EFFECT OF PHOTOPERIOD ON THE MEAN OVIPOSITION TIME OF 2 BREEDS OF LAYING HEN

1. Oviposition times were recorded for brown and white egg-laying hybr ids under 8, 10, 13 and 18 h photoperiods. 2. Mean oviposition time fo r both breeds was advanced relative to dusk by approximately 0.5 h for each 1 h extension of photoperiod. 3. Mean oviposition time for the b rown egg hybrid was 1.2 to 1.4 h earlier than that of the white egg hy brid under each lighting regimen. 4. A genetic difference in phase set ting of the Open Period for Luteinising Hormone (LH) release is the li kely reason for the difference in mean time of lay of the two breeds. The difference is possibly one between brown and white hybrids general ly, rather than between the particular varieties of hen used in this t rial. 5. The proportion of the day in which eggs are laid is shorter u nder long photoperiods presumably because light at the end of the phot operiod inhibits the pre-ovulatory surge of LH.