Model for the prediction of mean time of oviposition for hens kept in different light and dark cycles.

1. Evidence from 26 short-term experiments with laying fowls has been analysed to determine the relationship between onset of darkness and mean time of oviposition for light-dark cycles ranging from 21 h to 30 h incorporating various dark periods from 5 h up to 23 h. 2. When the light-dark cycle is equal to or longer than 24 h, mean time of lay (H) in hours from onset of darkness is predicted by the equation: H = 64.62-2.161C + 0.268S, where C = cycle length (h) and S = scotoperiod (h) (multiple correlation coefficient = 0.977). 3. For light-dark cycles shorter than 24 h, the best equation found was: H = -4.97 + 0.740C + 4.482S - 0.175CS (multiple correlation coefficient = 0.981). 4. The meaning of these equations in relation to events in the ovulation cycle of the fowl is discussed.