The detergent system of forage analysis, which was developed by Peter Van Soest at Cornell University, involves boiling forages in neutral or acid detergents. Neutral detergent (ND) solubilizes minerals, proteins, sugars, fats, and pectin, while the residue insoluble in ND, known as neutral detergent fiber (NDF), primarily consists of hemicellulose, cellulose, and lignin. Acid detergent (AD) solubilizes minerals, proteins, sugars, fats, pectin, and hemicellulose, leaving a residue known as acid detergent fiber (ADF) composed of cellulose and lignin. Although NDF should typically exceed ADF due to the inclusion of hemicellulose, exceptions exist.

In a recent study from our laboratory, for example, we fed some high-producing dairy cows alfalfa hay that contained 38% NDF (ash-corrected) and 36.8% ADF and others grass hay that contained 63.2% NDF (ash-corrected) and 29.4% ADF. A minimal difference existed between NDF and ADF in the alfalfa hay, whereas a substantial difference existed between NDF and ADF in the grass hay. Even more, there are instances in which a forage analysis report can provide an ADF concentration higher than the NDF concentration. This is a result that conceptually seems to make little to no sense.

So, how can we explain that ADF concentration is higher or similar to NDF concentration? In brief, while almost entirely solubilized in ND, not all pectin is solubilized in AD. In addition, while soluble in ND, tannins are insoluble in AD. Hence, the insolubility of some cell wall components in AD might result in residues containing more than just cellulose and lignin.

To finalize, what should we do when we observe these results? The best advice is to repeat the analysis in a sequential manner, meaning that the ADF analysis should be performed over the residue remaining after the NDF analysis. By doing this, the components insoluble in AD will be solubilized a priori in the ND during the NDF analysis.