The estimation of duodenal utilizable true protein in sheep fed mixed rations using the nitrogenous fractions of the Cornell Net Carbohydrate and Protein System

The objective of the experiment is to study the relationship between the nitrogenous fractions of the Cornell Net Carbohydrate and Protein System (CNCPS) and the duodenal utilizable true protein (uTP) in sheep, and the possibility of predicting duodenal uTP flow based on the CNCPS nitrogenous fractions intake. Twelve typical mixed rations for sheep were formulated as experimental rations and the chemical composition of the rations, i.e. CP (crude protein), SCP (soluble crude protein), NDICP (neutral detergent insoluble crude protein) and ADICP (acid detergent insoluble crude protein) was analyzed. The CNCPS nitrogenous fractions, i.e. PA (non-protein nitrogen), PB1 (rapidly rumen degradable crude protein), PB2 (intermediately rumen degradable crude protein), PB3 (slowly rumen degradable crude protein) and PC (bound crude protein) were calculated using the CNCPS based on the chemical composition of the rations. The duodenal uTP of sheep was determined in an earlier study using three sheep in four 3 × 3 Latin square designs by Li and Zhao [Li, Y.X., Zhao, G.Y., 2007. The prediction of the uTP of mixed rations for sheep using in vitro incubation technique. Arch. Anim. Nutr. 61(2), 103–113]. Statistical analysis indicated that the duodenal uTP flow (g/day) was closely correlated with the nitrogenous fractions intake of the sheep (g/day) in a multiple way:uTP=9.27PA−35.76PB1+10.12PB2−2.20PB3−37.38PC−16.82, r2=0.94, P=0.001, n=12uTP=9.27PA−35.76PB1+10.12PB2−2.20PB3−37.38PC−16.82, r2=0.94, P=0.001, n=12where the unit for all indices is g/day.It was concluded that the duodenal uTP flow of sheep fed mixed rations could be reliably estimated based on the CNCPS nitrogenous fractions intake using the model above.