Nitrogen Utilization and Excretion by Dairy Cattle

Nitrogen in the form of amino acids is required by dairy cattle to synthesize protein in body organs (liver, heart, intestine), muscle, and milk in the mammary gland. Protein contains an average of 16% N. Dairy cattle have two major N requirements: 1) the N required by rumen bacteria that digest feeds in the rumen and 2) amino acids required to synthesize protein needed for maintenance, growth, pregnancy, and milk production.

Energy is first limiting in meeting animal requirements for maintenance and production, and protein is second limiting. Therefore the energy and protein requirements must be discussed together, because the protein requirement is based on the energy allowable milk production in lactating cows and growth in replacement heifers. These requirements are described in detail in the National Research Council Nutrient Requirements of Dairy Cattle and The Cornell Net Carbohydrate and Protein System for evaluating diets and nutrient excretion. Energy and protein requirements are typically expressed as concentrations needed in the diet. These dietary requirements are determined as follows.

1. Energy feeds are added to the diet to meet target growth rates or milk production levels.
2. Microbial protein synthesized as the result of fermenting the sugars, starch, and fiber in the feeds fed to meet the energy and fiber requirements is computed.
3. Nitrogen in the form of ammonia and amino acids that will be available for microbial protein production from the energy feeds is computed.
4. Nitrogen in the form of protein that will escape degradation in the rumen but will be digested in the intestines is computed.
5. Feeds high in N sources are added to the diet as needed to meet deficiencies in (a) degradable “protein” (urea or natural protein) and (b) amino acid requirements of the animal that are not met by the bacterial protein and protein in the energy feeds that is digested in the small intestine.