To measure aNDF, samples are boiled in a neutral detergent solution with amylase and sodium sulfite. This detergent dissolves non-fiber components like sugar, starch, protein, and fat as well as some components of ash. The portion of the sample that does not dissolve is weighed and quantified as "aNDF" (Neutral Detergent Fiber).
In samples that have significant soil contamination aNDF may not be an accurate characterization of the fiber content of the feed; because soil components like silica do not dissolve in neutral detergent. When formulating rations this can lead to an overestimation of the fiber content and to an underestimation of the energy value, because soil contamination will also be accounted for in the ash component. Both of these errors will encourage a formulation of diets that are too "hot" with more energy and less effective fiber than intended.
In order to correct for soil contamination neutral detergent residues can be "ashed" at 550°C for 2 hours. By subtracting this final residue from the original aNDF value, we can determine a more accurate characterization of the fiber content of the feed "aNDFom".
For most feeds, the difference between aNDF and aNDFom is approximately 1% of DM. However, in hay and small grain silages the difference can easily be 5-10% of DM. Forages with high soil contamination can cause significant overestimations of fiber content when formulated for aNDF instead of aNDFom.
For example, this simplified ration is formulated to contain 28% aNDF. This may be a reasonable amount of fiber for a lactating dairy ration; however, if the forage is this specific sample from our wet chemistry dataset, the actual fiber content that the cow utilizes will be 23% instead of 28%.
|Ration Formulated using aNDF|
|% aNDF||% of Ration|
|Ration Experienced by the Cow|
|% aNDFom||% of Ration|
While this is an extreme example of a ration with only one forage and a very high soil contamination, forages like this are not uncommon on today's beef and dairy farms. For example, 6% of haylages and 15% of small grain silages submitted to Dairyland Laboratories contain more than 14% ash.
aNDFom can directly replace aNDF for most purposes including NRC and OARDC energy calculations, RFQ, and CNCPS based modeling programs for more accurate characterization of feeds.