Effects of excessive intakes of iodine upon growth and thyroid function of growing Holstein heifers.

Thirty Holstein heifer calves averaging 120 days of age and 102 kg of body weight were allocated to one control and four treatment groups of six each. Iodine, as ethylenediamine dihydriodide, was mixed 1:9 with dextrose and administered once daily atop feed at .625, 1.25, 2.5, or 5.0 mg iodine per kilogram body weight. Calves were housed individually in unheated, well-ventilated barns and fed complete mixed feeds containing less than 1 ppm iodine. Feed intakes were recorded daily and body weights weekly. Jugular venous blood was collected from iodine treated calves at 0, 4, 8, and 12 wk of the experiment and analyzed for iodine, thyroxine, and triiodothyronine of plasma. Feed intake per unit body weight and per unit gain were not significantly different between treated and control calves. However, daily feed intake and average daily gain decreased slightly at the highest iodine intake. Thyroxine and triiodothyronine were not different among treatments. Thyroxine declined in all calves from 0 to 12 wk. Thyroxine of calves fed 5.0 mg of iodine per kilogram body weight decreased more than of calves fed less iodine. Iodine intake as high as 5.0 mg/kg body weight was tolerated without morbidity, although a minor effect on performance and thyroid activity was indicated.