Regulatory mechanisms of nitrogen recycling in ruminants

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In beef and dairy cattle, collectively referred to as ruminants, protein that is provided in the diet is required for the production of edible products like meat and milk. However, the efficiency of converting dietary protein into milk or meat protein is poor at only 15 to 30%. In part, this inefficiency of dietary protein utilization arises because microorganisms in the ruminant's digestive tract degrade dietary protein to yield ammonia which they then use to synthesize their own proteins. This results in significant amounts of dietary protein being absorbed into the bloodstream in the form of ammonia, rather than as amino acids which are the building blocks of milk and meat protein. Ammonia that is aborbed into the bloodstream is detoxified via its conversion to urea by the liver, and most of the urea produced is excreted in the urine, thus resulting in an irreversible loss of nitrogen to the animal and also causing environmental degradation. However, ruminants can recycle some of this urea to the digestive tract, where it can be used by microorganisms as a source of nitrogen to synthesize their own microbial protein which, in turn, can be be used by the host animal as a source of amino acids. This recycling mechanism allows ruminants to have several opportunities to capture nitrogen that would otherwise be wasted.