Undegradable Protein in Ruminant Nutrition

The small intestine is the main place of digestion. Given the special physiology of ruminants, the protein reaching the small intestine is not identical to the protein contained in the feed. Rumen microbes utilize soluble dietary protein for synthesis of microbial protein. Hence, soluble proteins are degraded by proteolytic microbial enzymes to peptides, amino acids and ammonia, whereby also volatile fatty acids and carbon dioxide are formed.

Protein that is not degraded in the rumen and reaches the small intestine unmodified, is called rumen bypass protein or undegradable protein (UDP, see Figure). Here, it is digested by enzymatic hydrolysis as well as microbes contained in digesta. Consequently, the metabolizable protein in the ruminant is the sum of microbial protein and UDP.

It goes without saying that the ruminal microbes need an adequate supply of N to sustain the delicate balance between the ruminant and the ruminal microbes. In this context, it is noteworthy that microbes need sufficient energy supply for synthesis of microbial protein.

Important: protection of valuable proteins

Nonetheless, when it comes to costly protein sources, it is desirable to protect this valuable protein from rumen degradation. As each feedstuff has varying amount of rumen degradable protein, the content of UDP differs between varying feedstuffs (e.g. 35% with soybean meal, 15% with grass silage or wheat protein). Furthermore, the ammonia that results from the microbial protein degradation needs to be detoxicated. It is transported to the liver, transferred into urea and consequently excreted via urine or transported back into the rumen where it is converted back into ammonia where it can be again used by microbes.

Undegradable Protein in Ruminant Nutrition - Image 1

This ammonia circle seems pretty efficient at the first glance. And so it is - if poor protein sources are used. As soon as high quality protein sources are fed you don´t want the microbes to use the protein but the ruminant. Additionally, the described ammonia circle costs energy and if high amounts of degradable proteins are fed, it will result in an increased liver load.

Hence, some conditions were developed under which dietary protein escapes microbial breakdown:

Addition of secondary plant metabolites. They bind the proteins and the latter one become insoluble in the rumen but can still be digested in the small intestine
Chemical treatments, as formaldehyde
Heat treatments which also make the dietary protein highly insoluble

To sum it up: UDP is the part of dietary protein that passes intact from the rumen to the duodenum. It is important to supply the ruminal microbes with sufficient nitrogen while at the same time avoiding too high amounts of degradable protein. Several measurements can be undertaken to increase the amount of UDP.

Undegradable Protein in Ruminant Nutrition - Image 2