Maximum safe dietary magnesium and effects of high dietary magnesium on zinc metabolism in Holstein calves.

Fifteen male Holstein calves were fed diets containing .25 (control), .7, or 1.15% magnesium (from supplemental magnesium oxide) for 28 days. Feed consumption and growth rate were not affected adversely by .7% magnesium but were depressed with 1.15% magnesium. Fecal dry matter percentage was reduced slightly with .7% magnesium and substantially decreased with 1.15% magnesium. However, feces from calves fed .7% magnesium were more fluid in consistency. Urinary and fecal magnesium content increased in calves fed .7 and 1.15% magnesium, with changes closely related to dietary magnesium. Magnesium in plasma increased slightly with .7% magnesium and materially with 1.15%. Magnesium in liver, kidney, and heart was not affected by diets. Apparently calves can tolerate safely .7% magnesium, but 1.15% is detrimental. On day 21 of treatment, all calves received zinc-65 orally and were killed 7 days later. Calves fed .7 and 1.15% magnesium excreted less zinc-65 and retained more, especially in liver and large intestine. Liver and kidney of calves receiving higher percents magnesium had elevated stable zinc. Calcium and copper in tissue were not elevated. The effect of high dietary magnesium on zinc metabolism appears to be systemic in tissues.