Multidimensional welfare profiling of broiler leg health under probiotic or antimicrobial growth-promoter supplementation and environmental enrichment

Abstract Broiler leg health is a complex and multidimensional welfare concern that may be inadequately characterized using isolated indicators. This study developed an individual-level multidimensional welfare-response profiling framework to evaluate broilers subjected to 3 dietary strategies—negative control, zinc bacitracin as an antimicrobial growth promoter (AGP), or a multi-strain probiotic—combined with environmental enrichment in a 3 × 2 factorial experiment involving 1,440 male broilers housed in 48 pens. Welfare outcomes were organized into 5 biological domains: locomotor performance, skeletal mineralization, postural deformity, cartilage and joint disorders, and spinal and muscular disorders. Outcomes were directionally harmonized and normalized to a 0–1 scale, with greater values consistently representing a more favorable welfare response, before aggregation into domain-specific scores and a global welfare-response index. Mixed-effects models included dietary additive, environmental enrichment, and their interaction as fixed effects and pen as a random effect to account for clustering within the experimental unit. The global welfare-response index ranged from 0.727 ± 0.022 for the control with enrichment to 0.759 ± 0.013 for the control without enrichment. No significant effects of dietary additive (P = 0.599), environmental enrichment (P = 0.207), or their interaction (P = 0.715) were detected for the global index. At the domain level, probiotic supplementation with enrichment produced the greatest numerical skeletal-mineralization score (0.631 ± 0.026), but neither dietary additive (P = 0.150), enrichment (P = 0.933), nor their interaction (P = 0.758) significantly affected this domain. The additive × enrichment interaction for postural deformity showed a tendency (P = 0.073), whereas no significant effects were detected for the locomotor, cartilage/joint, or spinal/muscular domains (P > 0.05). Sensitivity analyses using alternative domain-weighting scenarios showed that treatment rankings varied with welfare priorities, reinforcing that the composite index should be interpreted together with domain-specific outcomes. Overall, probiotic-supplemented groups showed global leg-health welfare profiles within the range observed for the zinc-bacitracin groups, while treatment responses were primarily domain-specific rather than consistently expressed across the integrated welfare profile.