Effect of selenium and iodine supplementation on growth rate and on thyroid and somatotropic function in dairy calves at pasture

The effects of selenium and iodine supplementation on growth rate and on thyroid and somatotropic function were examined in heifer calves from 2 herds at pasture. Supplementation of calves with intraruminal Se pellets (designed to release Se about 3 mg/day) increased the basal plasma concentration of 3,5,3′-triiodothyronine and reduced the basal plasma concentration of thyroxine for both herds. For one herd, supplementation with Se increased the triiodothyronine response to challenge with thyrotropin-releasing hormone, increased body weight gain, and tended to increase the plasma concentration of insulin-like growth factor I. The plasma concentration of somatotropin was unaffected by Se supplementation. Supplementation with I increased the response of thyroid hormones to thyrotropin-releasing hormone but did not increase body weight gain. Interaction between Se and I treatment within herds was not apparent for any outcome variable. The data suggest that the effects of Se deficiency in grazing calves may be mediated by alterations in thyroid hormone metabolism but apparently are not mediated through modulation of the peripheral concentration of somatotropin.