Contrast in light intensity, rather than day length, influences the behavior and health of broiler chickens.

Day length and intensity are commonly manipulated aspects of the light environment in commercial broiler production. Both influence circadian rhythms, but it is unclear if they do this independently or synergistically. The effect of light:dark (20L:4D, 16L:8D) and intensity contrasts (1 lx:0.5 lx, 200 lx:0.5 lx) on broiler behavior and health (n=1,004, 4 replicates/treatment) was evaluated. Activity was measured using passive infrared detection, and feeding activity was measured by the amount of feed consumed/h over one 24-h period each week. Broilers were gait scored and weighed at 6 wk of age. Following euthanasia, eyes were dissected from 30 birds/treatment. Behavior and performance were analyzed using the GLM, gait score using the Kruskal-Wallis test, and eye measures using a MANOVA. The 200 lx birds were more active (P=0.03) and fed more (P=0.001) during the photophase but were less active (P=0.02) and fed less (P<0.001) during the scotophase, than the 1 lx birds. There were no differences in G:F (mean±SEM, 1.63±0.01 kg of feed/kg of BW). However, 1 lx birds were slightly heavier (2.79±0.01 kg; P=0.02) than 200 lx birds (2.72±0.01 kg). The 200 lx birds had better (P<0.001) mean gait scores than 1 lx birds, although treatment differences were small. One lux birds had greater side-to-side (18.86±0.11 mm vs. 17.63±0.11 mm, P<0.001) and back-to-front (13.39±0.09 mm vs. 12.89±0.09 mm, P<0.001) eye diameters and heavier eyes (2.42±0.03 g vs. 1.99±0.03 g, P<0.001) than 200 lx birds. There was only one effect of light:dark, with 16:8 having greater back-to-front eye diameters than 20:4 (13.30±0.10 mm vs. 13.00±0.10 mm, P=0.02). There were no interactions. These results indicated that light intensity, not day length, was the major factor affecting broiler behavior and health under these lighting conditions. Low contrast light intensity dampened behavioral rhythms and had possible health effects.