Research note: Stress responses in different genetic strains of laying hens housed in a cage-free environment.

Cage-free systems offer hens the opportunity to perform natural behaviors, but they also expose birds to more frequent environmental and social stressors, which can impact welfare. Therefore, this study assessed stress responses in four laying hen strains housed in a cage-free environment. Hy-Line W36 White, H&N White, Hy-Line Brown, and Bovan Brown laying hens were evaluated through plasma corticosterone levels, heterophil lymphocyte ratios, and heat shock proteins (HSP) 70, 90a, and 90b gene expression at five distinct time points throughout the laying hen cycle. There were no differences among these parameters found attributable to genetic strain. However, all strains exhibited similar stress responses at critical time points throughout the lay cycle. HSP70 expression was highest at the end of lay (p < 0.0001), HSP90a at the start of lay (p < 0.0001), and both HSP90b and heterophil lymphocyte ratios at peak lay (p < 0.0001), suggesting a potential relationship between the two. Corticosterone levels remained consistent across both genetic strains and time points. Therefore, although no distinctions were observed among genetic strains, the variability in stress levels over time provides insight into factors impacting the welfare of hens throughout egg production.