The effect of continuous light on prolactin secretion in prepubertal Holstein bulls.

Effects of 16 (16 light:8 dark) and 8 (8L:16D) h of daily light were compared with continuous light (24L:0D) exposure on prolactin (PRL) concentrations in serum of prepubertal bulls. Concentrations of PRL in serum were 2 to 3 fold greater in bulls exposed to 24L:0D or 16L:8D as compared with 8L:16D. However, PRL concentrations attained a maximum approximately 3 weeks later in calves exposed to 24L:0D than in calves given 16L:8D. Continuous low intensity (11 to 16 lux) lighting supplemented with 16 or 8 h of high intensity (449 to 618 lux) light per day increased PRL concentrations in serum of prepubertal bulls 1.5 to 2.5 fold relative to 8L:16D (470 lux). We found that relative to 8L:16D, 1) photoperiods of 16 or 24 h of light per day increased serum concentrations of PRL in prepubertal bulls; however, the time required to achieve maximum PRL concentrations was longer in animals exposed to 24L:0D, 2) continuous low intensity lighting supplemented with 16 or 8 h of high intensity daily light also increased concentrations of PRL in serum.