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Abstract
1. Broiler breeder pullets were exposed to constant 10-, 11-, 12-, 13-, 14- or 16-h photoperiods from 3 d of age. Egg weight, eggshell weight and shell thickness index were determined at 52 weeks of age.
2. Egg weight increased by 0·31 g, shell weight decreased by 30 mg and shell thickness index decreased by 0·57 mg/cm2 for each one-hour increase in photoperiod.
3. Whilst the changes in egg weight and eggshell thickness index might be overstated because eggs were collected at the same chronological time, the effect of time of egg-laying within the day was minimal in comparison, and did not negate the conclusion that egg weight increases, and shell weight and thickness index decrease with lengthening photoperiods.
4. The effect of photoperiod on eggshell quality was not due to differences in the rate of lay between treatments. Shell weight was unaffected by time of lay.
Acknowledgements
We acknowledge the generous supply of 1-d-old chicks and financial assistance from Cobb-Vantress for this research.