In these heat-induced chemical reactions, amino acids from protein react with sugar to form complex nitrogen-containing compounds that are similar to lignin and have very low digestibility. These compounds often smell sweet like caramel, produce various flavors, and can turn the forage to a tobacco-brown color.
Emeritus Professor / Extension Forage Specialist / University of Nebraska – Lincoln

Livestock often find such hay or silage palatable, but they may not provide as much nutrition as expected. The microbial activity that produces the heat consumes valuable sugars and the chemical reaction caused by this heat makes some of the protein become indigestible.

While standard forage tests for fiber and fiber digestibility predict energy available from heat-damaged forage just as well as they predict energy from undamaged hay and silage, tests for crude protein cannot distinguish between regular crude protein and heat-damaged protein. As a result, a standard forage test can mislead you into thinking you have more usable protein in your forage than actually is there.

Forage tests done using NIRS often report heat-damaged protein. If the heat-damaged protein is high enough (usually above 10 percent of the crude protein since all forages contain some unavailable crude protein), the test also will report an adjusted crude protein value that is lower than the regular crude protein value.

However, the NIRS test for heat-damage and the resulting protein adjustment may not be accurate enough if the ration contains much of this forage and the amount of crude protein in the ration is barely enough for the cattle. In these situations, a chemical analysis is needed for heat-damaged protein.


Fortunately, chemical lab tests for heat-damaged protein are available readily that can be used to effectively adjust crude protein values. Besides heat-damaged protein (HDP), labs also use terms like acid detergent fiber crude protein (ADF-CP), acid detergent insoluble nitrogen (ADIN), and insoluble crude protein (ICP) to describe this compound.

When heat damage is suspected, request an appropriate chemical analysis from the lab so crude protein adjustments can be made correctly then use these adjusted values to formulate rations.  FG

Bruce Anderson, Ph.D.
Agronomy and Forage Specialist

University of Nebraska – Lincoln