The role of liver Cu kinetics in the depletion of reserves of Cu in dairy cows fed a Cu-deficient diet

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AIM: To determine how the concentration of Cu in liver affects the rate of depletion of that Cu when cows are fed a Cu-deficient diet under experimental conditions, and to mathematically model the rate of depletion of Cu over time. METHODS: In June 2010, 25 non-lactating Friesian cows were assigned to three groups such that initial mean concentrations of Cu in liver were 265, 534 and 1,486 μmol Cu/kg fresh tissue (Day 0). All cows were managed as a single group and fed a Cu-deficient diet of primarily baled silage. No mineral Cu supplements were given. Liver biopsies were collected from cows on Days 0, 53, 98 and 161 to determine concentrations of Cu. At about the same time, samples of silage and pasture herbage were collected to determine Cu, Mo and S concentrations. RESULTS: Median concentration of Cu in silage was 6.5 (min 6, max 9) mg/kg DM. Concentration of Cu in liver decreased in all groups (p<0.001), over the duration of the study. The amount of Cu depleted from liver was greater in groups that started the study with higher initial concentrations of Cu in liver. The rate of decline followed exponential first-order kinetics with an elimination rate constant k of 0.0057 (CI 95%=0.0039–0.0074), meaning that about 0.57% of liver Cu reserves were depleted each day. For individual cows this loss amounted to 0.1–14 μmol Cu/kg liver/day. CONCLUSIONS AND CLINICAL RELEVANCE: Depletion of Cu from liver was dependent on initial concentration of Cu. These results can be used to predict how long an unsupplemented herd will remain in adequate Cu status, which adds confidence to decisions about when Cu supplementation should be withdrawn or reinstated. Cows with high concentrations of Cu in liver can maintain adequate Cu status for months without supplements. Intake of less Cu and more Mo would increase the rate of depletion, and seasonal factors would also have some influence.