Practical aspects of urea and ammonia metabolism in ruminants

Nitrogen was recognized over 200 yr ago as an element essential for normal function of farm animals. During the first half of the 19th century, the roles of proteins and urea in N metabolism were discovered. By the middle of the 20th century, the substrates, products, and enzymes of the urea cycle were elucidated. Work since then has quantified dietary crude protein requirements for specific production goals, protein synthesis and breakdown, ruminal ammonia production, endogenous urea synthesis, and urea recycling. In ruminants fed conventional diets, N absorbed as ammonia can be several times the amount of N absorbed in the form of amino acids or peptides. Nitrogen recycled to the digestive tract as urea in saliva or urea transported from blood ranges from 10 to 40% of N consumed in feed. Under production conditions, from 0 to 20% of N consumed by ruminants is retained as tissue N or excreted as milk protein. This review describes the quantitative aspects of urea and ammonia metabolism in ruminants and it relates the metabolic or economic costs of that metabolism to practical feeding situations. The review concludes with a discussion of conflicts and considerations among three main priorities in ruminant N metabolism: 1) maximizing microbial function in the rumen; 2) optimizing amino acid supply to the host ruminant; and 3) minimizing negative environmental effects of cycling N through ruminant production systems.