Copper and zinc balance in exercising horses fed two forms of mineral supplements

This study was undertaken to compare the absorption and retention of copper and zinc when supplemented to exercising horses in the form of sulfate or organic-chelate mineral supplements. Nine mature horses were used in a modified-switchback designed experiment consisting of seven 28-d periods. Horses were fed a diet consisting of 50% coastal Bermudagrass and 50% concentrate. All diets were balanced to meet the energy, protein, calcium and phosphorus requirements for horses performing moderate to intense exercise. Copper and zinc supplementation varied by period. During mineral depletion and repletion periods, horses respectively consumed diets with no supplemental mineral or Cu and Zn supplemented in the sulfate form to provide 100% of NRC (1989) values. In periods 4 and 7, horses were fed diets designed to provide 90% of NRC (1989) values for Cu and Zn supplied in the sulfate or organic-chelate forms. Horses were subjected to a standard exercise test on d 23 of periods 4 and 7 followed by a 4-d total fecal and urine collection. Blood samples were drawn every 28-d for determination of plasma Cu, Zn and ceruloplasmin concentration, and white blood cell counts and Cu,Zn-superoxide dismutase activity were evaluated in periods 4 and 7. Copper and zinc balance was determined from feed, fecal, urine and water samples obtained during the total collections in periods 4 and 7. Copper and Zn intake and fecal excretion were greater (P<0.05) for horses consuming the organic-chelate supplemented diet. Apparent Cu absorption as a percent of intake and retention as a percent of intake were also greater for this group. Plasma Cu, Zn and ceruloplasmin concentration was not different for horses consuming the two mineral supplement forms. White blood cell counts and superoxide dismutase activity were not affected by diet treatment. Formulation error and suspected sample contamination made it difficult to compare absorption and retention of Cu and Zn, but the use of a controlled repletion-depletion diet sequence appeared to be an effective experimental design component.