An article in Progressive Dairyman by Dr. Jeremy Schefers of the University of Minnesota Veterinary Diagnostic Laboratory, has shown a rise in the levels of copper concentration in liver samples of dairy cattle. With dairy operations that utilize copper sulfate in footbaths to control digital dermatitis in dairy cattle, the waste water is spread with manure. This has resulted in copper entering the food sources for dairy cattle resulting in this potential problem.
You can read the full article here for more information. Dairy ration copper concentrations: Should we be concerned?