Three multiparous Holstein cows in midlactation were fitted with rumin
al and duodenal cannulas. Cows were used in an experiment with a 3 x 3
Latin square design to study the effect of corn processing on nutrien
t flow from the rumen to the duodenum and on ruminal fermentation in l
actating cows. COWS were fed diets consisting of 40% forage and 60% co
ncentrate. The only difference in dietary ingredients was the type of
processed corn included at 24% of the total diet (as-fed basis). Treat
ments were dry-rolled corn with a bulk density of 0.54 kg/L and steam-
flaked corn with a bulk density of either 0.39 kg/L or 0.31 kg/L. Rumi
nal fluid pH was not affected by corn processing, but steam-flaking de
creased the molar percentage of acetate and increased the molar percen
tage of propionate. Digestibility of starch in the rumen was not affec
ted by processing. Digestibility of starch entering the duodenum as we
ll as apparent digestibility of starch in the total tract were increas
ed by steam-flaking. Flow of feed and microbial N from the rumen and m
icrobial N efficiency were not affected by grain processing. Yields of
milk and milk components as well as composition of milk were not affe
cted by grain processing.