This study aimed to determine the effects of maleic acid (MA) addition to corn at ensiling on silage quality and in vitro methane and total gas production, metabolisable energy (ME), and organic matter digestibility (OMD) parameters by using in vitro gas production techniques. Forage corn was ensiled either without (control group: MA 0) or with three different dosages of maleic acid, 0.5% (MA 0.5), 1.0% (MA 1.0), and 1.5% (MA 1.5) w/w of the fresh material for 60 days. As a result of this study, neutral detergent fibre level was decreased in the MA 1.5 group (P<0.05). The 0.5, 1.0, and 1.5% addition of maleic acid to forage corn at ensiling increased lactic acid concentration (P<0.05) in silage and reduced propionic acid (P<0.05). Iso-valeric acid concentration in the organic acids of the silage was decreased with maleic acid addition (P<0.05). The maleic acid addition decreased in vitro ruminal methane production (P<0.01). The silage pH value, and acetic, butyric and isobutyric acid concentrations and in vitro total gas production, OMD, and ME values did not change by MA addition (P>0.05). It was concluded that MA addition could reduce methane emission without any negative effects on silage nutrient composition or in vitro ruminal fermentation parameters.