Q&A: Understanding Fiber Types

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The fiber fractions found in plants are cellulose, hemicelluloses, and lignin. Some fiber is digestible (hemicellulose and some of the cellulose), and some is not (lignin). There is currently no exact method of measuring the proportion of digestible fiber (by microbial fermentation) to indigestible fiber in forage. The old “crude fiber” measure is grossly inaccurate. At this time, acid detergent fiber (ADF) and neutral detergent fiber (NDF) are used, and these measure combinations of fractions of fibers.

 NDF: includes low digestibility or indigestible cell wall/structural fiber so includes cellulose, hemicelluloses, and all lignin. High NDF means low digestibility (these are all elements within the cell wall).

 ADF: lignin, cellulose, some ash, and silica. High ADF means low digestibility.  Hemicellulose is estimated by subtracting ADF from NDF (these are elements within the cell wall of the plant).

 ADF and NDF measurements of fiber content give an indication of digestibility and palatablilty of forage.

 NDF is a measure of the plant’s cell wall content where the higher the value, the less the horse will eat of the hay (less digestible).

 ADF is a measure of the fiber concentration shown as a percent where, as ADF increases, digestibility and nutrient availability decreases.

Hays with ADF levels above 45% have little nutritive value (lower than 31% are excellent).  Hays with NDF values of 65% or greater are not very palatable (below 40% are excellent).

Essentially, the change that you are seeing from one hay test to the next has to do with the maturity of the hay at the time it is harvested. Different cuttings throughout the year will have different fiber fractions relative to the maturity of the plant at the time of cutting.  With alfalfa, the detergent fiber levels (ADF and NDF) will increase with an increase in the amount of stems in the hay. Consequently, the more stems, the more lignin (an indigestible fiber component), which gives stiffness to the stems, and the less digestible fiber will be present.