Effect of increasing pasture copper concentrations on the copper status of grazing Romney sheep

Abstract

Application of copper (Cu) fertiliser has previously been shown to increase and maintain pasture Cu concentrations, but to date the Cu status of grazing livestock has not been monitored. In this study, the application of 0.4 to 4 kg Cu ha−1 to twelve experimental paddocks on North Island, New Zealand hill country resulted in mean pasture Cu concentrations of 12.9 to 140 mg Cu kg−1 DM. Herbage Cu levels were maximal 65 days following application and had returned to pre‐experimental levels by 374 days. Romney lambs grazing treated pastures for 176 days had liver Cu concentrations as high as 7750 (imol (450 mg) Cu kg−1 liver FW after 99 days, but showed no clinical signs of chronic Cu toxicity. Herbage Cu concentration (and, by inference, Cu intake) was well correlated with liver Cu accumulation (R 2 = 0.84). An exponential function described the mean pasture Cu concentration and liver Cu concentration relationship. Bile and faecal Cu concentrations reflected Cu intakes, but caeruloplasmin activity, and Cu content of plasma, kidney, and muscle were not affected by treatments. An indirect method to assess animal Cu status, erythrocyte Cu/Zn superoxide dismutase activity, was compared with liver Cu concentration and found to be a predictor of liver Cu stores only in sheep having low to moderate Cu intake. As a method to increase the Cu intake of grazing sheep, Cu fertiliser applied at 1–2 kg Cu ha−1 per annum results in pasture Cu concentrations of 10–20 mg kg−1 DM and has an acceptable margin of animal health safety. In situations of Cu deficiency or chronic Cu toxicity, the most reliable indicator of sheep Cu status is changes in liver Cu concentration.