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The relationships between the severity of lameness, the presence of tibial dyschondroplasia, and the frequency of dustbathing behaviour and duration of tonic immobility were studied in 96 broiler chicks kept from day-old in groups of four in wire-floored cages. Dustbathing was observed when the birds were given access for one hour to a tray containing sand or straw. The duration of tonic immobility and the severity of lameness were scored during weeks 3, 4, 5 and 6. After slaughter at 41 or 45 days of age the birds were examined for tibial dyschondroplasia; 34 birds were classified as having tibial dyschondroplasia and 54 had detectable lameness problems. Birds with tibial dyschondroplasia had a higher lameness score (P < 0.001), dustbathed on fewer days (P < 0.0001), and had longer periods of tonic immobility (P < 0.03) at six weeks than birds which did not have the condition. However, almost all the chicks, including those with tibial dyschondroplasia dustbathed on day 27 after they had not been given access to the tray for three days, significantly more than on days when they had not been deprived of the tray (P < 0.0001). Furthermore, lame birds and birds with tibial dyschondroplasia also dustbathed less with age. Studies of the vertical wing-shaking element of dustbathing confirmed that tibial dyschondroplasia reduced dustbathing (P < 0.05) and also showed that during the one-hour tests, sand resulted in more dustbathing than straw (P < 0.01).

1. The aims of this study were to determine (1) the effect of light-dark schedules on the walking ability, the risk of tibial dyschondroplasia (TD) as well as the duration of tonic immobility (TI) reactions in commercial broiler flocks and (2) the effect of a daily dark period and reduced density on the behaviour of broiler chickens. 2. Experiment 1. Group 1 had a 2 to 8 h daily dark period from 2 to 26 d of age (light-dark programme A) at a stocking density of 28.4 chicks/m2. Group 2 had 8 h of darkness daily from 2 to 38 d of age (light-dark programme B) at 24 chicks/m2. The control group had 24 h continuous light at 28.4 chicks/m2. 3. Experiment 2. Behaviour was studied with and without a daily 8 h dark period and at high (30 chicks/m2) and low (18 chicks/m2) stocking densities. 4. Programme B reduced the prevalence of impaired walking ability, corresponding to gait score > 2, when compared with controls. The effect on walking ability corresponding to gait score > 0 approached significance. 5. Both light-dark programmes reduced the occurrence of TD. Programme B (combined with reduced stocking density), however, had the greater effect. 6. Both light-dark programmes reduced the duration of TI, compared with controls (mean = 426 s) Programme B resulted in a larger reduction (alpha = -156.9 s) than programme A (alpha = -117.0). 7. The proportions of chicks drinking, eating, pecking, scratching, standing and performing vertical wing-shakes increased--both when the 8 h dark period and the reduced stocking density were applied separately and in combination (experiment 2). 8. For all behaviours, except standing, the effect of the dark period was largest in broilers kept at the high stocking density (d 40).

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