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

Pour trials were conducted to investigate whether manipulations of photoper iod: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 abil ity by gait scoring (GS) and tibial dyschondroplasia by x-ray (TD). Foot bu rn, 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 fou r trials were consistent. Increased photoperiod was associated with increas ed BW and prevalence of TD. There was no clear relationship between photope riod and GS but foot pad bums were reduced by longer photoperiod. When the data were adjusted for differences in BW, increased photoperiod was associa ted with increased prevalence of TD, better walking ability (GS), and fewer hock and foot pad burns. Strong correlations were found between GS and liv e weight, and weak correlations with hock burn and TD. Tibial dyschondropla sia was weakly correlated with BW. The Linear regressions of GS on Live wei ght, within sex, across trials, were not different, but there was a differe nce between sexes, with males having a higher intercept but lower slope tha n females. It was concluded that shorter photoperiod affected walking abili ty and TD, but that these effects were largely a result of BW.