EFFECT OF SIZE AND TIMING OF PHOTOPERIOD INCREASE ON AGE AT FIRST EGGAND SUBSEQUENT PERFORMANCE OF 2 BREEDS OF LAYING HEN

1. ISA Brown and Shaver 288 pullets were changed from 8 h to 8, 10, 13 or 16 h photoperiods at 42, 63, 84, 105, 126 or 142 d of age. 2. Age at first egg (AFE) was curvilinearly affected by the size and timing o f the change in photoperiod. AFE was advanced most by a photoperiod ch ange from 8 to 13 h made at 63 or 84 d. ISA birds were generally more responsive than Shaver to the photoperiod changes. 3. Longer photoperi ods significantly increased survivors' egg production, but decreased l iveability to 504 d, so that eggs per hen housed were unaffected. Reta rding AFE by 10 d reduced survivors' egg numbers by 7.0, but increased mean egg weight by 126 g. Egg output by Shaver birds was unaffected b y AFE, but that of ISA was curvilinearly affected, with an apogee at a n AFE of 135 d. In both breeds, egg weight and egg output were greater following an early or late, rather than a mid-term photostimulation. 4. Photoperiod significantly increased mean daily food intake during l ay by 1.26 g/h. A 10 d retardation in AFE resulted in a reduction in f ood intake of 1 g/d. Efficiency of food conversion deteriorated accord ing to the square of the photoperiod, and changed curvilinearly accord ing to age at photostimulation. Food conversion efficiency improved by 0.05 g/g for each 10 d delay in AFE. 5. Shell quality was unaffected by AFE, but deteriorated with increasing photoperiod and was curviline arly affected by age at photostimulation with the smallest shell weigh ts associated with photostimulation at 63 d. The incidence of double-y olked (DY) egg production increased with photoperiod and decreased wit h delayed photostimulation. There was an exponential regression of DY eggs on AFE. 6. Body weight at first egg increased by 75 g/d delay in AFE, but body weight at 504 d of age was unaffected by AFE, photoperio d or age at photostimulation. Body weight gain during lay increased by 15 g/h increase in photoperiod, decreased by 6 g per 10 d delay in ph otostimulation and by 40 g per 10 d delay in AFE. Fat content at 504 d increased by about 10 g/kg and by 23 g/bird for each 10 d delay in AF E. 7. Mortality in lay increased by 0.8%/h increase in photoperiod, hu t was unaffected by either age at photostimulation or AFE.