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

Abstract

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.