Glucose Metabolism in Cows Fed Low— and High—Roughage Diets

Six lactating first-calf Holstein cows were used to test the effect of dietary roughage on glucose metabolism. Cows were fed either a low-roughage or high-roughage diet at isocaloric digestible energy intakes in a double changeover design experiment. Mean values (plus or minus standard deviation) for milk yield (kg/day), fat (%), lactose (%), and protein (%) for cows fed low-roughage were 19.0 plus or minus 4.4, 3.11 plus or minus .78, 5.19 plus or minus .27, 3.44 plus or minus .48; values for cows fed high-roughage were 17.5 plus or minus 5.1, 3.99 plus or minus .58, 4.94 plus or minus .25, and 2.78 plus or minus .33. One hour post-feeding on the 20th day of each period 2 mCi of tritiated glucose were administered to each cow by single injection to measure glucose kinetics. Mean values (plus or minus standard deviation) for plasma concentration (mg/100 ml) pool size (mg/kg), half-time (min), and utilization rate (mg/kg--75 per min) of glucose, and plasma insulin concentration (muU/ml) for cows fed low-roughage were 63.1 plus or minus 3.9, 17.9 plus or minus 3.4, 30.4 plus or minus 5.2, 8.55 plus or minus 2.44, and 22.0 plus or minus 3.9; for cows fed high-roughage values were 54.9 plus or minus 2.2, 114.5 plus or minus 17.2, 40.0 plus or minus 2.2, 4.06 plus or minus .38, and 16.2 plus or minus 2.4. A glucose load was administered intravenously to each cow on the last day of each period. Glucose half-times and mean plasma insulin following the clearance test were not affected by diet. Compared to high-roughage, low-roughage diets greatly affect metabolism in lactating cows when isocaloric intakes of each are fed. Fat depression, however, may or may not occur simultaneously.