Varying Levels of Urea for Dairy Cows Fed Corn Silage as the Only Forage

Ninety-one lactating Holstein cows were used in three experiments to study effects of dietary urea on milk yields and composition, voluntary consumption of corn silage, concentrations of rumen volatile fatty acids, and nutrient balances. The per cent of the total ration nitrogen furnished by urea ranged from 0 to 48%. The effect of concentrate intake in response to urea in high com silage rations also was investigated. Marked depressions in milk yields occurred when urea furnished 48 and 38% of the ration nitrogen. Significant decreases compared to controls were also noted when urea furnished 21 to 23% of the ration nitrogen, whereas no effect on production was detected at the 11% level. In general, voluntary intake of corn silage was not adversely affected by urea additions. However, silage consumption was depressed on high-concentrate rations which contained urea. Rumen valerate and isovalerate tended to be lower for urea-containing rations, but acetate and propionate were not affected. Small, but significant depressions in dry matter, crude fiber, and nitrogen-free extract (NFE) digestibilities resulted from urea additions as low as 11% of the ration nitrogen. Of the ingested nitrogen, the per cent recovered in milk was significantly lower and that in urine significantly higher when urea provided 22% of the dietary nitrogen compared to the control and 11% treatments.