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 13h of continuous light or to three skeletal photoperiods consisting of a total of 8h of light. Cows in the skeletal photoperiod light regimens received 6h of light from 0500 to 1100h and a 2-h light pulse at either 1500 to 1700h, 1800 to 2000h, or 2100 to 2300h. Cows exposed to the light regimen pulse at 1800 to 2000h 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 15h after subjective dawn. The expression of the circadian rhythm of these hormones depends on the photoperiod to which cows are exposed.
Published Version
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