Regulatory mechanisms of nitrogen recycling in ruminants

在肉牛和乳牛中，统称为反刍动物，日粮中提供的蛋白质是生产肉类和牛奶等可食用产品所必需的。然而，将饮食蛋白质转化为牛奶或肉类蛋白质的效率很低，仅为15%至30%。在一定程度上，这种食物蛋白质利用效率低下的原因是反刍动物消化道中的微生物将食物蛋白质降解为氨，然后再用氨来合成自己的蛋白质。这导致大量的饮食蛋白质以氨的形式被吸收到血液中，而不是以氨基酸的形式被吸收，氨基酸是牛奶和肉类蛋白质的组成部分。流入血液的氨通过肝脏转化为尿素来解毒，产生的大部分尿素被尿液排泄，从而导致动物不可逆转的氮素损失，也导致环境退化。然而，反刍动物可以将部分尿素循环到消化道，在那里微生物可以将其用作氮的来源，以合成自己的微生物蛋白质，而这些蛋白质又可以被宿主动物用作氨基酸的来源。这种回收机制让反刍动物有几次机会捕获原本会被浪费的氮。

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.