Changes in Thyroid I131 Release Rate during Growth in Holstein, Brown Swiss, and Jersey Calves at Constant Environmental Temperatures of 50 and 80° F

Abstract

Studies on the effect of age and temperature on thyroid I131 release rate on growing calves were conducted under the controlled environmental conditions of the Missouri Climatic Lab. At both temperatures and for all calves, thyroid I131 activity per unit weight decreased with age. Although all calves showed the same trends, breed differences in the latter relationship were highly significant, Jerseys having the highest ratios, followed by Brown Swiss and Holsteins. The thyroid I131 release rate of Jersey calves was approximately twice as great as the rate for Holstein and Brown Swiss, throughout this growth study; however, only the breed-age interaction was significant. Thyroid activity per unit weight was higher at 80[degree]F, with highly significant breed differences occurring, and varied significantly as a function of the interaction between temperature and age. Thyroid I131 activity per unit surface area showed no effect of temperature alone, but did increase slightly with increments in both age and temperature. Breed differences were significant in the ratio of thyroid I131 activity to surface area. The exponents (b values) of the parabolic equation expressing the relationship of thyroid I131 activity to body weight proved to be significantly higher at 80 than at 50[degree]F. There were significant breed differences in percentage increase in thyroid I131 activity per 1% increase in body weight, the Jerseys'' rate being higher than those of the Brown Swiss or Holsteins.