Brown and white egg-layer strain differences in fearfulness and stress measures

Different genetic lines of laying hens may show varying levels of fearfulness in response to stressful events or situations. It is important to select appropriate genetic strains when keeping hens in alternative housing systems to minimize health and production issues caused by fear and stress. In this study, data were obtained from two strains of Rhode Island Red (RIR1-2), two strains of White Plymouth Rock (WPR1-2) and three strains of White Leghorn (WL1-3) breeds. One hundred hens from each strain were used to compare strain differences in fear and stress responses. Each hen was scored for fearfulness based on her responses to the following tests: novel object, physical restraint, pencil, human reaction, inversion, and tonic immobility (TI). Stress was assessed based on physical asymmetry (ASYM) of the metatarsal and middle toe, and on corticosterone (CORT) concentrations in blood plasma and egg albumen. Differences between brown (RIR; WPR) and white (WL) egg layers were observed for all measures: WL had more head movements and took more time to right themselves during TI (P < 0.001), but higher flapping intensity was observed in RIR and WPR (P < 0.001). Hens in the WL strains also scored lower than RIR and WPR on the struggle test (P < 0.001), and had lower ASYM scores (P = 0.002) and concentrations of albumen CORT (P = 0.0065) and plasma CORT (P = 0.002), but scored higher during the remaining fear tests (P < 0.001). Differences among WL strains were also observed for all measures: WL2 had the most head movements and took the longest to right during TI (P < 0.001). During inversion, WP2 and RIR2 had the highest flapping intensity (P < 0.001) whereas WL2 and WL3 had the lowest flapping intensity (P < 0.001). There was variation among strains in plasma CORT, albumen CORT, and scores for the struggle test and pencil test. Only the WPR2 strain showed significant differences in ASYM (P < 0.001). Genetic strains of laying hens show clear variation between brown and white shell color varieties regarding fear and stress responses. Brown hens tend to actively avoid perceived threats whereas white hens use passive avoidance. Using several methods to test fear and stress to better understand individual birds' responses to stressors will help to design breeding strategies to maximize hens' success in alternative housing systems.