). All diets were isonitrogenous (15.9% CP) but differe d in their DVE and OEB values. In the supplemented diets, about 55% of tota l protein was supplied by whole lupin seed (WLS) or whole faba bean (WFB) p rotein. The amount of legume seeds per metabolic liveweight (WS: 20 g per L W0.75; WFB: 25 g per LW0.75) was kept the same throughout the study. Microb ial N (MNPD) estimated from the urinary excretion of total PD using the pre diction model of Chen and Gomes [Estimation of microbial protein supply to sheep and cattle based on urinary excretion of purine derivatives-an overvi ew of the technical details. Occasional Publication, International Feed Res ources Unit, Rowett Research Institute, Aberdeen, UK, 1992], was compared w ith microbial N (MNFOM) calculated according to the DVE/OEB model. Results showed that the diets had significant effects on the concentrations of urin ary creatinine, allantoin, the combined value for uric acid, hypoxanthine a nd 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 lo wer (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 v ersus 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 allant oin 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 MNPD based on total PD was also not affected and amounted to 11.2 g per day, varying from 5.4 to 16.2 g pe r day. Of the five concentrations of urinary products (creatinine, allantoi n, 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 corr elation between those five concentration parameters in urine and dietary in takes (OM, digestible OM, N, digestible N) and DVE and OEB values and N bal ance were also poor with the highest R-2<0.40. Comparing two different meth ods in their ability to predict microbial N, estimates of microbial N from urinary excretion of total PD was more variable than the potential microbia l N supply calculated according to the DVE/OEB model, which resulted in a p oor correlation between MNPD and MNFOM. Results from this study suggest that the concentrations of those five urina ry 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 ma rginally affected by large variations in DVE and OEB supply. (C) 2001 Elsev ier Science B.V. All rights reserved.