All amino acids, including the essential ones. are present in bacterial protein in proportion that is fairly close to the proportion of amino acids required by the mammary gland for milk synthesis. Thus, the conversion of dietary protein to bacterial protein is usually a beneficial process. The exception occurs when high quality protein is fed and the ammonia produced in the rumen cannot be utilized due to a lack of fermentable energy. On the average, for every increment of 1 kg of dry matter ingested by the cow, there is an increase of 33 g of body protein lost in the intestine and excreted in the feces. When fermentable energy is lacking or when crude protein in the diet is excessive or highly degradable, not all the ammonia produced in the rumen may be converted to microbial protein.
Excess ammonia cross the ruminal wall and is transported to the liver.
The liver converts the ammonia to urea which is released in the blood. It can be excreted into the urine by the kidneys. When urea returns to the rumen, it is converted back to ammonia and can serve as a nitrogen source for bacterial growth. Urea excreted in the urine is lost to the animal. With rations low in crude protein, most of the urea is recycled and little is lost in the urine.