Digestion kinetics of fiber: influence of in vitro buffer pH varied within observed physiological range

In vitro buffer pH reflective of the diurnal variation in ruminal pH was evaluated for its impact on digestion kinetics of NDF from three forage sources. Alfalfa hay, bromegrass hay, and corn silage were incubated in phosphate-bicarbonate buffer solution adjusted to pH 6.8, 6.5, 6.2, 6.0, 5.8, or 5.5 using 1 M citric acid. Ash-free NDF was measured after 0, 6, 12, 18, 24, 48, 72, and 96 h of fermentation. The experiment was replicated three times; kinetic parameters of fiber digestion were estimated by logarithmic transformation, and by linear and nonlinear regression procedures. Lag in NDF digestion increased as pH fell from 6.8 to 6.5 and again when pH decreased below 6.0. Decreasing buffer pH below 6.2 dramatically reduced NDF digestion rate for alfalfa hay and corn silage but had no significant effect on NDF digestion rate of bromegrass hay. Lag in NDF digestion increased below pH 6.0 for both alfalfa and corn silage but increased only when pH fell below 6.2 for bromegrass. Results suggest that lowered pH exerts its negative effect on NDF digestion between pH 6.2 and 5.8, as evidenced by increased lag and decreased rate, and that critical pH and specific, affected component of digestion varied among forages.