SOME REMARKS ON THE ANALYTICAL PROCEDURE OF VANSOEST FOR THE PREDICTION OF FORAGE DIGESTIBILITY

The solubility of ash in neutral and acid detergent was determined with 137 feeds. The solubility of soil particles was very low, so that almost all ash derived from soil contamination was determined as NDF [neutral detergent fiber] and ADF [acid detergent fiber]. The consequences for the cell contents and the different cell wall components and for the prediction of the digestibility with the summative equations were investigated and discussed. To determine the true digestibility of the cell contents and the quantity of endogenous excretions plus undigested microbial residue, the Lucas test was applied to the cell content of 161 diets. Despite high R2 and low Sy .cntdot. x N.A., the Lucas test gave unrealistic results, and evidence was given that this test was not appropriate to determine true digestibility of the cell contents nor the quantity of endogenous excretions plus undigested microbial residue. True digestibility of the cell contents apparently increased with increasing percentage of cell contents, or the quantity of endogenous excretions plus undigested microbial residue decreased with increasing percentage cell contents, or both happened together. Some possible explanations for these facts were given, and the influence on the prediction of apparent digestibility with the summative equations was discussed. At higher feeding levels apparent digestibility of the cell contents slightly decreased.