Temporal Dynamics of the Faecal Microbiota and Physiological Responses to Phytobiotics and Organic Acids in Broiler Chickens

ABSTRACT This study compared three alternative feed‐additive blends with an antibiotic control to assess growth performance, physiological responses and age‐related variation in the faecal microbiota of broiler chickens. A total of 240 male Cobb 500 broilers were allocated to four treatments with four replicate pens each: antibiotic control, organic acids (OA), plant extracts and essential oils (PE+EO), and essential oils combined with prebiotic/probiotic components and minerals (EO+PRE/PRO). Faecal bacterial profiles were assessed by 16S ribosomal RNA gene sequencing in 16 pooled libraries representing the four treatments at 7, 21, 35 and 42 days. Microbiome analyses were exploratory because only one library was available per treatment–age combination. Alpha diversity did not differ significantly by treatment or age. Permutational multivariate analysis of variance identified an age effect on Bray–Curtis community composition ( R 2 = 0.312, p = 0.0139), but no treatment effect ( R 2 = 0.201, p = 0.221). No genus showed a treatment effect after false‐discovery‐rate correction. Predicted functional analysis identified 31 pathways with a global age effect and no robust treatment effect. The PE + EO blend improved feed conversion relative to the antibiotic control during Days 1–21 and 1–35. Physiological, meat‐quality and intestinal‐morphometry responses varied among treatments. Thus, host responses to these additives occurred alongside predominantly age‐associated faecal microbial variation; the data do not establish that microbiota changes mediated the host responses.