influence of chemical form of zinc on the effects of toxic intraruminal doses of zinc to sheep

The EDTA, sulphate and oxide compounds of zinc were administered to sheep as a single intraruminal dose (480, 240 or 120 mg Zn per kg body weight) or as thrice‐weekly doses (240 mg Zn per kg body weight per dose) for 4 weeks. In the single‐dose experiment, serum zinc concentrations rose most rapidly, were highest and fell most rapidly in those animals receiving zinc EDTA. In the sheep receiving the sulphate, serum zinc concentrations were high and stayed high for 2 or 3 days. Zinc oxide caused slight, but prolonged, elevation of serum zinc. High concentrations of zinc were present in urine after administration of zinc EDTA. In the multiple‐dose experiments, the sheep dosed with zinc sulphate showed progressively higher elevations of serum zinc (first dose, 5–10 μg Zn ml−1; day 13, 30–60 μg Zn ml−1) and six of the seven sheep so dosed died before the final dose. At post mortem examination, many of these had lesions of the upper gastrointestinal tract and showed evidence of haemolytic crises. Sheep dosed with zinc oxide also had progressively higher peak serum zinc concentrations. In contrast, the peak serum concentrations for the zinc EDTA‐dosed sheep declined as the experiment progressed. No deaths occurred in the sheep dosed with zinc oxide or EDTA. Major pancreatic injury occurred in sheep dosed with the sulphate or oxide, but was only mild in the sheep dosed with the zinc EDTA. Diarrhea was mild and transitory in the EDTA‐dosed sheep, but more severe and persistent in those dosed with the sulphate. Organ zinc concentrations were very high in the sheep dosed with zinc sulphate or oxide and, although less elevated, were still high in the sheep dosed with the zinc EDTA. Mean pancreatic zinc concentrations were highest, with kidney and liver having slightly lower concentrations, for all the zinc compounds administered.