The search for practical antibiotic-sparing strategies in poultry is moving from single phytochemicals toward fermented plant matrices, where microbial biotransformation can modify nutrient availability, antimicrobial activity, and the delivery of bioactive compounds. This systematic review evaluated fermented agricultural botanicals in broiler production and performed a focused meta-analysis of feed conversion ratio (FCR). Searches through 20 September 2026 identified 23 in vivo reports meeting qualitative eligibility criteria; five non-overlapping trials provided compatible whole-period FCR data with measures of dispersion. The evidence covered fermented Ginkgo leaves, pine needles, Astragalus, ginger, grape and tomato pomace, mulberry, Chenopodium, dandelion, Artemisia, Moringa, chive, and shallot. Across studies, responses were more consistent for intestinal integrity, antioxidant status, pathogen control, and resilience under enteric or environmental challenge than for growth alone. Random-effects meta-analysis yielded a small point estimate favoring fermented botanicals (mean difference, -0.044 FCR units), but the Hartung-Knapp 95% confidence interval crossed zero (-0.093 to 0.005) and heterogeneity was high (I2=84.8%). Recent Allium studies illustrate a coherent development pathway linking host-derived probiotic selection, controlled fermentation, challenge testing, and practical delivery. Overall, fermented botanicals are promising components of antibiotic-sparing programs, but the evidence does not support universal replacement of therapeutic antimicrobials. Standardized fermentation, chemical and microbial quality control, direct antimicrobial-use outcomes, and multi-farm validation are priorities for the next phase of research.