Influence of Stocking Density on Tonic Immobility, Lameness, and Tibial Dyschondroplasia in Broilers

Abstract

This study assessed the effect of stocking density on the development of fear reaction (tonic immobility), lameness (gait), and tibial dyschondroplasia in broiler chicks of mixed sex. During 3 experiments, the study used 49,500 broiler chicks (Ross 208). Of these, 432 focal chicks were randomly selected and studied in 3 experiments (144 per experiment). The focal chicks were individually color-marked on the head and wings for identification. Series of stocking densities compared were 9 and 29, 13 and 25, 17 and 21, 18 and 30, and 20 and 28 chicks per m². From 8 to 42 days of age, the study tested focal chicks once a week for lameness (gait) using a scoring scale from 0 to 5 and for the duration of tonic immobility measured in seconds. Each trial terminated with the chicks' being humanely killed and examined for the expression of tibial dyschondroplasia, using a score from 0 to 3. For all series apart from 13 and 25 chicks per m², the average increase in the duration of tonic immobility per day was significantly higher, p < .05, for chicks kept at higher stocking densities. Tonic immobility durations also increased in chicks suffering from tibial dyschondroplasia, especially at 39 days of age, p < .01. The incidence of lameness was higher for high stocking densities among the series compared, p < .05 to p < .001. For most compared series, the growth rate also increased with a unit increase, p < .001. The incidence of tibial dyschondroplasia was significantly higher, p < .01, at stocking densities of 29, 25, 21, 30, and 28 birds per m² compared to those at 9, 13, 17, 18, and 20 birds per m², respectively. A total of 59.6% and 19.2%, respectively, of birds had tibial dyschondroplasia. The results show that high stocking densities adversely affect the welfare of broilers.