Keel bone fractures are associated with individual mobility of laying hens in an aviary system

Abstract

Keel bone fractures (KBF) in laying hens have been shown to cause pain and impair mobility under experimental conditions. However, it is not known how KBF relates to the mobility of individual hens housed in aviary systems. For the current study, 120 focal hens (60 Lohmann Brown (LB) and 60 Lohmann Selected Leghorn (LSL)) were kept in six identical pens equipped with a commercially relevant aviary system (20 LSL focal hens + 205 LB or 20 LB focal hens + 205 LSL per pen, respectively). Data on hen mobility were recorded at 21, 24, 27, 31, 35, 39, 44, 48, 52, 57 and 61 weeks of age. Infrared receivers were attached to the legs of focal hens. They recorded zone-specific codes between five zones (litter, lower tier, nest boxes, top tier, and wintergarden) at a frequency of 1 Hz for six consecutive days per week of age. At the end of each data collection period, hens were radiographed to assess keel bone fracture severity. Data were analysed using (generalized) linear mixed effect models. With increasing KBF severity, LB hens spent more time in the top tier (p = 0.005) and less time in the litter zone (p < 0.0001) and in the lower tier (p = 0.001). Independent of KBF, LB hens spent less time in the wintergarden (p = 0.011) and in the lower tier (p = 0.002) and more time in the top tier (p = 0.009) with increasing age. The likelihood of crossing more than one zone within a movement (e.g., jumping from the top tier to the litter directly) increased with increasing KBF severity (p = 0.036). Lohmann Selected Leghorn hens spent most time in the nest box zone and top tier and had few transitions between zones. With increasing age, LSL hens spent less time in the nest box zone (p = 0.018) and more time in the top tier (p = 0.006). Irrespective of strain, hens crossed fewer zones with increasing age (LB: p = 0.009, LSL: p = 0.002). Our findings indicate that hens having KBF prioritized paths among the upper tiers (i.e., mostly between nest box zone and top tier) over paths among the mid and lower tiers (i.e., between litter, lower tier and nest box zone). We conclude that behavioural adaptation to pain, the accessibility of resources as well as social factors might be important mechanisms driving individual mobility in response to KBF.