65Zn and Stable Zinc Absorption, Excretion and Tissue Concentrations as Affected by Type of Diet and Level of Zinc in Normal Calves

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Abstract
The effects of dietary zinc level (2 vs. 38 ppm) in a purified diet and of a practical versus a purified diet with the same zinc level (38 ppm) on 65Zn metabolism were determined following a single oral dosing in 2.5- and 4.5-month-old male Holstein calves. The 12 animals were killed 14 days after 65Zn dosing and before those given the zinc-deficient diet developed any clinical indication of a deficiency. Calves fed the low zinc purified diet absorbed considerably more 65Zn, but there was little overall difference between groups fed the practical-type diet and the purified diet with added zinc. Those given the practical diet had a higher fecal endogenous 65Zn excretion the second week after dosing. Younger calves in the first experiment absorbed a much higher percentage of 65Zn from diets with adequate zinc than older ones in the later experiment. There was no difference in net 65Zn retention due to age among those given the zinc-deficient diet, suggesting that differences associated with age were not caused by reduced ability of animals to absorb zinc but rather to effects of other factors on the homeostatic control mechanism. Tissue samples from calves given the deficient diet retained much more of the administered 65Zn dose in all tissues than those given zinc-adequate diets. When results were calculated relative to net absorbed dose, the more active metabolic tissues including liver, heart, lung, kidney, spleen, testicles and digestive tract sections from calves fed the zinc-deficient diet still retained much more 65Zn than comparable tissues of the other calves: this effect was greater in the younger calves. As percentage of absorbed dose, diet did not have marked effects on 65Zn in tissues which metabolize 65Zn more slowly, including hair, scrotum, muscle and bone. Most tissues from calves fed the zinc-deficient diet contained somewhat less stable zinc than those from animals fed adequate zinc. Dry matter content of tissues was not affected by the diets.