Commercially fast-growing broiler chickens can experience poor welfare conditions due to barren conditions and their rapid growth. As fast-growing broilers grow and gain weight, their health can deteriorate and behavioral repertoire limited. The use of slower-growing strains and/or providing a more complex environment, could benefit broiler welfare.

This study examined the impact of body weight and environmental complexity on broiler chicken affective states behavior in two genetic strains. We tested two genetic strains (fast-growing and slow-growing) and two housing conditions (high-complexity and low-complexity) and assessed affect and behavior at age- or weight-matched timepoints.

Slow-growing broilers were less anxious than fast-growing broilers. However, anxiety increased with weight gain in both strains. Chronic stress, measured by feather corticosterone, varied between strains and weights, with higher concentrations in fast-growing birds at low body weights. Environmental complexity did not impact chronic stress responses. Age negatively impacted consummatory, exploratory, play, and active behaviors similarly for both strains. Fast-growing birds showed fewer locomotive, play, and social behaviors compared to slow-growing birds. Environmental complexity only influenced locomotive behavior, with more observed in a simple environment.

In conclusion, slow-growing broilers generally showed improved affect and behavior compared to fast-growing broilers. The effects of weight gain on affective states depended on genetic strain and environmental complexity. Overall, producers may improve affect and behavior of broilers by using slow-growing strains, processing broilers at lower weights (when housed with enrichments), or increasing environmental complexity using slow- or fast-growing strains (as anxiety was reduced at 1 and 2 kg body weights).