The objective of this study was to evaluate the effects of 0, 4, 8, and 12 MM DL-malate on the in vitro mixed ruminal microorganism fermentation of alfalfa hay and Coastal bermudagrass hay. When alfalfa hay was the substrate, 4 and 8 MM DL-malate numerically increased propionate concentration, and 12 mM DL-malate increased (P < 0.10) propionate. All three concentrations Of DL-malate decreased (P < 0.05) the acetate: propionate ratio. In Coastal bermudagrass hay fermentations, all three DL-malate concentrations increased (P < 0.05) propionate and decreased (P < 0.05) the acetate: propionate ratio, while 4 and 12 mM DL-malate numerically increased in vitro dry matter disappearance. When mixed ruminal microorganisms were incubated with 6.25 MM DL-lactic acid and alfalfa hay, 8 and 12 mm DL-malate increased (P < 0.05) final pH, and 12 MM DL-malate increased (P < 0.10) propionate and decreased (P < 0.10) the acetate:propionate ratio. DL-Malate treatment had little effect on in vitro dry matter disappearance. Addition of 8 and 12 MM DL-malate to Coastal bermudagrass hay Plus DL-lactic acid fermentations increased (P < 0.05) final pH, and 8 mM DL-malate increased (P < 0.10) in vitro dry matter disappearance. Even though DL-malate treatment consistently increased final pH values in fermentations that included DL-lactic acid, there was not a corresponding increase in in vitro dry matter disappearance of either alfalfa hay or Coastal bermudagrass hay in the 48-h batch culture incubations.