Glucose Metabolism in Cows Fed

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 highroughage diet at isocaloric digestible energy intakes in a double changeover design experiment. Mean values (+ standard deviation) for milk yield (kg/day), fat (%), lactose (%), and protein (%) for cows fed low-roughage were 19.0 + 4.4, 3.11 + . 78, 5.19 + . 27, 3 . 4 4 + . 48;values for cows fed high-roughage were 17.5 + 5.1, 3.99 + . 58, 4.94 + . 25, and 2.78 + . 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 (+ standard deviation) for plasma concentration (mg/lO0 ml) pool size (mg/kg), half-time (min), and utilization rate (mg/kg -75 per min) of glucose, and plasma insulin concentration (~tU/ml) for cows fed low-roughage were 63.1 + 3.9, 17.9 + 3.4, 30.4-+ 5.2, 8.55 -+ 2.44, and 22.0 + 3.9; for cows fed high-roughage values were 54.9 -+ 2.2, 114.5 + 17.2, 40.0 + 2.2, 4.06 + . 38, and 16.2 -+ 2.4. A glucose load was administered intravenously to each cow on the last day of each period. Glucose halftimes and mean plasma insulin following the clearance test were not affected by diet. Compared to high-roughage, lowroughage diets greatly affect metabolism in lactating cows when isocaloric intakes of each are fed. Fat depression, however, may or may not occur simultaneously. I N T R O D U C T I O N Increasing the proportion of grain or decreasing the proportion of roughage in a lactating cow's diet is a primary cause of depressed milk fat secretion. Reasons are usually related to the fact that decreased roughage intake affects carbohydrate digestion which increases availability of glucogenic precursors and reduces concentrations of circulating precursors required for milk fat synthesis. Although low-roughage diets increase plasma glucose and insulin in lactating cows, until recently there has been no evidence to show that low-roughage diets also increase glucose utilization (or entry) rates in lactating cows or ruminants in general. Evans and BuchananSmith (9) have reported that low-roughage diets increased plasma concentration, pool size, and utilization rate of glucose, and depressed halftime in sheep. Bickerstaffe et al. (4) have reported that low-roughage diets increased glucose utilization rates in lactating cows. Decreasing the roughage content of a ruminant 's diet is usually associated with increased digestible energy (DE) concentration, which is correlated positively with rates of glucose utilization in ruminants (16). Therefore, the effect of a low-roughage diet on glucose metabolism in ruminants may be attributed simply to in . creasing intake of digestible energy alone, rather than any effect on carbohydrate digestion. The purpose of our experiment was to compare the effects of isocaloric intakes (DE basis) of a lowand high-roughage diet on glucose metabolism in lactating cows. Glucose metabolism was evaluated using both [2-3H] glucose as a tracer and with a clearance test. E X P E R I M E N T A L PROCEDURES Received October 8, 1974. Animals and Diets Six lactating first-calf Holstein cows between 7 and 14 wk postpartum were used in an