Storing livestock manure is the primary stage of manure management where microbial processes and chemical reactions result in the release of methane (CH4), nitrous oxide (N2O), ammonia (NH3), and carbon dioxide (CO2). This study examined the reduction of CH4 emissions from slurry storage under two temperatures (cool [10°C] and warm [30°C]) when a glucose‐rich substrate (brewing sugar) and activated effective microorganisms were applied at 10% (w/w) and 5% (v/w), respectively. Brewing sugar addition influenced microbial anaerobic respiration, resulting in a reduction of slurry pH to <5.0, through “self‐acidification” caused by lactic acid production. Subsequently, CH4 emissions were significantly reduced by 87 and 99% in the cool and warm environments, respectively. The effective microorganism treatment did not change the chemical characteristics of the slurry but reduced CH4 emissions by 17 and 27% (P < 0.05) in the cool and warm environments, respectively. These results suggest that self‐acidification after addition of a carbon source may be a promising alternative to slurry acidification using concentrated acids.