Characteristics of developmental changes in the kinetics of glucose and urea in Japanese Black calves: comparison with Holstein calves

The current study was conducted to clarify the characteristics of glucose and urea kinetics in male Japanese Black calves, using a glucose and urea dilution method with stable isotopes, at preweaning (3 wk of age) and postweaning (13 and 26 wk of age) stages, in comparison with the kinetics of glucose and urea in male Holstein calves. Six Japanese Black and 6 Holstein calves were arranged in a 2 (breed) x 3 (stage) factorial block design. These 12 suckling calves were fed only whole milk, offered concentrate and orchardgrass hay after 3 wk of age, and weaned at 6 wk of age. Under steady-state conditions, glucose challenges (1.0 mg/kg of BW; [U-[sup.13]C]D-glucose) and urea challenges (both 2.2 mg/kg of BW; [[sup.13]C]urea and [[sup.15][N.sub.2]]urea) were performed at the 3 stages examined. There were no significant differences in plasma glucose concentrations between Japanese Black and Holstein calves at any stage, but the glucose concentrations at 3 wk of age were greater (P < 0.05) than those at 13 wk of age in both breeds. The glucose pool size in Japanese Black calves was smaller (P < 0.05) than that in Holstein calves at all stages. Within each breed, there were no significant differences between the glucose pool sizes at 3 and 13 wk of age, but the glucose pool size was larger (P < 0.05) at 26 wk of age. Moreover, Japanese Black calves had greater glucose irreversible loss rates (P < 0.01) than Holstein calves at 3 wk of age, and the glucose irreversible loss rates were less (P < 0.05) on and after 13 wk of age in both breeds. Japanese Black calves had greater plasma urea N concentrations (P < 0.05) than Holstein calves at all stages, and in both breeds, the urea N concentrations increased (P < 0.05) with age. The urea pool size was smaller (P < 0.01) in Japanese Black calves than in Holstein calves at all stages, and in both breeds, the urea pool size increased (P < 0.05) with age. In comparison with Holstein calves, Japanese Black calves had greater urea irreversible loss rates (P < 0.05) on and after 13 wk of age and greater urea recycling rates (P < 0.05) at 26 wk of age. In addition, in both breeds, urea irreversible loss rates and urea recycling rates increased (P < 0.05) with age. We conclude that Japanese Black calves have partially different glucose and urea kinetics from Holstein calves and that the kinetics of these metabolites in both Japanese Black and Holstein calves are strongly influenced by weaning. Key words: breed, glucose kinetics, irreversible loss rate, Japanese Black calf, recycling rate, urea kinetics