Effects of the rearing period and the production phase on fear-related responses in adult laying hens

In 2013, almost seven billion laying hens (Gallus gallus domesticus) produced eggs for human consumption (CIWF, 2013). Under commercial egg production, specialised rearing farmers keep the birds until they are nearly sexually mature at 16–18 weeks of age. Prior to the onset of lay, the birds are transferred to an egg production farm. The housing systems used in commercial rearing and egg production farms are confined systems (cages) and loose-housed systems (single-tier floor or multi-tier aviary systems) (Landbrug og Fødevarer Erhvervsfjerkræsektionen, 2015). One of the main differences between these housing systems is the level of environmental complexity. In Norway, the majority of laying hens are reared in aviaries. As a consequence, some of the aviary-reared birds are transferred to furnished cages for egg production. The literature suggests that the environment during early life affects physical and behavioural development. However, little knowledge exists on how aviary rearing, as opposed to cage rearing, affects fear responses in adult laying hens. Fearfulness is the predisposition of an individual to be easily frightened (Boissy, 1995; Jones, 1996). Fear is the emotion experienced by an individual when exposed to dangerous stimuli. Fear results in behavioural, physiological, and cognitive responses. The conscious component of fear cannot be assessed verbally in animals, but measures of neural, physiological, and behavioural responses can provide information about their emotional state. In the wild, fearfulness is adaptive as it increases the chance of survival. Although fearfulness has been reduced after generations of domestication, predator-avoidance behaviour is still observed in laying hens exposed to humans or novelty (Boissy, 1995; Jones, 1996; Waiblinger et al., 2006). In commercial egg production, high levels of fearfulness and subsequent fear responses are associated with problems such as feather pecking (Uitdehaag et al., 2009; de Haas et al., 2010; de Haas et al., 2013; Kops et al., 2013; Rodenburg et al., 2013), cannibalism (Newberry, 2004) and panic induced smothering (Mills and Faure, 1990; Boissy, 1995; Bright and Johnson, 2011; Richards et al., 2012; Barrett et al., 2014). The core welfare aspect of this thesis was to investigate the effects of different levels of environmental complexity during the rearing period on fear responses in adult laying hens. Papers I and II compared the effects of aviary versus cage rearing in birds after transfer from the rearing farm to the egg production farm. Fear responses were assessed at 19, 21, and 23 weeks of age, tested in the home cage (paper I) or in a test arena (paper II). The results from papers I and II suggest that aviary rearing reduces the expression of fear responses compared to rearing in cages. Paper III investigated whether the provision of a pecking substrate (a thin layer of paper covering the wire mesh floor) from the first day of life affected fear responses in aviary-reared birds. The control group was reared on the wire mesh, without access to a pecking substrate until later in the rearing period. Fear tests were conducted at the egg production farms when the birds were at the peak of lay. The results from paper III indicate that access to a pecking substrate during the early rearing period reduces the fear response to a novel object. However, the effect was only evident in birds that were not provided with environmental enrichment at the egg production farm. In other words, provision of environmental enrichment at the egg production farm can counteract a lack of a pecking substrate during the early rearing period. The results of paper III therefore emphasise that both the environmental complexity during rearing and the environment and management during the egg production phase affects fear responses in laying hens. In paper IV, risk factors associated with problem behaviours in adult hens were evaluated. The risk factors identified were related to management and housing systems at the production farm. Combined, the results from papers III and IV emphasise that both the rearing period and the conditions at the production farm affect problem behaviours in adult laying hens. The complete elimination of fear is neither desirable nor required to safeguard the best welfare possible for laying hens in commercial egg production. However, by including fearfulness in the breeding programs and focusing on optimising environment and management during rearing and production, problem behaviours can be reduced. By increasing environmental complexity and the level of stimulation the chicks and pullets experience during rearing, and by providing environmental enrichment during the production phase, farmers can improve the welfare of commercial laying hens