Beliveau, R M. and McKinnon, J. J. 2009. Effect of graded levels of wheat-based dried distillers' grains with solubles on rumen fermentation in finishing cattle. Can. J. Anim. Sci. 89: 513-520. A 4 x 4 Latin Square experiment using ruminally cannulated heifers (388 +/- 25 kg, mean +/- SD) wits conducted to evaluate the effects of graded levels (0, 7, 14, 21%, DM basis) of wheat-based dried distillers' grains with solubles (DDGS) on rumen fermentation characteristics and eating behaviour of cattle fed barley-based diets. Mean rumen pH at cutoff values of pH 5.8 or less (P = 0.02) and pH 5.5 or less (P<0.01) and pH area between 5.2 and 5.5 (P=0.02) decreased cubically as DDGS content increased with maximum pH at 2.6% and minimum pH at 14.7% DDGS inclusion levels. A cubic effect of DDGS inclusion level (P=0.03) was noted for time below pH 5.2 where local minima and maxima were found at 2.9 and 15.0% DDGS. Ammonia N (P<0.01) and the acetate:propionate ratio (P=0.01) increased, while propionate decreased (P<0.01) linearly as DDGS content increased. It was concluded that despite the high fibre, low starch nature of wheat-based DDGS, substitution for barley did not improve the acidic rumen fermentation conditions associated with feeding barley-based finishing diets.