A KINETIC-MODEL OF COPPER-METABOLISM IN LACTATING DAIRY-COWS

A model of Cu metabolism in dairy cows was developed using a stable isotope, Cu-65, as an in vivo tracer. Copper-65 was administered intravenously to six dairy cows fed a diet of corn silage plus grain concentrate containing 10 mg Cu, 1.1 mg Mo and 2.2 g S kg-1 diet dry matter. The distribution of tracer and the natural concentration of Cu in liver, plasma and milk were monitored for 140 d (urine and feces for 5 and 7 d). A kinetic modeling program (SAAM/CONSAM) was utilized for development of the model. Seven compartments, including liver, two plasma compartments, remaining body, gut, milk (including udder) and feces were required to describe the experimental data. The model included five routes of Cu loss: biliary and nonbiliary fecal excretion, urinary excretion and two routes into the milk/udder compartment. The solution to the model indicated that 5.1 +/- 1.5% (mean +/- SD, n = 6) of dietary Cu was absorbed from the gut; 92.2 +/- 5.0% of the absorbed Cu was removed directly by liver; 38.4 +/- 13.3% of Cu leaving the liver entered the nondirect-reacting (NDR) Cu compartment of plasma; 80.7 +/- 9.0% of NDR Cu entered the remaining body compartment while the balance of the NDR Cu supplied the milk/udder compartment. The transfer of Cu from liver to the NDR Cu in plasma appeared to be a point of metabolic control in Cu metabolism, and was represented in the model by an empirical nonlinear function.