Insoluble Nitrogen for Milk Production in Holstein Cows via Increases in Voluntary Intake and Nitrogen Utilization

Influences of insoluble and soluble N on milk production and N use were studied in 24 Holstein cows. The basal diet (grain: corn silage plus urea, 1:1 dry matter) contained 12% crude protein (60% insoluble N). Urea or soybean meal was added to the basal diet, increasing the crude protein to 15% with 43 and 67% insoluble N, respectively. Balance studies were performed during wk 4, 10, and 22 of lactation. Multiple regression analyses were performed on balance data adjusted for metabolic size to determine coefficients of soluble and insoluble N use. Intake of N, partitioned into insoluble and soluble N, accounted for 76, 30, 61, and 10% of the variation in dry matter intake, milk yield, milk N, and body weight^.75, respectively. Utilization coefficients for insoluble N were higher for dry matter intake, milk N, and weight^.75 but lower for milk yield. Urea supplementation increased dry matter intake; however, much of the milk yield by cows fed urea was attributed to a greater weight^.75 loss. In contrast, cows fed soybean meal as a source of insoluble N maintained both weight^.75 and higher milk production because of higher dry matter intake and more efficient use of N.