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Endogenous Zinc Excretion and ⁶⁵Zn Metabolism in Holstein Calves Fed Intermediate to High but Nontoxic Zinc Levels in Practical Diets¹

Department of Dairy Science, University of Georgia, Athens, Georgia 30601

Zinc and ⁶⁵Zn metabolism, from a single intravenous dose (given 7 days after diets were initiated) were studied in Holstein bull calves fed a practical diet containing 38 ppm zinc or the same diet with 200 or 600 ppm supplemental zinc (zinc oxide). The supplemental zinc increased endogenous fecal ⁶⁵Zn excretion by 30% with 200 ppm added zinc having as much influence as 600 ppm. The supplemental zinc had a very variable effect on ⁶⁵Zn distribution in different tissues 14 days after dosing. With some tissues including testicle, rumen wall and abomasum (fundic) each increase in dietary zinc reduced ⁶⁵Zn content. In muscle, dietary zinc level had no effect on ⁶⁵Zn concentration. Adding 200 ppm zinc to the diet increased ⁶⁵Zn and stable zinc in pancreas, liver and kidney; however, a further increase to 638 ppm caused very large increases in these tissues indicating a breakdown in homeostatic control for both injected and absorbed zinc. With the 638 ppm diet smaller increases in ⁶⁵Zn occurred in bone and small intestine. Serum ⁶⁵Zn increased with the high zinc diet. Except for muscle, where there was no change, specific activity in every tissue studied decreased with each increase in dietary zinc.

KEY WORDS: • zinc • endogenous excretion • metabolism

¹ Journal Series Paper no. 1098, University of Georgia College of Agriculture Experiment Stations, College Station, Athens. Supported in part by Public Health Service Research Grant no. AM-07367-NTN from the National Institute of Arthritis and Metabolic Diseases.

Manuscript received 1 June 1971.