Chronic stress and fear in fast- and slow-growing broilers as they age and gain weight when raised in simple or complex environments.

The objective was to assess how environmental complexity influences fear and chronic stress in fast- and slow-growing broiler chickens as they age and gain weight. Six-hundred fast-growing (Ross 708) and 600 slow-growing (Hubbard Redbro Mini) broilers were raised in 24 pens with simple (standard) or complex (permanent and temporary enrichments) pens. Tonic immobility (TI) tests were performed on 6 broilers/pen to assess fear, with 3 sampled for primary wing feathers to assess feather corticosterone (fCORT) concentration at 4, 5, and 6 weeks of age and 1, 2, and 3 kg of body weight, resulting in 4 testing points per strain due to overlapping weights and ages (TI n=576, feather n=288). Feather brightness (indicating melanin content) was calculated from images. fCORT was assayed using an ELISA. Fast-growing broilers remained in TI longer (106±6 sec) than slow-growing broilers (82±6 sec) at equal ages (p=0.013). Heavier birds remained in TI longer (105±8 sec) than lighter birds (73±8 sec; p=0.011). Feather brightness and fCORT concentration were not correlated in slow-growing broilers (ρ=0.085, p=0.320), but were negatively correlated in fast-growing broilers (ρ=-0.286; p=0.001). Young (7.5 ng/g; p=0.002) and light (8.8 ng/g; p<0.001) broilers had higher fCORT concentrations than older (5.0 ng/g) and heavier (4.7 ng/g) ones. Fear increased with age, suggesting reduced self-efficacy. fCORT concentration declined with age and weight gain, suggesting stress response downregulation.