Investigations of Ultradian and Circadian Rhythms in the Concentration of Cortisol and Prolactin in the Plasma of Dairy Cattle

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Abstract
The circadian and ultradian pattern of secretion of cortisol and prolactin in cattle was analyzed and from this a meaningful sampling technique for these hormones was suggested. Two Friesian and 2 Jersey heifers, non-pregnant and 18 mo. of age, were bled (2 ml blood) every 10 min for 24 h via an indwelling jugular cannula inserted 6 h before sampling commenced. The concentration of cortisol in plasma showed an ultradian rhythm with an amplitude of .apprx. 30 ng/ml and a frequency of .apprx. 0.6 cycles/h. Elimination of the ultradian component of variation, by taking the mean of the rolling means (last 3 values) for individual heifers, indicated a diurnal variation in cortisol concentration which was high between midnight and mid-morning and low in the afternoon. There were frequent sporadic bursts of prolactin secretion but these were inconsistent. There was no evidence of diurnal variation in prolactin concentration or of a temporal relationship between cortisol and prolactin secretion.