Characterization of proteins in feeds according to the CNCPS and comparison to in situ parameters

Several methods were used for the evaluation of feed protein for rumen degradability and intestinal digestibility of rumen undegraded protein (RUP). The aim of this work was to explore the possibility of using the Cornell method of predicting the fraction RUP. An in situ method was adopted for estimation of degradability parameters (a, b, c) and effective degradable crude protein (EDCP), and five nitrogen fractions (A, B1, B2, B3, and C) were determined according to the Cornell Net Carbohydrate and Protein System (CNCPS). Fifty-one feedstuffs – 13 cereals grains, 5 legume seeds, 3 oilseeds, 11 oilseed by-products, 4 distillers dried grains with solubles (DDGS), and 15 silages (maize, lucerne, grass, and grass-clover) – were used in this work. The examined feedstuffs varied widely in nutrient composition. Fraction B1 (soluble true protein) in forages was small (2.5–5.7% of total N), but it varied to a large extent in concentrates (DDGS 0.9–1.2, legumes 46.5–63.7, oilseeds meal 17.1–51.8% of total N). Fraction B2 represented a large proportion of the total protein in oil seed meals (44.3–82.6% of total N) and in DDGS (55.8–77.8% of total N), too. Fraction B3 was relatively small (less than 10% of total N) in all feedstuffs and declined with increasing acid detergent insoluble nitrogen (ADIN) concentration. The concentration of ADIN fraction in feeds affected ruminal degradability. Lucerne silage, with a high content of ADIN (30.9% of total N), had a low effective crude protein (CP) degradability (57%). Correlation between EDCP and fraction A was r = 0.76. A weaker correlation (r = 0.67) was found between in situ parameter “b” and fractions B2 and B3 (r = 0.59), respectively. The results show that much more samples of all feed types should be analyzed to obtain results allowing a more exact prediction of CP degradability and RUP.