Protein and energy requirements and responses : a United States perspective

A comparison of UK (Agricultural Research Council; ARC) and USA (National Research Council; NRC) feeding standards for dairy cows is made. The two standards appear similar in terms of food energy required for milk production, even though the ARC uses metabolizable energy and the NRC uses net energy. The two systems have the same basic approach to calculating protein supply, although ARC incorporates information on level of food intake in calculating rumen microbial protein synthesis and in estimating undegraded food protein. The maintenance requirement for protein is much less with the ARC system because NRC assigns a large maintenance charge to metabolic faecal nitrogen. The result is that ARC has lower dietary protein recommendations than NRC. Milk production response to incremental additions of energy (grain) or protein diminishes as the point of maximum response is approached. The most profitable level of energy and/or protein supplementation is often below the supplementation level required for maximum milk production. More emphasis must be given to lactation studies for information to improve accuracy of the feeding standards and for construction of milk response curves to incremental additions of supplements.