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Iodine Toxicity. Physiological Effects of Elevated Dietary Iodine on Calves¹

North Carolina State University at Raleigh

Abstract

Three trials were conducted to study the effects of elevated dietary iodine intake on the performance, thyroid gland development, certain blood constituents and other measures in calves. Each trial consisted of 32 individually fed Holstein bull calves fed four levels of iodine. Iodine, as calcium iodate, was added to a basal diet at the rate of 0, 10, 100 and 200 ppm; 0, 25, 50 and 100 ppm; and 0,10, 25 and 50 ppm for Trials 1 through 3, respectively.

Elevated dietary iodine depressed gain and feed intake, the magnitude being significant for diets containing 50, 100 or 200 ppm added iodine in at least one trial. The feeding of either the 100 or 200 ppm diets, and in some cases lower levels, produced symptoms of toxicity including coughing and profuse nasal discharge. In Trial 1, the addition of 200 ppm iodine tended to produce heavier thyroid glands, on a body weight basis, and in Trial 2 this occurred at all levels. In Trial 3 there was a tendency toward lighter thyroids in calves fed all levels of added iodine. Calves receiving added iodine also tended to have heavier adrenal glands at the conclusion of Trials 2 and 3. All calves fed supplemental iodine had markedly higher concentrations of serum iodine throughout the trials, and those fed the 200 ppm diet had significantly lower blood hemoglobin and serum calcium and tended to have higher serum BEI levels at the end of Trial 1. Considering all trials, no consistent differences in rumen volatile fatty acid patterns were observed.

It appears that the minimum toxic iodine level for calves is near 50 ppm although a portion of the animals were adversely affected at lower levels. Differences in adrenal weights and trends in growth rate indicate that levels as low as 25 ppm were undesirable. Sub-toxic levels (as measured by performance) tend to increase serum iodine and may or may not result in slightly enlarged thyroids and a transient period during which the animals exhibit signs of iodinism.

¹ Paper No. 4053 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Experiment Station of the products named, nor criticism of similar ones not mentioned. Acknowledgment is given to Morton Salt Company, Chicago, Illinois, for supplying the calcium iodate.

² Present address: Animal Science Department, University of Kentucky, Lexington 40506.

³ Present address: Animal Science Department, Virginia Polytechnic Institute and State University, Blacksburg 24061.