Are changes in behavior of fast-growing broilers with slight gait impairment (GS0-2) related to pain?

Impaired walking ability in terms of slight or definite defects is more common in broiler production than lameness that obviously hinders movement, but it has received limited scientific attention. This study aimed to compare behavior of conventional broilers with impaired walking ability (assessed as gait score (GS) 2) with those walking normally (GS0) and those with only a slight gait defect (GS1). Behavior in the home environment was registered, and an analgesic intervention to quantify changes in time budgets indicating pain relief was applied. The study included 192 Ross 308 broilers. On day 27 of age, the birds were distributed as evenly as possible into birds of GS0 and GS2 of each sex based on obtained gait score. Following this, each experimental bird was housed with 3 companion birds. On days 30 and 32 of age, the behavior in the home pens was recorded. All experimental birds were injected with the NSAID carprofen on one of the 2 d and saline on the other. The statistical analyses used the GS scored on the day of recording as explanatory factor. Compared to GS0 birds, GS2 birds tended to be more inactive (mean (CI): 4,193 (3,971-4416) vs. 4,005 (3,753-4,257) s; P = 0.074), spent more time sitting while feeding (306 (266-353) vs. 213 (180-251) s; P = 0.026), were less likely to perch (probability: 0.78 (0.69-0.85) vs. 0.91 (0.85-0.95); P = 0.012), and spent less time performing comfort behavior (749 (689-814) s vs. 875 (792-967) s; P = 0.043). Compared to GS1 birds, GS2 birds spent more time inactive (GS1: 4,022 (3,818-4225) s; P = 0.027), less time foraging (289 (253-329) vs. 347 (309-388) s; P = 0.047), and were less likely to perch (GS1: 0.90 (0.86-0.93); P = 0.001). For some of these behavioral variables, administration of carprofen led to behavioral changes across the GSs, which may suggest that the behavioral expression of the broilers was limited by pain. These findings are of relevance to animal welfare, but the underlying causes are still not fully clarified.