Nature and nurture: genetic strains of broiler chickens housed in varying levels of environmental complexity

Abstract

Conventional broilers show welfare concerns associated with their fast growth. Using slower-growing strains or adding environmental complexity has been shown to improve broiler welfare. However, less is known about slow-growing broiler affective state, and the impact environmental enrichment will have on affective state in different strains. Poor health can cause pain, and barren environments can limit broilers in expressing natural behavior, resulting in negative affect. Through measuring affective state, we can get an overall view of the animal's internal experience, thus welfare.

In chapter 3, anxiety in fast- (Ross 708) and slow-growing (Redbro M) broilers was assessed when birds were raised with varying environmental complexities and as they aged and gained weight. Redbro M broilers showed lower anxiety through more broilers feeding and showing less vigilance during the attention bias test. Complex environments decreased anxiety in early life but anxiety increased to similar levels as those raised in simple environments as broilers gained weight. Anxiety, shown through vigilance, increased as broilers gained weight. These results indicate that affective state, specifically anxiety, can change as birds gain weight, depending on environmental complexity and genetic strain. Overall, slow-growing broilers (Redbro M) showed reduced anxiety compared to fast-growing broilers (Ross 708), across housing treatments and weights. In chapter 4, fear and chronic stress were compared between fast- (Ross 708) and slow-growing (Redbro M) broilers with different environmental complexities. Fear increased as broilers gained weight, with Ross 708 broilers showing more fear at equal age points, but not weight points, compared to Redbro M broilers. Younger and lighter broilers showed lower chronic stress than older and heavier broilers. Fear may have increased due to weight-gain associated factors, including lameness and increased inactivity, which may have resulted in a lower sense of self-efficacy. Chronic stress may have decreased as broilers aged and gained weight due to broilers having their stress response downregulated, allowing them to maintain homeostasis and function properly even while experiencing stressors.

In chapter 5, chronic stress and anxiety were assessed between six broiler strains raised with either a hut or platform. Redbro M and Ja57NH broilers had lower anxiety compared to other strains. Redbro M had lower fCORT concentration, indicating lower chronic stress, than Ja57NH broilers and tended to have lower fCORT concentrations than Cobb 500 broilers. For secretory immunoglobulin-A (SIgA), with higher concentrations indicating less chronic stress, fast-growing broilers showed higher concentrations than Redbro M and Ja57NH. Enrichment type did not impact anxiety or chronic stress. Anxiety and chronic stress (fCORT) were lower in slow-growing broilers, especially the Redbro M broilers. SIgA concentrations may be impacted by genetic selection, thus it does not accurately reflect chronic stress between different genetic strains.

In chapter 6, production, contact dermatitis, and lameness were assessed in broilers strains with different growth potentials when raised with either a hut or platform. Fast-growing broilers showed fast growth and high feed conversion efficiency compared to slow-growing broilers. Contact dermatitis and lameness increased in prevalence over age but was had higher prevalence at younger ages in fast-growing broiler strains. Slow-growing broilers showed lower prevalence of breast burn and lameness, but Redbro-Yield broilers tended to show high prevalence of footpad dermatitis compared to Ja57NH and Ross 308 broilers. Generally, Redbro-Yield and Ja57NH broilers showed the lowest prevalence while Redbro M broilers were more intermediate. Lameness was more prevalent in Ross 708 and Ross 308 broilers, intermediate for Cobb 500 and Redbro M broilers, and lowest in Redbro-Yield and Ja57NH broilers. Enrichment type did not impact production or contact dermatitis, but platforms decreased lameness prevalence across strains in week 6. Fast-growing broilers showed high productivity but experienced higher rates of health issues. However, health outcomes varied by strain and aspect assessed.

In conclusion, slow-growing broiler strains showed overall better welfare, but there are differences between slow-growing broiler strains. Redbro M and Ja57NH broilers showed lower anxiety, Redbro M broilers showed lower chronic stress, and Redbro-Yield and Ja57NH broilers showed lower incidence of contact dermatitis and lameness compared to fast-growing strains assessed. Complex environments may reduce anxiety in early life, but they did not show benefits on affective state through other measures or later in life. Both huts and ramps had similar impacts on health and affective state, indicating they are equally suitable.