The effect of photoperiod:scotoperiod on leg weakness in broiler chickens.

Four trials were conducted to investigate whether manipulations of photoperiod:scotoperiod affected the prevalence of leg weakness in broiler chickens. Modified photoperiods were applied from 3 until 21 d of age, followed by gradual or immediate return to 23 h light. The photoperiods tested were 8, 16, 21, and 23 h light. Leg weakness was assessed by measuring walking ability by gait scoring (GS) and tibial dyschondroplasia by x-ray (TD). Foot burn, hock burn, angulation of the hock joint, and BW were also measured. In total, 4,640 birds were assessed. The responses of the birds across the four trials were consistent. Increased photoperiod was associated with increased BW and prevalence of TD. There was no clear relationship between photoperiod and GS but foot pad burns were reduced by longer photoperiod. When the data were adjusted for differences in BW, increased photoperiod was associated with increased prevalence of TD, better walking ability (GS), and fewer hock and foot pad burns. Strong correlations were found between GS and live weight, and weak correlations with hock burn and TD. Tibial dyschondroplasia was weakly correlated with BW. The linear regressions of GS on live weight, within sex, across trials, were not different, but there was a difference between sexes, with males having a higher intercept but lower slope than females. It was concluded that shorter photoperiod affected walking ability and TD, but that these effects were largely a result of BW.