If you’ve noticed that your forage is testing better based on Neutral Detergent Fiber (NDF) levels, don’t get too excited. It may be just dirt.
Forage labs are using a new technique to determine NDF which removes soil contamination. It’s referred to as “aNDFom” or “ash free NDF” or NDF on an organic matter (om) basis.
Most minerals are dissolved by neutral detergent, so they don’t end up in NDF (sometimes referred to aNDF). However, sand or silica is left behind so it is reported as part of the NDF. Sand is obviously not fiber, so there has been some inherent inaccuracy in NDF, depending on the amount of contamination from dirt. The new ash corrected NDF, or dirt corrected NDF will more accurately predict cattle performance. It is used in the latest modeling programs.
Corn silage, grass, and alfalfa differ in the average amount of soil contamination. According to Dairyland Labs, the aNDFom averaged 0.84% less for corn silage, 1.23% less for grass and 1.98% less for alfalfa than the old NDF method.
Some forage samples have been as much as 10% lower for aNDFom compared to aNDF. This means there’s a lot of dirt contamination in those samples. Small grain forages like wheat, rye, and oats and also sorghum silage have had the greatest discrepancies between aNDF and aNDFom.
Now, if you change one side of the equation, you need to change the other side. If we have redefined what NDF is, we also need to redefine what standards we use for NDF in our rations. Hence, current recommendations for NDF as a percent of ration dry matter, NDF intake and forage NDF intake on a percent of body weight also must be redefined. Most ration NDF levels are 1-2% lower on a dry matter basis when aNDFom is used, but they could be even lower if dirt contamination has been a problem. A ration that used to be 29% NDF, may now be 27-28% aNDFom.
So, your forages may not necessarily be better. Just the numbers have changed. If the lab report shows both aNDF and aNDFom, the difference is dirt. The new method will better estimate performance, since dirt doesn’t make milk.