Mathematical linkage of total-tract digestion of starch and neutral detergent fiber to their fecal concentrations and the effect of site of starch digestion on extent of digestion and energetic efficiency of cattle

Published regression equations relating digestibility of starch to fecal starch concentration have slopes that differ by over 5-fold. Hence, nutritionists have questioned their legitimacy. Total-tract starch digestibility and concentration of starch in feces are interlocked mathematically by 2 factors-starch content of the diet and digestibility of diet DM. Digestibility data compiled from the published literature including 201 diets fed to lactating dairy cows and 191 diets fed to feedlot cattle were employed to re-examine the relationships of digestibility to fecal concentrations of starch and NDF. Regression analyses and plots clearly illustrated imprecision of this relationship when diet starch content and digestibility of diet DM were ignored. Furthermore, because fecal starch and diet digestibility are related inversely, mathematics implies that starch digestibility is related curvilinearly to fecal starch concentration. Effects of site of starch digestion on extent of digestion and energetic efficiency also were examined. Direct digestibility measurements for nutrients and energy can preclude errors involved with in vitro availability assays, prove more economical than laboratory procedures to predict nutrient digestibility, and provide more applicable data concerning energy availability when compared with summing tabular means for feedstuffs from publications or from computerized diet formulation programs. When combined with DMI, direct digestibility measurements should markedly improve precision for quantifying amounts of nutrients and energy available for maintenance and performance by productive ruminants in dairies or feedlots. As with all analytical procedures, accurate digestibility measurement requires representative sampling and proper analysis of diets and feces.