Too little Copper
Copper (Cu) has many roles in metabolism: Copper contributes to enzymes which are used for synthesis of elastic connective tissues, in mobilization of iron from body stores, and in pigment synthesis.
Copper deficiency may lead to osteochondrosis and other skeletal diseases.
Solutions: Good sources of copper are usually the commercial vitamin/mineral mixtures, and commercial concentrates produced for horses.
Too much Copper
Horses can tolerate moderate oversupplies of copper without known negative effects. You should nevertheless always try to formulate rations which provide copper in recommended amounts. "No red lines please!"
Solutions: Study your ration to try and reduce the amount of concentrates or vitamin/mineral mixtures with a high copper content.