Investigations of ultradian and circadian rhythms in the concentration of cortisol and prolactin in the plasma of dairy cattle.

This experiment analysed the circadian and ultradian pattern of secretion of cortisol and prolactin in cattle, and from this a meaningful sampling technique for these hormones has been suggested. Two Friesian and two Jersey heifers, non-pregnant and 18 months 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 approximately 30 ng/ml and a frequency of about 0.6 cycles/h. Elimination of the ultradian component of variation, by taking the mean of the rolling means (last three values) for individual heifers, indicated a diurnal variation in cortisol concentration which was high between midnight and midmorning 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.