Influence of corn and sorghum starch on the in vitro kinetics of forage fiber digestion.

In vitro digestion experiments were conducted to examine the interactions of starch source and pH on kinetics and apparent extent of ruminal NDF digestion. Alfalfa hay, bromegrass hay, and each hay with either raw sorghum, raw corn, or pure corn starch in ratios to simulate a 28% NDF diet were incubated at pH 5.5, 6.2, or 6.8. Ash-free NDF was measured at 0, 6, 12, 18, 24, 30, 36, 48, 72, and 96 h of fermentation; logarithmic transformation and linear regression were used to estimate digestion parameters. Significant forage by starch interactions on lag and rate of NDF digestion and significant forage and starch by pH interactions on rate and potential extent of NDF digestion indicated that forage type must be considered when the effect of pH and starch source on NDF digestion is predicted. For both forages, raw or pure corn starch increased lag more than sorghum starch, especially at low pH. Raw sorghum starch decreased the rate of NDF digestion more for bromegrass hay than for alfalfa hay, and pure corn starch had the largest negative impact on fiber digestion, especially at low pH. Sources of starch influenced lag and rate of NDF digestion differently at pH from 6.8 to 5.5, which led to dramatic differences in apparent extent of ruminal NDF digestion.