MP is not crude protein

Crude protein is a convenient lab number: nitrogen × 6.25. It does not tell you how much true protein the cow absorbs after rumen microbes and bypass protein finish their work. That absorbed supply is closer to what nutritionists call metabolizable protein (MP) — and the amino acid profile of that MP is what builds milk protein.

Much of dietary protein is broken down in the rumen. Microbes rebuild amino acids into microbial protein, which later digests in the small intestine. Some feed protein escapes (“bypass”) and also contributes amino acids downstream. The cow’s usable amino acid pool is the sum of those streams — not the crude protein on the tag.

When diets are short on key essential amino acids relative to MP — lysine and methionine are the usual first limits in many North American dairy diets, and histidine can be co-limiting — milk protein yield and nitrogen efficiency suffer even if crude protein looks “high enough.” NASEM (2021) moves the conversation toward essential amino acid supply and response, not a single CP finish line.

Exact MP and amino acid targets belong with the herd nutritionist and the model they use. The atlas lesson is directional: stop treating crude protein as the finish line. Ask whether the rumen is fed for microbial protein, whether bypass protein is intentional, and whether Lys/Met (and sometimes His) balance is part of the conversation.