Forage analysis is the first step to understanding animal protein supplementation requirements. Most producers are familiar with crude protein and understand how this value compares with livestock needs. However, forage reports also show different fractions of the crude protein. Understanding these fractions allows us to adjust animal diets for heat damage, or the protein associated with the fibrous portion of forage which is therefore unavailable to the animal. Further, using the protein fractions aside from the standard crude protein value, we can move forward with balancing diets based on metabolizable protein (MP) as recommended by the National Academies of Science Engineering and Medicine, Nutrient Requirements of Beef Cattle.

Kern rebecca
Animal Scientist / Ward Laboratories Inc.

Crude protein (CP)

First, let us brush up on crude protein. Protein is a large molecule made up of smaller amino acids folded together to form the protein. Amino acids are molecules that contain nitrogen, a carbon bonded to two oxygens and a functional group of carbons. Animals have a requirement for not only protein, but specific amino acids. This is where the concept of the ideal protein comes in. An ideal protein is a protein that provides the right balance of amino acids needed by the animal.

However, when we see crude protein on a forage analysis, it is not referring to amino acids folded into protein. Instead, the emphasis should be placed on the word crude. Crude protein on a forage report is a measure of the percent nitrogen multiplied by 6.25 to get an estimate of protein content. This value can be compared to the animal’s nutrient requirement for crude protein.

Heat-damaged protein (HDP)/acid detergent insoluble crude protein (ADICP)

The first fraction of crude protein you will encounter on a forage analysis is HDP. This is the protein associated with the indigestible acid detergent fiber portion of the feed. This protein often becomes associated with the fibrous portion of the forage through some type of heat damage. Often, this can be caused by baling at the wrong moisture content. Haylages are also more susceptible to heat damage, being stored at a higher moisture content and heating during the ensiling process.

The heating reaction that causes the protein to bind to the fiber in the forage is known as the Maillard reaction. This reaction ties up protein with the lignin in the forage, making that protein unavailable for fermentation by rumen microbes. In addition to reducing the rumen-degradable protein (RDP), it also causes a caramelization of the sugars in the forage. This results in the sweet tobacco smell of heat-damaged forage and makes it more palatable for livestock consumption – therefore, typically increasing forage intake.


Under some circumstances, an adjusted crude protein (ACP) should be used when balancing the ration.

  • If HDP/CP is less than 0.14, then no adjustment is needed.
  • If HDP/CP is between 0.14 and 0.20, then:


  • If HDP/CP is greater than 0.20, then:


Neutral detergent insoluble crude protein (NDICP)

This fraction of the crude protein contains HDP plus the protein bound to the cell wall. The HDP is unavailable to the animal while the difference between NDICP and HDP is slowly digestible protein. Under some nutritional models, NDICP – HDP is used to represent the rumen-undegraded protein (RUP) in the metabolic protein system. 

Insoluble protein

The next two fractions of protein are interrelated. First, insoluble protein is the fraction of the protein that remains after extraction in a laboratory solution. This solution is meant to simulate rumen degradation. This fraction of the protein can be used as an estimate for RUP. This is the portion of the protein that does not get degraded into ammonia, amino acids and nitrogen in the rumen. This portion is also referred to as bypass protein and is absorbed by the animal in the small intestine.

Soluble protein

Soluble protein is the inverse of insoluble protein. It is the fraction of the feed that is removed by a laboratory solution simulating rumen degradation. This fraction is determined by subtracting insoluble protein from total crude protein. Further, soluble protein represents the portion of the feed that rapidly degrades in the rumen. This fraction of protein can be used to estimate rumen-degradable protein (RDP). 

It should be noted that there is some controversy over the use of insoluble protein and soluble protein to represent RUP and RDP in non-forage feeds. These analyses are correlated with RUP and RDP, but they do not account for some attributes of feedstuffs. More work is needed to provide a rapid accurate parameter to determine RUP and RDP.

Metabolizable protein

While the above protein fractions may not be perfect, they do allow for more precise ration balancing and supplementation based on metabolizable protein. Metabolizable protein is the protein absorbed in the small intestine. Also referred to as true protein, this is provided by the RUP and microbial protein. This system of ration balancing considers both the animal’s nutrient requirements and the requirements of the rumen microbes. According to a set of researchers, “Many research works have reported improved herd health, reproduction and profitability when diets were balanced with MP.”

Further, when producers and nutritionists start considering the metabolizable protein as opposed to simple crude protein, they can start to consider amino acid requirements. Ruminant livestock do not utilize the amino acids provided by their diet, but rather those provided by the microbial protein and RUP protein fractions. Supplementation of rumen-protected limiting amino acids may be the next opportunity to improve performance and reach the genetic potential of our beef cattle, sheep and goats. The dairy industry, as usual, has already started utilizing this precision management tool.

In conclusion, there is more to protein than just total crude protein when it comes to livestock nutrient requirements. Crude protein is a great place to start. However, if we understand the protein fractions within our forages, we can ensure livestock requirements are met and improve performance. Starting by adjusting the total crude protein to accommodate protein tied up in fiber, we can ensure animals are not short on protein. By using the soluble and insoluble portions of protein to balance diets and plan supplements around MP, we can ensure livestock health and more efficient forage and feed utilization to improve profitability.

References omitted but are available upon request by sending an email to the editor.