THE INFLUENCE OF THE DIETARY BALANCE BETWEEN ENERGY AND PROTEIN ON MILK UREA CONCENTRATION - EXPERIMENTAL TRIALS ASSESSED BY 2 DIFFERENT PROTEIN EVALUATION SYSTEMS

Twentythree dairy cows were fed rations with different proportions of energy and digestible crude protein (DCP). When the ration was balance d for energy and DCP according to Swedish standard the cows' milk urea concentration was 4.66-4.92 mmol/1 (95% CI of mean). With increasing intakes of DCP, fed together with standard levels of energy, the mean milk urea concentration increased in proportion to the surplus of DCP. In contrast, the concentration of urea decreased when the cows were o verfed with energy at the same time as they were underfed with protein . When the rations were recalculated in accordance with the AAT/PBV sy stem for dietary protein evaluation the 95% CI for the mean milk urea concentration of the cows receiving a balanced ration was 3.76-4.56 mm ol/1. The concentration of urea was dependent primarily on the PBV Whe n the 2 protein evaluation systems were compared there was a strong co rrelation between PBV and DCP. Ammonia was the only constituent of the rumen whose concentration was strongly correlated with the milk urea concentration. Taken together with earlier data the present results su ggest that a milk urea concentration between 4.0 and 5.5 mmol/1 should be regarded as normal at least when cows are fed conventional feedstu ffs.