Zeolite supplementation modulates ruminal ammonia and methane production without compromising animal performance: A meta-analysis of in vitro and in vivo studies

Zeolite has potential to improve nitrogen utilization and modulate rumen fermentation through its ion-exchange and adsorption properties; however, reported responses in ruminants remain inconsistent. This meta-analysis quantified the effects of dietary zeolite supplementation on rumen fermentation, nutrient digestibility, productive performance, milk characteristics, and blood metabolites. Data were obtained from 34 eligible articles identified through Scopus, PubMed, and Semantic Scholar. Effect size was calculated as natural logarithm of response ratio using a random-effects model and converted to relative mean difference (RMD). Zeolite supplementation reduced ruminal NH₃-N concentration under both in vitro (RMD = -11.13%; P < 0.001) and in vivo conditions (RMD = -11.40%; P = 0.009). Methane production was also reduced in vitro (RMD = -14.53%; P = 0.035) and in vivo (RMD = -9.70%; P = 0.001). Both in vitro and in vivo, zeolite increased total VFA (mmol/L), acetate (%), propionate (%), while decreasing butyrate concentration, the acetate-to-propionate ratio. In vivo, zeolite improved the digestibility of dry matter, organic matter, crude protein, ether extract, total digestible nutrients, neutral detergent fiber, and hemicellulose. Dry matter intake, average daily gain, milk yield, and milk composition were affected, whereas growth (RMD = 12.52%; P = 0.011) and milk efficiency (RMD = 13.66; P = 0.05) increased. Zeolite reduced blood urea nitrogen (RMD = -7.96%; P = 0.003), although aspartate aminotransferase activity increased modestly. Meta-regression indicated a dose-dependent reduction in ruminal NH₃-N, while subgroup responses varied according to zeolite type and animal species. In conclusion, Zeolite supplementation at inclusion levels of 0.1-5% showed beneficial effects on ruminal fermentation, including reductions in ruminal NH₃-N and methane production, improved nutrient digestibility, and maintained or improved animal productive performance, with responses in fermentability being more evident in small ruminants. However, an optimum inclusion level could not be definitively established, and further studies are needed to clarify the underlying microbial, enzymatic, and post-ruminal mechanisms, which were insufficiently addressed in the available literature.