Five multiparous lactating Holstein-Friesian dairy cows fitted with rumen cannula were allowed to graze perennial ryegrass swards. Next to a control treatment of grazing only, pelleted barley (PB), pelleted maize (PM), toasted and subsequently pelleted barley (TPB), and toasted and subsequently pelleted maize (TPM) were fed as a supplement in two equal portions. Before and after 3 h of grazing the rumen content was evacuated, weighed, sampled and returned to the animals. Then the cows were kept inside the barn and starved for 6 h, after which rumen evacuations were repeated. The estimated clearance rates of starch showed significant differences (P<0.001) between grain types and compared to unsupplemented animals the apparent ruminal clearance of nitrogen was significantly (P<0.001) reduced. Supplementation with processed grains significantly increased (P<0.05) the milk production and significantly (P<0.001) decreased milk fat percentage. It is concluded that supplementing grass with high-energy low protein feeds, such as grains, substantially improves the N utilization and reduces the urea output in milk.