Changes in circulating plasma levels of cortisol in lactating and non-lactating dairy cattle during the estrous cycle.

Plasma cortisol levels were determined by radioimmunoassay for five lactating cows and five open heifers from blood samples collected daily during the course of a complete estrous cycle. Each animal was fitted with an indwelling jugular catheter to minimize stress from bleeding. Mean plasma cortisol levels were 5.67 ng/ml for the lactating cows and 5.87 ng/ml for the open heifers. Mean values for individuals ranged from 3.79 ng/ml to 6.94 ng/ml in the lactating group, and 3.37 ng/ml to 11.69 ng/ml in the open group. These differences, both between and within groups, were not significant. Mean cortisol values during the estrous cycle ranged from 2.26+/-0.93 ng/ml on day one to 9.49+/-2.11 ng/ml on day six for the lactating group, and 2.74+/-0.52 ng/ml on day six to 14.66+/-10.78 ng/ml on day twelve for the open group. Group by day interactions were not significant. Attempts to correlate plasma cortisol with lactation or day of estrus were not significant.