title: Hormonal responses of lactating dairy cattle to TRH and ACTH in a shade management system within a subtropical environment.
abstract: Lactating cows (64) were balanced by breed (54 Holstein and 10 Jersey) and assigned randomly to shade (S) or no shade (NS) management treatments for a continuous 20 wk trial beginning 5-5-76. A sub-sample of Holstein cows, five S and five NS, were fitted with jugular catheters 84 days after initiation of experiment. Thyrotropin Releasing Hormone (TRH; 100 mug) was administered intravenously at 1200 h to evaluate prolactin responses. Two days later each cow received intravenously 200 IU of ACTH at 1100 h to compare acute corticoid responses to ACTH. Mean prolactin response to TRH was greater for NS cows (291 vs 169 ng/ml; P < .01) as was peak plasma concentrations at 20 min (467 vs 267 ng/ml; P < .01). Mean corticoid response to ACTH injection was less for NS cows (52 vs 70 ng/ml; P < .10). Corticoid concentrations of plasma in both treatments had declined 65% by 7 h postinjection. These endocrine differences may be associated with thermoregulation and/or metabolic adjustments of cows exposed to different environmental systems of management during a seasonal period of thermal stress.
authors.affiliation: Dairy Science Department Institute of Food and Agricultural Sciences University of Florida Gainesville, FL 32611 USA.
authors.forename: H
authors.initials: H
authors.lastname: Roman-Ponce
authors.forename: W W
authors.initials: WW
authors.lastname: Thatcher
authors.forename: R J
authors.initials: RJ
authors.lastname: Collier
authors.forename: C J
authors.initials: CJ
authors.lastname: Wilcox