BACKGROUND:Oversupply of dietary copper (Cu) is common among UK dairy herds, but studies on the long-term outcomes of this oversupply are scarce. METHODS:A longitudinal study was undertaken to determine the long-term implications when 80 Holstein‒Friesian heifers with a mean (±standard error) age of 4.1 ± 0.1 months and a mean liveweight of 137 ± 2.4 kg were fed a recommended (R; 16 mg/kg dry matter [DM]) or high (H; 32 mg/kg DM) dietary Cu concentration until 6 weeks prior to calving. RESULTS:Hepatic Cu concentrations in both treatment groups were elevated into the ranges used to diagnose chronic Cu toxicity in cattle at 6.9 months of age (798 ± 46.4 mg/kg DM for H vs. 643 ± 35.4 mg/kg DM for R), with associated evidence of liver damage. Hepatic Cu concentrations then returned to normality but remained higher (p < 0.001) for heifers fed H than for those fed R and were associated with a reduced (p = 0.044) conception rate to first and second services (73.7 ± 8.05% for H vs. 91.2 ± 7.68% for R). LIMITATION:This retrospective analysis identified pre-study liver damage, which may have affected results. CONCLUSIONS:Supplying Cu in excess of requirements resulted in liver damage and reduced conception rates.

To test the hypothesis that Cu metabolism in dairy cows is affected by dietary starch concentration and additional sulfur S and Mo, 60 Holstein-Friesian dairy cows that were [mean ± standard error (SE)] 33 ± 2.5 days postcalving and yielding 41 ± 0.9 kg of milk/d were fed 1 of 4 diets in a 2 × 2 factorial design experiment over a 14-wk period. The 4 diets had a Cu concentration of approximately 15 mg/kg of dry matter (DM), a grass silage-to-corn silage ratio of 1:1, a dietary starch concentration of either 150 g/kg of DM (low starch, LS) or 220 g/kg of DM (high starch, HS), and were either unsupplemented (−) or supplemented (+) with an additional 0.8 g of S/kg of DM and 4.4 mg of Mo/kg of DM. We found an effect of dietary starch concentration on mean reticular pH, which was 0.15 pH units lower in cows fed the high starch diets. The addition of S and Mo decreased intake by 1.8 kg of DM/d, an effect that was evident beginning in wk 1 of the study. Mean milk and fat yields were 37.0 and 1.51 kg/d, respectively, and were not affected by dietary treatment. We found an effect of dietary starch concentration on milk protein concentration, protein yield, and urea nitrogen, which were increased by 2.8 g/kg, 0.09 kg/d, and 2.1 mg/dL, respectively, in cows fed the high starch diets. We found no effect of dietary treatment on either cow live weight or body condition. Mean plasma Cu, Fe, and Zn concentrations were 15.3, 42.1, and 14.4 µmol/L, respectively, and were not affected by dietary treatment. In contrast, we found an interaction between dietary starch concentration and Cu antagonists on plasma Mo, where feeding additional S and Mo increased plasma Mo to a greater extent when cows were offered the high versus low starch diet. We also found that increasing dietary starch concentration increased serum ceruloplasmin activity, but serum haptoglobin concentration was not affected by dietary treatment. The addition of S and Mo decreased hepatic Cu concentration, whereas in cows fed the higher dietary starch concentration, hepatic Cu concentration was increased over the period of our study. We concluded that increasing dietary starch concentration decreases rumen pH and increases milk protein yield and hepatic Cu concentration, whereas feeding additional S and Mo decreases intake and hepatic Cu concentration.