Metabolisable Protein (MP)
Metabolisable Protein (MP): is a measure of total amino acids that are available for intestinal digestion to meet maintenance, growth, production and foetal growth. While most feed analysis reports crude protein (CP) as the base units available, the way protein is utilised by the cow is quite complex, meaning that not all CP is actually utilised. Lactating cows have a requirement for absorbed amino acids, not CP.
MP is evaluated by the sum of microbial crude protein (MCP) flowing from the rumen plus the digestible rumen undegraded protein (dRUP) from feed consumed. MCP is either limited by the amount of effective rumen degradable protein (ERDP) available in a ration or by the amount of fermentable carbohydrate (fCHO) available to the rumen bugs to capture ERDP. If there is inadequate fCHO relative to ERDP, nitrogen (CP) is recycled as urea, which is often excreted via faeces. Forages are often high in soluble protein which is very difficult for rumen bugs to utilise, even with available fCHO.
It is possible to have a ration very high in CP but still not meet the MP requirements of a high production dairy cow. Plus fecal nitrogen excretion is an energy burning exercise. Hence rations should be assessed for MP supply to ensure that requirements are met for the breed, stage of lactation and level of productivity.