Understanding Crude Protein and Crude Fiber in Horse Diets

Crude protein is called “crude” because the assay used in its determination doesn’t actually measure protein at all. Instead, the analysis used by most laboratories measures nitrogen. Protein value is calculated by multiplying the figure for nitrogen by 6.25. There are other substances in forages that contain nitrogen, so this analysis is subject to some error.

Most of the protein in forages is in the cell contents. This protein is readily digested by the horse’s proteolytic enzymes. The digestibility of protein found in the cell contents is 80% or higher. Some protein in forages, however, is incorporated into the cell wall. This protein is called unavailable protein because it is completely indigestible. Unavailable protein is measured by running a nitrogen analysis on the acid detergent fiber (ADF) fraction of the forage. It can be expressed as ADIN (acid detergent insoluble nitrogen). ADIN is usually produced from heat damage in a chemical reaction between carbohydrate and protein.

Crude fiber is a very old assay that is part of the labeling requirements for many animal feeds. This assay doesn’t really measure anything in particular, and it fails to account for some indigestible cell wall components in different types of forages. For example, mature alfalfa (lucerne) hay contains 45% cell wall, but only 27% crude fiber. Beet pulp contains 47% cell wall, but only 17% crude fiber. Crude fiber always underestimates the actual fiber content of a forage, and while the crude fiber measurement must appear on labels, it is not a number that is particularly helpful in understanding the analysis of a horse feed.

Information in this article was published in Advances in Equine Nutrition. This volume is available from Kentucky Equine Research.