A mechanistic model is presented to adjust the fractional rate of fermentation of available fiber based on the estimate of unavailable fiber and the computed theoretical unavailable fiber using lignin content. The object of this computer program is to generate a system by which values for rate of digestion and its retardation can be predicted and used in the field. At the present time there is no system for deriving digestion rates apart from those in the feed dictionary. Analytical values available in the field, include NDF, ADF, lignin, crude protein (CP) and sometimes in vitro digestibility at 24 or 30 h. Rates of digestion and DU will need to be predicted from these observations.