Effect of day length on flock behavioural patterns and melatonin rhythms in broilers.

1. Ross × Ross 308 male broilers were used to study the impact of 14, 17, 20 and 23 h of light (L) on flock behavioural patterns and 24 h melatonin rhythm synchrony during the photophase. 2. Behaviour in two pens (n = 53) per lighting treatment was digitally recorded for 24 h in trial 1 (27-28 d of age (d 27)) and two (42-43 d (d 42)), and quantified using a scan sampling technique at 10 min intervals. Regression procedures were used to test flock trend analysis between behavioural (resting, standing, walking and feeding) variables and time during the photophase. 3. The presence of a flock melatonin rhythm was determined by radioimmunoassay of blood samples collected at 6 times for birds raised on 23L and 8 times for 14, 17 and 20L birds (n = 6 per time) over a 24 h period (d 21) in experiment 1. 4. Quadratic and linear relationships between time and behaviour during the photophase were frequent in 14L and 17L birds, sporadic in 20L birds and non-existent in 23L birds. Relationships were noted in inactive resting (d 27: 14L and 17L; d 42: 14L and 20L), walking (d 27: 14L and 17L), standing (d 27 and d 42: 14L, 17L and 20L) and feeding (d 27: 14L and 17L). A quadratic time × melatonin relationship existed in 14L, 17L and 20L flocks only. 5. Behaviour during the photophase and 24 h melatonin production indicates that flocks exposed to 23L do not develop synchronised rhythms, increasing the risk of suffering from sleep fragmentation.