The inefficiency of starch utilisation by ruminants fed readily fermentable cereal grains, such as barley, decreases the metabolic potential of such feeds and can cause serious dysfunctions related to acidosis or pre-acidosis status in animals with higher energy requirements. The rate and extent of ruminal disappearance of grain dry matter (DM) are determined largely by the morphological features of the seed endosperm, but the presence of polyphenols may also constitute a limiting factor in ruminal hydrolysis. To assess the impact of tannins on the rate and extent of ruminal fermentation of barley grain, commercial grade tannic acid (TA) was included at 0, 1.0, 2.5 or 5.0% (wt/wt, DM basis) with ground barley grain in incubation in situ, and disappearance of DM and crude protein were monitored over 48 h. A dose-dependent effect of TA on in situ degradation of barley meal was observed; significance was attained (p < 0.05) at the 5% treatment level. Scanning electron microscopy revealed limited microbial hydrolysis of endosperm cell walls in TA-treated samples, although TA did not prevent microbial attachment to or hydrolysis of starch granules. Tannins may be effective for slowing ruminal disappearance of barley to improve starch utilisation by ruminants. Copyright © 2005 Society of Chemical Industry