Assessment of mobility and pain in broiler chickens with identifiable gait defects

Gait defects, assessed as gait score (GS), is a common welfare problem in broiler chicken production. The aim of this study was to examine whether birds with GS2 experience pain and poor locomotor ability compared to birds of lower GS. From day 0-27 of age, 600 Ross 308 birds were housed in simulated commercial conditions (stocking density 40 kg/m2). On day 27, all birds were gait scored and 192 birds (4 blocks of 48 birds) were selected as experimental birds based on gait score (GS0 or GS2). On day 28, a runway test with and without obstacles was performed to examine the locomotor ability of the birds. Approximately 3 hours before testing, the experimental birds were injected with the NSAID carprofen (25 mg/kg s.c.) or saline (equivalent volume). A Conditioned Place Preference (CPP) test was conducted on day 38, after 4 days of conditioning to a chamber (70 × 70 cm; painted either green with vertical stripes or orange with horizontal stripes) after injection with carprofen or saline. The broilers were expected to show a preference for the place in which they experienced a positive emotional state as induced by the mitigation of pain by carprofen. In the runway test, GS0 birds reached the end of the runway with obstacles faster compared to GS2 birds (P = 0.04) and tended to be faster than GS1 birds (P = 0.08). No effect of carprofen was found on the behaviour in the runway and no effect of GS on the behaviour in the runway without obstacles. In the CPP test, there was no effect of GS or carprofen on chamber preference (P > 0.05). The results showed clear differences between birds of GS2 and lower GSs in terms of locomotor response in the runway, indicating that locomotor ability was compromised with increased GS. The results could not confirm whether these differences in locomotor ability are related to or due to increased pain with increased GS.