Body weight thresholds for sexual maturation are conserved despite mild feed restriction and differences in photostimulation timing in laying hens

As the laying hen industry transitions to cage-free systems, changes in growth trajectories and body composition may influence sexual maturation. This study investigated the interaction between photostimulation (PS) timing and body weight (BW) on sexual maturation, endocrine response, and early egg production. At 12 weeks of age (woa), 600 Lohmann LSL-Lite pullets were distributed across four rooms, each containing six aviary-style pens reared under 9L:15D. From 13 weeks of age (woa), three pens per room were assigned to ad libitum (AL) or 20% restricted (R) feeding treatments. Photostimulation (14L:10D) was applied when each treatment reached a 1.45-kg BW target, occurring at 18 woa (PS18) for AL hens and 20 woa (PS20) for R hens, generating four treatment combinations: AL-18, R-18, AL-20, and R-20. All hens transitioned to AL feeding at 21 woa. R hens remained lighter than AL hens from 16 woa onward ( P < 0.001) and showed a 6.6-day delay in age of first egg (AFE; P < 0.001), despite only a 21 g BW difference at AFE between feeding treatments ( P = 0.319). Weekly blood sampling revealed that estradiol (E 2 ) rose prior to photostimulation across feeding treatments ( P < 0.001), with only a 20 g BW range at onset ( P = 0.259), consistent with activation of the reproductive axis at a BW threshold. PS18 hens exhibited a shorter interval between AFE and 50% egg production than PS20 hens ( P < 0.001), indicating enhanced responsiveness to photostimulatory cues with earlier photostimulation. Both R and PS20 hens reached established egg production at a later age and produced fewer cumulative eggs through 26 woa ( P < 0.05). R hens produced lighter eggs at 21 and 23 woa ( P < 0.001), whereas later photostimulation resulted in thicker, stronger eggshells ( P < 0.05). Collectively, this study suggests that attainment of a conserved BW threshold initiates reproductive activation, whereas photostimulation influences the rate of progression toward peak production, underscoring the importance of strategic BW and lighting management in cage-free rearing systems.