Laying hen's mobility is impaired by keel bone fractures and does not improve with paracetamol treatment

Keel bone fractures (KBF) in laying hens pose a severe problem for animal welfare as they are likely to be associated with pain and suffering. Furthermore, they are suspected to hinder or restrict hens in their performance of natural species-specific behaviour. The aim of this study was to determine whether KBF affect laying hen behaviour in a non-cage system and whether this alteration is caused by pain. Forty Brown Nick and 40 Nick Chick hens were individually marked, and video recorded in eight experimental pens containing two perches with a platform attached to a ramp to facilitate access, an elevated nest box with platform for landing, drinker, feeder and litter. Data was collected at 37 and 39 weeks of age (WoA) on two consecutive days each at three time points per day. Paracetamol was administrated via drinking water either during the 37th or the 39th WoA. Additionally, activity assessments were done at 26, 28, and 30 WoA to detect differences pre-fracture. To assess the keel bone (KB) state, hens were radiographed at 30, 37 and 39 WoA and KB categorised as having no fracture, healed fracture(s) or fractures with a visible fracture gap. The odds of perch or nest access (i.e., vertical locomotion) decreased when a fracture gap was visible (p = 0.004) but the walking pace on ramps leading to the perches was not affected by KBF. Paracetamol did neither reverse the decrease in mobility behaviour nor did it affect the walking pace on the ramp. Of the hens with fractures featuring a visible fracture gap, those receiving paracetamol performed less rapid comfort behaviours (e.g., wing flapping) than hens receiving vehicle (i.e. water) only (p = 0.04). Preening and dustbathing duration was increased with the administration of paracetamol (p = 0.02) though was unaffected by KBF in this study. No difference was found in activity before the fracture(s) occurred. In conclusion, we show that KBF affect laying hen mobility behaviour in non-cage systems and thus a reduction of KBF would improve laying hen welfare.