EFFECT OF DIETARY IRON CONCENTRATION, AGE, AND LENGTH OF IRON FEEDINGON FEED-INTAKE AND TISSUE IRON CONCENTRATION OF BROILER CHICKS FOR USE AS A BIOASSAY OF SUPPLEMENTAL IRON SOURCES

Three experiments were conducted with day-old chicks to study the effe cts of dietary Fe concentration and age on Fe accumulation in tissues as an estimate of supplemental Fe bioavailability, and of delaying the time of initial high Fe supplementation up to 7 d of age on feed inta ke to 3 wk of age. In Experiment 1, chicks were fed a basal corn-soybe an meal diet (188 mg/kg Fe, DM basis) or the basal supplemented with 4 00, 600, or 800 mg/kg added Fe as reagent grade FeSO4 . 7H(2)O for eit her 1, 2, or 3 wk. Dietary Fe depressed (P < 0.001) feed intake and bo dy weight gain, especially at 3 wk. Kidney Fe concentrations increased linearly (P < 0.001) with increasing dietary Fe. Liver Fe concentrati on also increased linearly, but reached a plateau in birds fed 600 mg/ kg Fe. Bone Fe increased linearly (P < 0.05) at 1 wk, but not at 2 or 3 wk. Liver and kidney Fe regressed on daily Fe intake had the best fi t to a linear model at 2 wk. In Experiment 2, chicks were fed either a basal diet (320 mg/kg Fe, DM basis) continuously, the basal supplemen ted with 800 mg/kg added Fe as FeSO4 . 7H(2)O continuously, or were st arted on the control diet and switched to the high Fe diet on Day 3, 5 , or 7. Feed intake was lower (P < 0.05) in birds started on Fe on Day s 1 or 3, but delaying feeding of high Fe diets until Day 5 resulted i n intake at 3 wk similar to that of birds fed the basal diet. In Exper iment 3, the basal diet (123 mg/kg Fe) was fed to chicks for 6 d, then experimental diets were fed for 14 d. Diets were the basal or basal s upplemented with 400, 600, or 800 mg/kg added Fe as reagent grade or f eed grade Fe sulfate or an Fe methionine complex. When estimated from regression of log(10) liver Fe concentration on total analyzed dietary Fe concentration, relative bioavailability was set at 100% for reagen t grade Fe sulfate, and the feed grade sulfate was 92.3% and Fe methio nine was 88.3%. Liver Fe concentrations may be useful criteria for det ermining Fe bioavailability and 2 wk of feeding was the optimal time r equired for such a bioassay. Delaying feeding high Fe diets until 5 d of age alleviated the decreased feed intake associated with high Fe di ets.