Protein supply from undegraded dietary protein

A summary of in vivo estimates of the amount of dietary protein from individual feedstuffs that escapes microbial degradation in the rumen is presented. Values range from approximately 20% for protein in barley, oats, wheat, and alfalfa silage to 65 to 70% for protein in fish meal and animal by-products. In vitro or in situ methods for estimating protein degradation can be used, but at this stage of development, the methodology is more useful in providing a relative ranking of feedstuffs on the basis of protein degradation than in providing absolute estimates of protein degradation. A number of factors influence protein breakdown in the rumen, including extent of crosslinking in the protein (disulfide bonds), retention time in the rumen, protein solubility, and processing and storage effects on protein. It is important to consider the amino acid content of the undegraded dietary protein, particularly lysine and methionine, two amino acids likely to be limiting for milk production. Strategies for using protected proteins in dairy cattle diets are discussed.