EFFECT OF PHOTOPERIOD ON THE CONCENTRATIONS OF CORTICOSTERONE AND LUTEINIZING HORMONE IN THE PLASMA OF THE DOMESTIC HEN

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Abstract
Ovipositions in hens maintained under schedules of 16 h light: 8 h darkness (16L : 8D) or 8L : 16D were restricted to a 12 h period of the day while in hens maintained on continuous light, ovipositions occurred at times throughout the whole of the 24 h day. In hens maintained in continuous light a significant rise in the plasma concentration of corticosterone was observed 11–9 h before ovulation/oviposition. In hens in 16L : 8D there was an increase in the concentration of corticosterone in the plasma during the first 1·5 h of darkness which, under that schedule, coincides with the onset of the 'open period' for the occurrence of the preovulatory release of LH. In hens ovulating the first egg of a sequence the rise in the secretion of corticosterone was only transient and levels fell as preovulatory concentrations of LH rose to a peak. However, in hens undergoing an ovulation late in a sequence or in those not expected to ovulate, the concentration of corticosterone in the plasma continued to rise to attain a maximal concentration 8 h later. In hens maintained in 8L : 16D there was a tendency for the concentration of corticosterone in the plasma to increase at the beginning of the open period, 4 h after the onset of darkness, though this rise was much smaller than that observed at the beginning of the open period of hens on 16L : 8D. A further, more pronounced, increase in the concentration of corticosterone was observed between 11 and 15 h after the onset of darkness. It is suggested that the rise in the concentration of corticosterone at 11–9 h before ovulation in hens in continuous light reflects the beginning of an open period with a free-running circadian periodicity of 26–27 h.