EFFECTS OF TEMPERATURE AND FEED INTAKE ON PLASMA CONCENTRATION OF THYROID HORMONES IN BEEF CATTLE

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Abstract
In each of two experiments, plasma concentrations of thyroxine (T4) and triiodothyronine (T3) and the Free Thyopac Index (FTI) were determined, at intervals during winter, in beef steers housed either indoors in a heated barn or in outdoor pens. During experiment 2, a comparison was also made of the effects of four feed intake levels ranging from one to two times maintenance. In both experiments, plasma concentrations of T4 and FTI values were higher in steers kept outdoors than those kept indoors. The highest plasma T4 concentrations and FTI values were recorded in outdoor steers on a maintenance level of feed intake during February when the mean outdoor temperature was −15 °C. Under these circumstances, increasing the level of feed intake to twice maintenance reduced plasma T4 concentrations and FTI values. Level of feed intake had no effect on either plasma T4 or FTI values in steers kept in a heated barn. Plasma T3 concentrations were higher in the outdoor than the indoor steers during experiment 2. Reducing feed intake from twice maintenance to maintenance levels resulted in a decrease in plasma T3 concentrations in steers kept outdoors, but not in those kept indoors in a heated barn. These results indicate that thyroid hormone responses of cattle to cold environments are influenced by the level of feed intake.