Effects of Nitrogen Fertilization on the Thyroid Function of Rats Fed 40% Orchard Grass Diets12

A total of 108 weanling male rats was assigned to a 3 × 3 factorial experiment with three dietary iodine levels: 0.68 (high), 0.23 (medium), 0.08 ppm (low); and 3 diets: a basal diet, a high-nitrogen-hay diet containing 40% hay from a plot treated with 450 kg nitrogen/hectare and a low-nitrogen-hay diet with 40% hay from a plot treated with 34 kg nitrogen/hectare. An attempt was made to equalize many essential nutrients. After a 30-day feeding period, there were no significant differences in body weight gain among treatments. For rats fed the basal diet, the high-nitrogen-hay diet and the low-nitrogen-hay diet, respectively, average weights of the thyroid glands were 16.7, 22.0 and 19.0 mg; average heights of the thyroid epithelial cells were 3.9, 5.7 and 4.9 μ; and average 24-hour postdose 131I thyroid levels were 15.2, 23.5 and 24.4% of dose. For groups receiving the high, medium and low dietary iodine, respectively, average weights of thyroid glands were 17.7, 17.4 and 22.6 mg; average heights of epithelial cells were 3.6, 4.2 and 6.7 μ; and average 131I levels in the thyroids were 9.9, 18.7 and 34.5% of the dose. Goiter was associated with inclusion of forage in the diet and, for two parameters, was intensified by intensive nitrogen fertilization of the soil but responded to iodine supplementation. It is suggested that nitrate is partly, but not entirely, responsible for the changes associated with intensive nitrogen fertilization.