Nitrogen use in dairy cows

N use in dairy cattle basically consists of two elements:

• The first of these is to provide an adequate supply of N and carbohydrates to support the growth of rumen microorganisms and the production of microbial crude protein (MCP).
• The second part of this system involves the use of amino acids found in the small intestine to meet the cow’s nutritional requirements.

Nitrogen fractions in feed

Although all foods contain N, the amounts in which it found in different feeds, its availability and how it is used by dairy cows can be highly variable. Forage analysis laboratories determine the amount of N in analyzed samples and multiply this value by 6.25 to obtain the crude protein (CP) value.

Calculation example: Alfalfa silage = 3% N * 6.25 = 18.75% CP (both on a DM basis)

Although different feeds may have the same CP value, the nutritional value for the dairy cow will be different depending on the feed. Let’s look at the following examples:

• Alfalfa hay, alfalfa silage and alfalfa grass – All have 20% CP.
• Raw and roasted soybeans – Both with 40% CP