The influence of zinc oxide utilization in ruminants: A systematic review and meta-analysis

Zinc (Zn) is an essential trace mineral involved in rumen microbial activity, nutrient metabolism, and animal productivity. This meta-analysis quantitatively evaluated the effects of ZnO supplementation on rumen fermentation characteristics, nutrient utilization, and productive performance in ruminants using 17 eligible studies published between 1971 and 2025. Standardized mean differences (SMD) were pooled using random-effects models. Heterogeneity was assessed using I² statistics, while subgroup analysis were performed according to ZnO form. Publication bias was evaluated using funnel plot inspection, Egger's regression test, and trim-and-fill correction. ZnO supplementation significantly improved several in vitro fermentation parameters by reducing methane production (SMD = -1.281; 95% CI: -1.818 to -0.743; P < 0.001), increasing propionate concentration (SMD = 0.521; P = 0.026), decreasing the acetate-to-propionate ratio (SMD = -1.226; P < 0.001), and increasing metabolizable energy (SMD = 2.034; P < 0.001). In vivo, ZnO supplementation reduced ruminal ammonia concentration (SMD = -0.869; P < 0.001) while increasing total volatile fatty acids (SMD = 0.520; P = 0.018), dry matter digestibility (SMD = 9.944; P < 0.001), average daily gain (SMD = 0.565; P = 0.004), dry matter intake (SMD = 0.649; P = 0.020), and alkaline phosphatase activity (SMD = 0.822; P = 0.033). Subgroup analysis revealed that nZnO consistently produced larger effect sizes for methane mitigation and growth responses than ZnO. Although publication bias was detected for methane outcomes, trim-and-fill analysis confirmed the robustness of the antimethanogenic effect (adjusted SMD = -1.082; P < 0.001). These findings provide quantitative evidence supporting that the supplementation of ZnO is a feasible option to improve productivity and sustainability in ruminant feeding systems.