PERFORMANCE AND TISSUE ZINC AND METALLOTHIONEIN ACCUMULATION IN CHICKS FED A HIGH DIETARY LEVEL OF ZINC

Four experiments were conducted to identify several factors that might improve the accuracy and reproducibility of Zn bioavailability assays for chicks. Response of tissue Zn and metallothionein (MT) concentrat ions to various elevated levels and soluble sources of dietary Zn were measured, as well as the effect of delaying high Zn administration un til 7 d posthatching to alleviate the detrimental effect of Zn sulfate on feed intake to 3 wk of age. Bone Zn increased (P < 0.01) in all ex periments in response to increasing dietary Zn concentrations. Liver a nd pancreas MT were affected (P < 0.01) by a source by age interaction and variability that made this criterion unsuitable for bioavailabili ty assays. Lastly, 1-d-old chicks were used to study the effect of del aying feeding of a high-Zn diet up to 7 d of age. The basal diet was f ed continuously for 21 d as a control. A diet containing 1,000 ppm Zn was either fed continuously from Day 1, or started on Day 3, 5, or 7. Chicks given high Zn on Day 3, 5, or 7 decreased (P < 0.01) feed intak e within 24 h of feeding. Delayed feeding of high dietary Zn might hel p to alleviate decreased feed intake observed in previous studies. Del aying the onset of high Zn feeding by several days may help alleviate feed intake problems observed with Zn sulfate. Use of either Zn glucon ate or Zn acetate as a standard in assays or use of MT synthesis as a bioavailability criterion will probably not be useful to improve accur acy of the estimates.