In ovo green light remodels the hepatic transcriptome and shapes sex-dependent growth and stress behaviour in meat-type ducks.

Light during incubation can program poultry growth and stress resilience, but the duck eggshell transmits only a small, variable fraction of external light, so conventional incubation lighting is ineffective and inconsistent. We developed an in ovo lighting (IOL) system that implants a light-emitting diode into the air cell to deliver a defined photic dose, and used it to optimise incubation lighting for Cherry Valley meat ducks and to define its mechanism and post-hatch consequences. We quantified eggshell transmittance, screened light colours and photoperiods for effects on hatchability, incubation duration and embryonic growth, profiled the embryonic day (ED) 24 hepatic transcriptome by RNA-sequencing, and assessed post-hatch sex-specific growth, behaviour, antioxidant status and stress hormones in a 2 × 2 design. Transmittance was low (3-6%), wavelength-selective (red > white > green) and intensity-independent. IOL did not impair hatchability but shortened incubation by 5.7-8 h and widened the hatch window. Green light most increased ED24 body weight (39.6 vs 32.5 g; P < 0.05) and metatarsal length. Daily durations ≥18 h were embryolethal, whereas 12 h produced the largest hepatic response, down-regulating extracellular-matrix and collagen genes and up-regulating CYP7A1, PPAR and steroid-hormone pathways, indicating a shift from structural construction to energy mobilisation. Post-hatch, green light raised 14-d male body weight, improved female leg developmental stability and antioxidant capacity, but green-light females showed reduced exploration and the highest corticosterone, serotonin and interleukin-1β. Rhythmic in ovo green light is a precise, safe intervention that accelerates incubation and growth through hepatic metabolic reprogramming, but its photoperiod must be calibrated to limit a sex-specific stress cost.