Effects of moderate heat and milk yield on plasma thyroxine in cattle.

The effects of milk yield (lactational intensity) and short (18 h) moderate heat exposure (30 C) on plasma thyroxine were studies in spring (March) and fall (October) in the University of Missouri dairy herd. Spring and fall thyroxine did not differ at thermoneutral temperature of 15 C. The relatively short moderate heat exposure had no effect within production groups either in the spring or fall sampling. As stage of lactation progressed, thyroxine also increased. The effect of stage of gestation was an elevation in the first trimester (compared to nonbred) that progressed into the second and third trimesters. In both spring and fall (15 C), the high production group (adjusted and unadjusted) had lower plasma thyroxine compared with middle and low production groups, while no difference in adjusted means was significant between middle and low production groups. Correlations were --.51 between plasma thyroxine and lactational intensity in both spring and fall. The short exposure to moderately high environmental temperature intensified the relationship of lowered plasma thyroxine in hibh producing cows, and thyroxine was negatively correlated (--.63 and --.64) with lactational intensity. Because total plasma thyroxine is inversely related to lactational intensity, studies are warranted to quantitate free hormonal concentrations and utilization by dairy cattle with varying productivity.