Understanding Analysis for Horse Forages

Two values that can be calculated from a forage analysis are especially useful in evaluating its quality for use by horses.

First, the amount of soluble carbohydrate or sugar can be calculated. The original way to calculate this was as nitrogen free extract (NFE) = 100 – crude protein – crude fat – crude fiber – ash. This is not a good way to estimate soluble sugar because a lot of the cell wall components end up in the NFE estimate because they aren’t recovered in the crude fiber fraction. True cell soluble sugars are completely digestible by horses, so any of the cell wall that escapes crude fiber detection is falsely estimated to be completely digestible. A much better way to estimate this fraction is to substitute neutral detergent fiber (NDF) for crude fiber. By doing this, what is left by subtraction is likely to be readily digestible by the horse.

The second value that is often calculated for forages is energy content. This is expressed as either digestible energy (DE) in calories or joules per kilogram or pound or as percent total digestible nutrients (TDN %). For all practical purposes, these various expressions of dietary energy are interchangeable. TDN % X 4.4 = DE (Mcal/kg) or TDN % X 2 = DE (Mcal/lb). What is important to realize is that both DE and TDN are only estimates of the actual digestible energy content of forages. They are calculated based on the relationship between selected assay results and the digestible energy content of feeds determined in actual digestion trials with animals.