title: Effect of chronobiological alteration of the circadian rhythm of prolactin and somatotropin release in the dairy cow.
abstract: The objective of this study was to determine if a skeletal photoperiod administered at the appropriate time of a 24-h day could stimulate prolactin and somatotropin release in dairy cattle. Cows in mo 8 of gestation were exposed to either 12 to 13 h of continuous light or to three skeletal photoperiods consisting of a total of 8 h of light. Cows in the skeletal photoperiod light regimens received 6 h of light from 0500 to 1100 h and a 2-h light pulse at either 1500 to 1700 h, 1800 to 2000 h, or 2100 to 2300 h. Cows exposed to the light regimen pulse at 1800 to 2000 h exhibited a circadian rhythm of prolactin and somatotropin release. The mean prolactin and somatotropin concentrations were also higher in this treatment. It is concluded that a circadian rhythm with a photosensitive phase is present for prolactin and somatotropin release in the lactating dairy cow. The photosensitive phase for both hormones occurs between 13 and 15 h after subjective dawn. The expression of the circadian rhythm of these hormones depends on the photoperiod to which cows are exposed.
authors.affiliation: Department of Animal and Poultry Science, University of Guelph, ON, Canada.
authors.forename: N M
authors.initials: NM
authors.lastname: Evans
authors.forename: R R
authors.initials: RR
authors.lastname: Hacker
authors.forename: J
authors.initials: J
authors.lastname: Hoover
mesh_terms: D000818:Animals; D002417:Cattle / Q000378:metabolism*; D002940:Circadian Rhythm; D005260:Female; D013006:Growth Hormone / Q000097:blood / Q000378:metabolism*; D007774:Lactation / Q000378:metabolism; D008027:Light; D011247:Pregnancy; D011270:Pregnancy, Animal / Q000378:metabolism*; D011388:Prolactin / Q000097:blood / Q000378:metabolism*
chemical_list: D011388:Prolactin; D013006:Growth Hormone