The effect of high pasture molybdenum concentrations on the copper status of grazing horses in New Zealand

The objective of this study was to determine whether Mo causes secondary Cu deficiency in pasture-fed horses. Twelve Thoroughbred weanling fillies wer e divided into two groups and fed either high Mo pasture (8 to 15 mg Mo kg( -1) DM) of a low Mo pasture (0.6 to 1.2 Mo kg(-1) DM) for 84 days. Both pas tures contained 6 to 9 mg Cu kg(-1) DM. The animals were challenged with Cu over the last 14 days to determine change in their Cu status to 1.0 mg Cu kg(-1) liveweight d(-1) given orally three times a week. Copper status was assessed from liver, plasma, blood cell, and plasma trichloroacetic acid-in soluble Cu concentrations as well as the enzyme activities of plasma caerul oplasmin and red blood cell superoxide dismutase. None of these indices was affected by the increase in Mo intake. Horses grazing pasture high in Mo r esponded no differently to Cu challenge than did those on low Mo pasture. T he results of this study suggest that Mo is unlikely to increase dietary Cu requirements and induce secondary Cu deficiency at the Cu and Mo concentra tions normally found in New Zealand pastures grazed by Thoroughbreds.