Influence of nonfiber carbohydrate concentration on forage fiber digestion in vitro

We quantified the effect of dietary nonfiber carbohydrate (NFC) concentration (30, 35, 40, or 45% of DM) on in vitro digestion kinetics of neutral detergent fiber (NDF) from alfalfa and corn silages at pH 5.8 or 6.8. The objective was to simulate the effect of diets differing in effective fiber content on the optimal NFC-to-NDF ratio that resulted in maximal fiber digestion. Ash-free NDF was determined at 0, 3, 6, 9, 12, 24, 30, 36, 48, 72, and 96 h of fermentation. Kinetic parameters were estimated using logarithmic transformation and linear regression. The optimal NFC-to-NDF ratio for maximal NDF digestion differed between the two forages. For alfalfa fermented at pH 6.8, apparent extent of ruminal NDF digestion was greatest between 30 and 40% NFC, but at pH 5.8 NDF digestion was greatest at 35% NFC. For corn silage fermented at either pH 6.8 or 5.8, apparent extent of NDF digestion was greatest at 30% NFC. An NFC-to-NDF ratio of 0.70-1.20 maximized NDF digestion for alfalfa only when fermentation pH was maintained at 6.8. These in vitro results demonstrate that the optimal dietary NFC content for maximum NDF digestion in the rumen for a particular forage will be a function of fermentation pH that reflects the physically effective NDF content of the diet. (C) 2000 Elsevier Science B.V. All rights reserved.