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.