Increased stocking density impacts negatively on broiler chicken bone quality

Because broiler chickens are slaughtered before reaching physical maturity, and are thus constantly undergoing skeletal development, stocking density during rearing may influence their bone quality. A negative influence on bone quality predisposes broilers to several welfare problems. For instance, increased tibia curvature may cause lameness which compromises the bird’s ability to move around and to get sufficient feed and water, whereas decreased bone strength increases the risk of fracture during catching and transport. In the present study we evaluated the effect of stocking density on bone quality. Birds were housed in 3.3 m2 pens at 2.4, 5.8, 8.8, 12.1, 13.6, 15.5, 18.5, or 21.8 birds/m2 (4 pens per density). At 39 days of age the birds were euthanized and several aspects of bone quality were measured. The effects of stocking density, sex and weight were analysed in a linear mixed model including pen as a random factor. Tibia curvature (measured on a 0 to 3 scale, 0 being nearly perfectly straight and 3 severely bent) was greater in males than in females (0.74 vs. 0.60, F1,382=9, P=0.004) and increased by 34% from the lowest to the highest stocking density (0.70 vs. 0.94, F1,19.9=9, P=0.006). Tibia strength (as measured in a shear test) decreased by 10% from the lowest to the highest density (0.65 vs. 0.59 kN, F1,11.5=12, P=0.005). Tibias were also slightly shorter when stocking density was higher (10.9 and 10.7 cm at the lowest and highest stocking density, respectively, F1,23.7=6, P=0.020). This last effect may be due to the increased curvature, as length measurements were made in a straight line. In contrast, there were several other bone quality measures for which no effect of stocking density could be detected (i.e., tibia weight, torsion, and dyschondroplasia, and femur curvature and epiphysis shape, P>0.10). Group size was small in the present study, but it is unlikely that the results are scale-specific as an effect of stocking density on bone quality was previously found in commercially sized flocks (Hall, 2001). As such, changes in bone quality indicated a negative impact of stocking density on broiler welfare, although this effect was only apparent for certain specific indicators of bone quality.