Postprandial changes of selected blood and ruminal metabolites in ruminating calves fed diets with or without hay

Eight ruminally cannulated Holstein bull calves were used in a switchback design to evaluate effects of hay on postprandial changes in ruminal pH, ammonia and VFA, blood ketones, and plasma glucose, NEFA, VFA, and urea N. Calves were fed 4.5 kd/d of calf starter with or without hay for ad libitum consumption. Blood and ruminal fluid were sampled every 2 h for 12 h on d 21 (periods 1 and 2) or d 13 (period 3). Ruminal pH declined from 6.6 at 0 h to 5.3 by 2 h. Changes in ruminal and plasma VFA were consistent with low pH and rapid fermentation of soluble carbohydrate. Ruminal ammonia, molar percentage butyrate, and blood ketones, plasma urea N, and plasma molar percentage butyrate were lower when hay was fed. Postprandial increases in blood beta-hydroxybutyrate and acetoacetate were reduced when hay was fed, and correlated .65 and .50, respectively, to ruminal butyrate. Concentration of beta-hydroxybutyrate averaged 1.24 and 1.87 mmol/L at 4 h postfeeding. These data suggest that rapid consumption of limited amounts of grain increased in ruminal VFA and blood ketones; increase in ketones was smaller when hay was included in the diet.