Protein

Proteins are required by the dairy cow for many metabolic functions e.g. growth, lactation, and reproduction.

The protein content in feed is commonly reported as crude protein (CP). Crude protein is not a direct measure of the protein, but rather an estimate based on the nitrogen (N) content. Protein contains 16% N, so crude protein is calculated by multiplying the N content in the feed by 6.25.

Crude Protein requirements

The amount of crude protein (CP) a dairy cow requires depends on:

stage of lactation
milk production
energy status of the cow
quality of the feed
amino acid profile of the diet
ambient temperature
activity

Generally, in pasture-based systems, dietary protein does not limit milk production.

Exceptions to this are when:

high levels of low-protein supplement (e.g. barley, fodder beet, maize silage) are feed
during dry summers when the crude protein content of pasture declines, and low-protein supplements are fed.

Crude Protein components

Crude protein includes the true proteins (nitrogen plus amino acids) which can be further broken down into rumen degradable proteins (RDP), which includes soluble proteins and, undegradable dietary proteins (UDP). Crude protein also contains non-protein nitrogen (NPN) and undigestible dietary protein.

Cows cannot store protein, so they need a constant supply from the diet; however, they differ from single-stomached mammals in that they do not need this protein to contain pre-formed amino acids this is because they can use non-protein nitrogen (NPN) sources (e.g. urea) to produce amino acids and generate microbial protein.

Ruminants are also able to recycle nitrogen for later use. If cow protein requirements are greater than protein supply, large amounts of nitrogen can be recycled back to the rumen for use by the rumen microbes, whereas in non-ruminants, nitrogen is almost entirely lost in the urine.