EFFECT OF CONSTANT AND OF CHANGING PHOTOPERIODS ON AGE AT FIRST EGG AND RELATED TRAITS IN PULLETS

1. The effects of constant photoperiods and of single (5 h) changes in photoperiod applied at 12 or 17 weeks of age upon age at first egg (A FE) were studied using ISA Brown and Shaver 288 pullets. 2. Birds rear ed from 2 d of age until after maturity on constant 10 h photoperiods matured 8 d earlier than birds reared on constant 8 h and 5 d earlier than the average for 13 or 18 h photoperiods. 3. A single increment in photoperiod from 8 to 13 h advanced AFE by 23 d (compared to 8 h cons tant day controls) when applied at 84 d, but by only 6 d when given at 119 d. An increase in photoperiod from 13 to 18 h advanced AFE by onl y 4 d, averaged across breeds and age at increase. A reduction in phot operiod from 13 to 8 h delayed AFE by 22 d when given at 84 d and by 1 6 d at 119 d. A similar 5 h reduction in photoperiod, but from 18 to 1 3 4 retarded maturity by 11 d in ISA Brown pullets, but only when give n at 84 d, and delayed AFE in Shaver 288 by 12 d, but only when given at 119 d. This interaction may be partly explained by the different ph ysiological stages reached by the two breeds when the photoperiod was changed. 4. Under constant daylengths cumulative food intake before fi rst egg was positively correlated with photoperiod, but the early AFE for birds on 10 h photoperiods resulted in this group having the lowes t cumulative food intake to first egg. 5. A 5 h increase in photoperio d at 84 d significantly reduced the food consumed to first egg, but ha d no effect when given at 119 d.A 5 h decrease in photoperiod generall y increased the food consumed to first egg, but the effect was only si gnificant when the daylength was reduced from 13 to 8 h at 119 d. Food intake to first egg in birds subjected to a change in photoperiod was highly correlated with AFE. 6. The data confirm that sexual developme nt in growing pullets responds more to changes in photoperiod than to the absolute daylength, that changes made at different daylengths are not equivalent and that sensitivity changes with age.