The relationship of dietary protein intake to metabolizable protein supply. The two branch points (indicated by 1 and 2) constitute the major variables relating the dietary crude protein supply to the metabolizable protein supply. The first branch point represents the proportion of protein that is degraded in the rumen. This branch point is influenced by inherent properties of the protein and the rate of ingesta passage through the rumen. The second branch point represents the proportion of nitrogen from degraded protein that is recaptured as microbial protein. This is influenced by the microbial growth rate, which depends on the supply of rumen available energy. Nitrogen that is not recaptured as microbial protein is absorbed from the rumen as ammonia and converted to urea by the liver. Some urea is recycled back to the rumen, but a large portion is excreted in urine. RUP, rumen undegraded protein; RDP, rumen degraded protein; N, nitrogen; MCP, metabolizable crude protein; MP, metabolizable protein.