Effect of Changes in Forage Ration on the Thyroidal I131 Release Rates of Young Dairy Animals

Abstract

Summary Previous reports showed that a sharp decline in milk production following certain forage ration changes could not always be explained on the basis of reduced feed intake and digestibility, or both. In one experiment the thyroidal I 131 release rates of six Holstein heifers were significantly higher shortly after they were changed to legume-grass silage than when they were fed alfalfa hay. In subsequent experiments the thyroidal I 131 release rates were significantly higher when the heifers were fed legume-grass silage from a plastic-covered stack or mixed grass-clover-alfalfa hay than when they were fed legume-grass silage from a tower silo. When the forage was changed from alfalfa hay to legume-grass silage during the time I 131 release rate was being determined, three of five calves exhibited an increased release rate, whereas their controls showed no change. When reutilization of iodine was blocked with thiouracil, the two calves switched to silage showed a significant increase in I 131 release rate, whereas one control calf exhibited a decrease and the other an increase. These changes in I 131 release rate indicated that some as yet unidentified factor(s) in certain forages exerts an influence on the thyroid activity, so that an adjustment in thyroid activity of the animal is induced when a sudden change in forage rations is made. It is suggested that changes in thyroid activity associated with changes in forage ration, at least in part, may explain the sharp decline in milk production observed when similar ration changes were made in previous experiments.