Protein Utilization

Knowledge Nuggets

• Forages contain crude protein (nitrogen x 6.25) in the form of both true protein and non-protein nitrogen (NPN), both of which can be used by cattle.

• Young cattle with immature rumens cannot efficiently utilize non-protein nitrogen.

• Forages provide protein to cattle either as protein that is degraded by the ruminal microbes (degraded protein) or as small nitrogen bearing compounds such as peptides, amino acids and ammonia that are absorbed by the intestine (undegraded protein).

• Peptides, amino acids and ammonia are utilized by the cow, but are also used by the microbes to form new protein and to provide protein that escapes microbial breakdown in the rumen, called bypass protein.

• Rapid and extensive ruminal breakdown of crude protein in forages leads to decreased protein efficiency because the ruminal microbes do not use the degraded protein as fast as it is broken down. This leads to formation of ammonia.

• Although some ammonia is used by microbes to form new protein, most ammonia is absorbed from the rumen, carried by the blood to the liver and converted to urea and excreted in the urine.

• Some urea will "recycle" to the rumen, either via the saliva or directly from the blood where it is broken down to ammonia. In cattle fed low protein diets, urea recycling is an important means of salvaging nitrogen that would otherwise be lost from the animal.

• Sometimes it is beneficial to feed a protein source that is resistant to microbial breakdown in the rumen. Protein bypasses the rumin and is absorbed in the intestines which improves the overall availability of it to those animals with high protein demands (growing calves, lactating cows). Higher bypass protein feeds include alfalfa dehy pellets, beet pulp and distillers grains.

• Forages typically have high levels of easily degraded protein and low levels of undegradeable or bypass protein. Once processed with heat, such as when forages are pelleted, most of the protein becomes denatured making it less digestible by microbes. As a result, a greater percentage of the protein stays intact during the digestive process and gets absorbed in the small intestine.

• Because of the higher cost of feeds that contain bypass protein, the economic advantages do not exist in most beef feed scenarios. Bypass protein may offer economic advantages when feeding high producing dairy cows.