The effect of rising environmental temperatures (35°–95° F.) On thyroid 131I release rate of holstein, brown swiss and jersey heifers

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Abstract
1. There were many distinct, consistent, individual differences in the thyroid activity of several of the animals used in this study. Graphic indications of breed differences were present at several high temperatures, and statistical analyses supported differences between the Jerseys raised at each temperature and the other two corresponding breeds. The Jerseys displayed higher rates. In no instances were there thyroid 131I release rate differences between Holsteins and Brown Swiss.2. There was a negative correlation between thyroid 131I release rate and temperature for each breed raised at each temperature. Correlations for the 50° F. animals were statistically significant below the 0·05 level of probability and for the 80° F. animals they were significant below the 0·01 level. As the environmental temperature increased from 35° to 80° F. there was a gradual decrease in thyroid 131I release rate. Above 80° F. there was a sharp decline in thyroid 131I activity.3. Within the range of environmental temperatures from 35° to 70° F. the animals raised at 80° F. displayed thyroid release rates higher than those of the animals raised at 50° F. At 80° and 90° F. the 50° F. Jerseys exhibited higher activity. The Jersey differences approached statistical significance. At the lower temperatures graphic differences between the Brown Swiss and the Holsteins raised at the two temperatures appeared to be present, but they were not generally supported by statistical analysis.4. At temperatures of 80° F. and above, when regulatory mechanisms could not control body temperature, both groups showed considerable rises in body temperature concomitant with decreases in thyroid 131I activity and TDN consumption. The 80° F. group showed a rise less sharp than that of the 50° F. group.

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