| dc.description.abstract | Metabolic status and photostimulatory cues interact to regulate sexual maturation, reproductive development, and early egg production in cage-free laying hens. Ongoing genetic selection, combined with the transition to alternative housing systems, has altered growth patterns and energy expenditure of modern laying hens, highlighting the need to define the physiological thresholds governing reproductive activation. At 13 weeks of age (woa), pullets reared in aviary-style pens distributed across four rooms were subjected to differing feeding regimens of either ad libitum (AL) or 20% feed restriction (R), which was lifted at 21 woa. Photostimulation (PS) was initiated in two rooms once each feeding treatment achieved the target body weight (BW) of 1.45 kg, occurring at 18 woa for AL birds and 20 woa for R birds. This design generated both synchronized (AL-PS18 and R-PS20) and desynchronized (AL-PS20 and R-PS18) relationships between BW and PS timing. Reproductive activation was associated with narrow physiological thresholds. The initial rise and peak in circulating estradiol occurred within an approximate 21 g BW range, while the onset of lay occurred within a similarly constrained 34 to 44 g window across treatments. Despite differences in feed intake and growth trajectory, hens initiated reproductive function only after reaching this defined physiological state, indicating that BW reflects, rather than determines, readiness for reproduction. Body composition provided further resolution of this threshold, with hens entering lay within a consistent abdominal fat pad range of 22.56 to 37.28 g, corresponding to 1.70 to 2.34% of BW. The probability of hens being in lay reached 50% at approximately 1.45 kg in BW, aligning closely with the BW threshold associated with the peak in estradiol production, as well as for the age of first egg. These findings demonstrate that the abdominal fat pad represents the biologically relevant adipose depot for metabolic signaling for reproduction. Desynchronization between metabolic status and PS delayed the onset of lay, reduced early egg production, and altered endocrine responsiveness, whereas synchronization between these factors improved uniformity of reproductive development. Mild feed restriction resulted in lighter eggs during early lay, while later PS improved eggshell thickness and breaking strength, indicating a trade-off between early production and egg quality. Oviposition timing and location, as well as spatial distribution within the cage-free system, were influenced by feeding treatment and PS age. Across treatment groups, hens laid a greater proportion of eggs in the morning compared to the late afternoon. A consistent developmental shift from PM- to AM-dominant egg laying was observed, with this shift occurring earlier in the PS18 hens compared to the PS20 hens. Nestboxes were the preferred oviposition site for both AL and R hens. Spatial distribution of the cage-free environment differed by feeding treatment, as R hens spent more time on the litter and AL hens more time on the perches. However, these differences dissipated with age. Despite treatment-related differences in daytime distribution, location of hens during the early scotophase did not vary between feeding treatments, suggesting that energy-conserving behaviors in hens with reduced feed intake do not extend to influence roosting location. Collectively, metabolic readiness governs reproductive activation, and precise alignment of feeding and PS strategies is critical in optimizing flock uniformity, production efficiency, and egg quality in modern cage-free laying systems. | en_US |
