title: Changes in circulating plasma levels of cortisol in lactating and non-lactating dairy cattle during the estrous cycle.
abstract: 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.
authors.affiliation: Department of Dairy Science, Louisiana Agricultural Experiment Station, Baton Rouge, LA 70803, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Roussel
authors.forename: T J
authors.initials: TJ
authors.lastname: Clement
authors.forename: T J
authors.initials: TJ
authors.lastname: Aranas
authors.forename: S H
authors.initials: SH
authors.lastname: Seybt