Effects of corn processing on the site and extent of digestion in lactating cows.

Three multiparous Holstein cows in midlactation were fitted with ruminal 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 nutrient flow from the rumen to the duodenum and on ruminal fermentation in lactating cows. Cows were fed diets consisting of 40% forage and 60% concentrate. The only difference in dietary ingredients was the type of processed corn included at 24% of the total diet (as-fed basis). Treatments 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. Ruminal fluid pH was not affected by corn processing, but steam-flaking decreased the molar percentage of acetate and increased the molar percentage of propionate. Digestibility of starch in the rumen was not affected by processing. Digestibility of starch entering the duodenum as well as apparent digestibility of starch in the total tract were increased by steam-flaking. Flow of feed and microbial N from the rumen and microbial N efficiency were not affected by grain processing. Yields of milk and milk components as well as composition of milk were not affected by grain processing.