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Diurnal Variation in Plasma Corticosteroid Concentration in Dairy Cattle

Department of Veterinary Science, The Pennsylvania State University, University Park, 16802

ABSTRACT

Adrenal corticosteroid concentrations were determined by a competitve protein-binding technique in blood samples taken at various times of the day via indwelling jugular cannulae from six cows in each of three groups, pregnant nonlactating, nonpregnant lactating, and pregnant lactating. The results showed a pattern of plasma corticosteroid concentration suggestive of a daily rhythm with higher concentrations between 2:30 and 6:30 AM and lower concentrations at 8:30 PM in the three groups. Pregnancy and lactation status had no significant effect on the daily pattern, but the daily corticosteroid mean was higher in the pregnant, lactating group (.64 ± .05 µg/lOO ml; mean ± SE) than in the pregnant, nonlactating group (.50 ± .03 /µg/100ml).