Linking egg size, embryonic development, and post-hatch thermoregulatory responses in broilers from young and old breeders

This study evaluated how breeder age influences egg traits, embryonic development, and post-hatch thermoregulatory responses in broilers reared under thermoneutral conditions. Fertile Cobb 500® eggs from 34- and 63-week-old breeders were incubated under standardized conditions in a split-plot design (breeder age × incubation day: 0, 7, 14, and 18 d; n = 160). Initial (IW) and final egg weight (FW), embryo weight (EW), ratio between net internal egg weight and embryo weight (NW/E) and embryo length (EL) were recorded. A second experiment reared chicks from the same breeder ages to 42 d in a completely randomized 7 × 2 factorial design (week of age × breeder age; n = 48), with weekly measurements of body weight (BW), respiratory rate (RR), cloacal temperature (CT) and surface temperatures (back, head, eye, wing, and foot). Eggs from 63-wk breeders showed greater IW and FW (p < 0.001) and higher NW/E (p < 0.01). EW and EL increased with incubation day (p=0.026 and p=0.004, respectively), with breeder age × day interactions (EW: p = 0.001; EL: p = 0.009). Factor analysis consistently yielded two components representing egg size/reserves (IW, FW, and NW/E) and embryonic growth (EW and EL) in both breeder age groups. Post-hatch, RR was influenced by breeder age (p = 0.04), week (p < 0.001) and their interaction (p = 0.001), whereas EyeT showed modest breeder-age effects (p = 0.01). CT, HeadT and WingT remained stable (p> 0.05). BW was affected only by week (p < 0.001), with no effect of breeder age (p = 0.95) or interaction (p = 0.99). CHAID decision trees highlighted IW (incubation) and BW (post-hatch) as the primary split variables. Practically, breeder age should primarily guide egg grading and incubation management, whereas broilers from young and old breeders can be managed similarly in the grow-out phase under adequate thermal conditions.