Effects of High but Nontoxic Levels of Zinc in Practical Diets on 65Zn and Zinc Metabolism in Holstein Calves

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Abstract
Zinc and 65Zn metabolism was studied in 16 Holstein bull calves fed a practical diet (33 ppm Zn) or the diet supplemented with 200 or 600 ppm zinc from zinc oxide or 600 ppm from zinc sulfate. None of the supplemental zinc treatments substantially affected zinc content of muscle, heart, tibia joint, tibia shaft or testicle samples obtained 21 days after initiation of dietary treatments. Even though 233 ppm zinc caused large increases in zinc content of pancreas and liver, 633 ppm resulted in increases many times as great. High zinc diets decreased 65Zn absorption. Thus, high zinc diets decreased the proportion while increasing the total amount of zinc absorbed. In all tissues studied, feeding the 233 ppm diet reduced 65Zn content 14 days after a single oral dose. In contrast, increasing dietary zinc from 233 to 633 ppm greatly increased 65Zn accumulation in liver, pancreas and kidney. The 65Zn and stable zinc data indicated that some control mechanism which limited zinc build-up was overcome with the 633 ppm diet in these tissues but not in many others. Zinc oxide and zinc sulfate had comparable effects on zinc levels in most tissues. Relative to zinc oxide, zinc sulfate tended to reduce 65Zn content of most tissues and to increase endogenous 65Zn excretion.