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SummaryFollowing duodenal dosing, 65Zn was rapidly accumulated by several calf tissues, with a considerable amount in the liver and other tissues within 1 hr. With adequate dietary zinc, liver contained a substantial quantity of labile zinc which was greatly depleted in calves fed a zinc-deficient diet for 2 weeks. Other soft tissues had a very limited amount of labile zinc. Zinc turnover rate was much slower in muscle than in the softer tissues, and was not greatly affected by dietary zinc level. Transfer of zinc from the intestinal mucosa to the blood appeared to be the rate-limiting step in zinc absorption. This step was accelerated by feeding a zinc-deficient diet for 2 weeks. However, 65Zn uptake by the mucosa from the intestinal contents was vastly increased by the zinc-deficient diet.

SummaryAbsorption and metabolism of 65Zn in the natural chemical form of forage and as 65ZnCl2 were studied in Holstein calves following single tracer oral doses. Chemical form did not affect 65Zn excretion or total retention but 65Zn was higher in calves given 65Zn as forage in metabolically more active tissues (liver, spleen, heart, lungs, duodenum, and small intestines, p < .05). Thus, chemical form of 65Zn dose affected tissue distribution but not absorption. It is theorized that 65Zn in the two chemical forms was metabolized differently after absorption due to different 65Zn-ligand combinations being transported from the intestine to the tissues.