Lighting schedule and dimming period in early life: consequences for broiler chicken leg bone development.

Prolonged (>20 h) light periods during grow-out of broiler chickens have been shown to increase the occurrence of skeletal abnormalities, but the effects of early life light-dark schedules are not well known. The present experiment investigated the effect of lighting schedule and light-dark transition during the first days of a broiler chicken's life on leg bone development. In 2 experiments, Ross-308 broiler chicks (n = 2,500 per experiment) were subjected to 1 of 5 treatments for 4 d: 24L; 2L:1D lighting schedule with either an abrupt or gradual light-dark transition ("dimming"); and a 2L:6D lighting schedule with an abrupt transition or dimming. At d 4, tibia and femur weight, length, and diameter, yolk free body mass, organ weights, realized weight gain, feed intake, feed conversion ratio, and mortality were determined. In Experiment 2, chick length and relative asymmetry of the femur and tibia were determined additionally. Data were analyzed using orthogonal contrasts. 24L resulted in higher femur diameter (P<0.028; both experiments), tibia diameter (P<0.001; Experiment 1), relative asymmetry of tibia length (P=0.002; Experiment 2), and relative asymmetry of femur length (P=0.003) than applying a light-dark schedule. A 2L:1D lighting schedule resulted in higher femur length (P=0.039; Experiment 1) and relative asymmetry of tibia length (P=0.032; Experiment 2) and lower relative asymmetry of tibia diameter (P=0.016) than a 2L:6D lighting schedule. An abrupt light-dark transition resulted in higher relative asymmetry of tibia length (P=0.004; Experiment 2) and relative asymmetry of tibia diameter (P=0.018) than dimming. To conclude, leg bone development in the first 4 d of a broiler chicken's life was higher for 24L than when a lighting schedule was applied, but relative asymmetry was higher as well, suggesting developmental instability. The effect of dimming on leg bone development was less pronounced, but the decreased relative asymmetry levels in the dimming treatment suggested lower environmental stress than for the abrupt light-dark transition.