Nitrogen (N) is a key nutrient for dairy cattle and is the building block of amino acids and proteins in animals and plants. Nitrogen in feed is either used to support milk and milk protein production or it is excreted via urine and feces. Crude protein (CP) is the term commonly used to describe the N content of feeds and rations and is calculated as N content * 6.25 (e.g., a forage with 2.88% N contains 18% CP). Nitrogen is also important for soil nutrient cycling, crop growth, and the environment. Excess N excreted by the cow is a concern for both air (ammonia) and water quality. In addition, excreted manure N contributes to greenhouse gas emissions by contributing to the production of nitrous oxide, a potent greenhouse gas. The goal of feeding dairy cows is to provide adequate N to support maintenance, growth, reproduction, and milk production while minimizing excretion of N to the environment.