Prolactin and Growth Hormone Responses to Photoperiod in Heifers*

Abstract

Serum PRL, GH, and progesterone were measured at 1300 h twice weekly in heifers exposed to natural length or 16-h light (16L)-8-h dark (8D) during Nov. 11 to March 9 (n = 20) and April 30 to Aug. 13 (n = 18). Heifers exposed to 16L–8D received fluorescent light between 0600–2200 h daily. Serum PRL increased 4-fold in response to 16 h of artificial illumination between Nov. 11 and March 9 (P < 0.01), and increased 1.6-fold between April 30 and Aug. 13 (P < 0.01). Serum PRL fluctuations in natural and 16L–8D photoperiods were synchronized and positively correlated (r = 0.36–0.53) with changes in ambient temperature. Overall, serum PRL concentrations increased (P < 0.01) within 16L–8D and natural length photoperiods as ambient temperatures increased from -7 to +29 C. When ambient temperatures were below 0 C, serum PRL concentrations were similar in heifers exposed to natural and 16L–8D photoperiods. As ambient temperatures increased, serum PRL concentrations increased synergistically in animals exposed to 16L–8D photoperiods. Thus, the interactions between length of photoperiod (16L–8D vs. natural) and ambient temperature were significant (P < 0.02) between Nov. 11 and March 9 and between April 20 and Aug. 13. Serum GH concentrations did not respond to 16L–8D photoperiods or changes in ambient temperature during the fall-winter or springsummer seasons. As determined from serum progesterone concentrations, 3 of 10 heifers exposed to 16L–8D photoperiods and 0 of 10 exposed to natural photoperiods initiated estrous cycles between Nov. 11 and March 9 (P ≃ 0.10). Photoperiods of 16L–8D did not affect onset of estrous cycles between April 30 and Aug. 13. We conclude that 16 h of illumination daily increase serum PRL in comparison with heifers exposed to natural length photoperiods of 9–12 h. However, ambient temperatures below 0 C suppress the ability of 16L–8D photoperiods to increase serum PRL, but as temperatures increase, serum PRL increases synergistically in response to 16L-8D photoperiods.