© 2022 Japanese Society of Grassland Science.The study aimed to determine the effect of fumaric and maleic acids on the ensiling process. The corn herbage was ensiled without additives (control silage) or with 0.25% fumaric acid (FA25), 0.50% fumaric acid (FA50), 0.25% maleic acid (MA25), 0.50% maleic acid (MA50), FA25 + MA25 (FA25MA25) and FA50 + MA50 (FA50MA50) (treatment silages) as wet basis. The starch contents of FA50 and MA50 silages were lower than the content of the control silage (p <.05). The lactic acid content of the FA50MA50 silage was lower than that of the control silage (p <.05). Percentages of linoleic acid of MA25 and MA50 silages were higher than that of the FA50 silage (p <.05). The in vitro methane production and metabolic energy (ME) value of FA50MA50 silage was lower than that of the control silage (p <.05). The in vitro organic matter digestion (OMD) values of FA25, MA25 and FA50MA50 silages were lower than that of control silage (p <.05). The molarities of acetic (AA), propionic (PA), butyric (BA), isovaleric (IVA), isobutyric (IBA) and total volatile fatty (TVFA) acids of the in vitro ruminal fermentation fluid for the FA50MA50 silage were lower than that of the control silage (p <.05). As a result, the addition of fumaric and maleic acids (0.25% alone and together, or 0.5% alone) in ensiling of corn herbage did not change the quality, acidity (except for propionic, butyric and isobutyric acids) and nutrient contents (except for non-fiber carbohydrates and starch) and in vitro ruminal fermentation values (gas–methane production and organic acids). The combined use of 0.5% fumaric plus 0.5% maleic acid harmed silage quality and in vitro ruminal fermentation values. However, 0.5% maleic acid was partially more effective than those other organic acid additions in preserving linoleic and α-linolenic acids in the silage material.