Effects of Layer Breeder Age and Early Hypoxic Stimulation (ED 7-9) of the Chorioallantois Membrane on Eggshell Decalcification, Neovascularization of Heart Tissue, Mineralization and Morphometrics of Hatchlings

Oxygen concentration (O₂) during incubation is crucial for embryo development, and hypoxic conditions can influence phenotypic plasticity in poultry. Although low O2 (hypoxia) can be detrimental, it may also promote adaptive responses. Breeder age, a known genetic determinant of egg quality and embryonic development, is likely to interact with O₂ levels during incubation, however, this relationship remains understudied in layer breeders. This study examined how layer breeder age and reduced oxygen (O₂) levels during early embryonic development affect various factors, including eggshell decalcification (Dcal-SHL), chorioallantoic membrane (CAM) weight, heart tissue vascularization, egg weight loss (EWL), eggshell temperature (EST), and calcium (Ca) and phosphorus (P) content in bone and blood and tibia and femur morphometrics. A total of 900 eggs from 33 and 50-week-old ISA brown layer breeders were incubated in a 2x3 factorial design with O₂ levels of 15%, 17% (hypoxic), and 21% (control). Oxygen was reduced for 1hr/day from embryonic days (ED) 7-9 using air-N₂ flushing. Results showed increased CAM weight and heart tissue vascularization under hypoxia, especially in older breeders (50 weeks). Hypoxic conditions (15% and 17% O₂) reduced embryo weight loss and eggshell temperature compared to controls during the post-exposure phase (ED 15-18). There was an interaction between breeder age and O₂ levels on mineral absorption, with reduced oxygen leading to lower Ca and P absorption in bones, higher eggshell P retention, and decreased tibia morphometrics (weight, length, diameter, and seedor index) in hatchlings. Additionally, CAM weight correlated negatively with Dcal-SHL Ca at 15% O₂. The study concluded that reduced oxygen during early embryonic development increases CAM weight, heart neovascularization, and Ca mobilization from eggshell to blood. However, older flocks exhibited reduced Ca transfer to bones, likely due to homeostatic imbalance.