ZINC STORES IN CHICKENS DELAY THE ONSET OF ZINC-DEFICIENCY SYMPTOMS

Three chick assays were conducted to evaluate Zn depletion rates in whole body and various tissues. In Assay 1, chicks fed a corn-soybean meal diet containing 1,037 mg Zn/kg had twice as much Zn in intestine and bone and 50% more Zn in liver and whole body than chicks fed 37 mg Zn/kg (P < .01). In Assay 2, the minimum Zn requirement for growth of chicks consuming a chemically defined, amino acid diet was 10.6 mg Zn/kg. In Assay 3, chicks were fed the chemically defined diet containing either 300 or 10.6 mg Zn/kg during an 8-d pretest period, after which they were fed either 0 or 10.6 mg Zn/kg for 9 d. Tissue Zn concentrations were determined on Days 0, 1, 2, 3, 4, 5, 7, and 9 of the test period. Following the pretest period, Zn was higher in the liver, tibia, and small intestine, but not in muscle, of chicks fed 300 mg Zn/kg than of those fed 10.6 mg Zn/kg (P < .01). Growth of chicks switched from 10.6 to 0 mg Zn/kg was less than for chicks maintained on 10.6 mg Zn/kg by Day 5 (P < .05), whereas chicks switched from 300 to 0 mg Zn/kg did not display a growth depression relative to the control until Day 8 (P < .05). Zinc that accumulated in the liver and small intestine of chicks fed the 300 to 0 Zn regimen declined until Day 3 of the test period, whereas tibia Zn decreased until Day 8. Chicks fed the 10.6 to 0 regimen did not display a substantial decrease in small intestine, liver, or tibia Zn. The data confirm that Zn can accumulate in bone, liver, and intestine and can subsequently be released for use during a period of Zn deficiency. The data also indicate that chicks fed a diet containing the minimum requirement of Zn cannot accumulate reserves of Zn that become available for use during a subsequent period of Zn depletion.