Effect of temperature-season on bovine adrenal cortical function, blood cell profile, and milk production

Data collected monthly for one calendar year from Holstein cows lactating under Louisiana ambient climatic conditions comprised a total of 264 cow-months. The year was divided into seasons of cool, intermediate, and hot temperatures. The hot season elicited depressions in circulating corticoids and percent hematocrit. Leukocytosis was a response to increasing ambient temperature primarily accounted for by an increase in circulating neutrophils. Progression from intermediate to hot season gave evidence of eosinophila, and in addition, a relative eosinopenia may have been induced by increased adrenal cortex activity. Milk production was depressed during the hot temperature-season and showed no significant relationship with corticoids of plasma. Rectal temperatures and respiration rates were indicative of heat stress in the hot temperature-season and gave significant negative correlations with circulating corticoid concentration.