EFFECT OF SELENIUM AND IODINE SUPPLEMENTATION ON GROWTH-RATE AND ON THYROID AND SOMATOTROPIC FUNCTION IN DAIRY CALVES AT PASTURE

The effects of Se and I supplementation on growth rate and on thyroid and somatotropic function were examined for heifer calves from two her ds fed pasture. Supplementation of calves with intraruminal Se pellets increased the basal plasma concentration of 3,5,3'-triiodothyronine a nd 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 BW gain, and tended to increase the plasma concentration of IGF-I. The p lasma concentration of growth hormone was unaffected by Se supplementa tion. Supplementation with I increased the response of thyroid hormone s to thyrotropin-releasing hormone but did not increase BW gain. Inter action between Se and I treatment within the herds was not apparent fo r any outcome variable. These data suggest that the effects of Se defi ciency in grazing calves may be mediated by alterations in thyroid hor mone metabolism but apparently are not mediated through modulation of the peripheral concentration of growth hormone.