Standardization of procedures for nitrogen fractionation of ruminant feeds

The Cornell Net Carbohydrate Protein Model (Chalupa et al., 1991; Sniffen et al., 1992) has developed the need for uniform procedures to partition feed nitrogen into A, B, and C fractions (Pichard and Van Soest, 1977). While carbohydrate fractions are relatively standardized (based on NDF, ADF with corrections for ash, protein, and lignin), the fractionation of plant nitrogen has been open to considerable variation in procedures. This has led to non-uniformity among reported values for nitrogen fractions. This paper recommends reliable procedures for nonprotein nitrogen (NPN) and buffer-soluble protein. These procedures have been examined for reproducibility and relevance to biological expectations. Procedures for acid-detergent insoluble nitrogen (ADIN), and neutral-detergent insoluble nitrogen (NDIN) are also included as they are required for the model. Some alternatives in certain procedures are offered.