Crude protein

What is crude protein?

Crude protein is an analysis value. The lab measures the nitrogen content of the feed and converts it to protein with a factor of 6.25, because protein contains 16 percent nitrogen on average. The value includes true protein made of amino acids and non-protein nitrogen such as urea or ammonia. Crude protein is abbreviated as CP or XP and is usually given in grams per kg of dry matter or as a percentage of dry matter.

How does a cow use protein?

In ruminants, protein supply runs along two routes. A large part of the feed protein is broken down to ammonia by the microbes in the rumen. The microbes use it to build their own protein, as long as they have enough energy at the same time. Later they are digested in the small intestine. Microbial protein has a very good composition.

A smaller part of the feed protein passes through the rumen unchanged. This bypass protein, also called UDP, is digested directly in the intestine. Rumen-protected protein feeds contain a higher share of it.

Metabolisable protein and rumen nitrogen balance

Ration planning uses two values. Their names differ by country: in the Netherlands they are DVE and OEB, in Germany nXP and RNB:

• Metabolisable protein, the protein available in the small intestine: microbial protein plus bypass protein. This is the value that limits the cow’s production.
• The rumen nitrogen balance: it shows whether there is nitrogen left over in the rumen (positive) or a shortage (negative). A balance around zero to slightly positive is considered balanced.

With a negative balance the microbes lack nitrogen, they grow less well and fibre digestion suffers. With a clearly positive balance more ammonia is produced than the microbes can use. The liver converts it into urea, which costs energy, and the urea is excreted in urine and milk.

How much crude protein does a cow need?

The requirement depends on milk yield. Rations for high-yielding cows are often around 16 to 17 percent crude protein in the dry matter. Dry cows need clearly less. For calf starter around 20 percent is common, because young animals need a lot of protein for growth.

Grass and grass silage often contain a lot of crude protein, maize silage little. Rations with a lot of maize are therefore supplemented with rapeseed or soya meal. Young spring grass, on the other hand, is so rich in protein that the rumen nitrogen balance can become strongly positive.

Milk urea as a check

The milk urea content from milk recording shows how well energy and protein match in the rumen. A guide value is about 150 to 250 mg per litre. If it stays higher, there is usually too much crude protein or too little energy in the ration. If it is lower, the rumen microbes are short of nitrogen. Look at the value together with the milk protein content: low milk protein points more towards an energy shortage.

Too much and too little

A protein surplus puts a load on the liver and metabolism, costs money and increases nitrogen excretion via the slurry. It is also linked to poorer fertility. A shortage lowers feed intake, fibre digestion and milk yield. Around calving, a good protein supply is important so that the cow does not break down too much body tissue.

Frequently asked questions

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