The utilization of natural plant secondary compounds as feed additives in animal nutrition has been extensively studied because of their ability to modify digestive and metabolic functions. Condensed tannin (CT) supplementation can potentially alter ruminal fermentation, and mitigate methane (CH4) emissions. The objective of this study was to determine the effect of quebracho CT extract (QT; Schinopsis balansae) within a roughage-based diet on overall fermentability and CH4 production utilizing the in vitro gas production technique (IVGP). Twenty rumen cannulated steers (227 19 kg) were randomly assigned to four dietary treatments (n=4): QT at 0, 1, 2, and 3% of DM (QT0, QT1, QT2, and QT3). A roughage-based diet containing 88% bermudagrass hay and 12% concentrate was fed daily at 2.1% of shrunk body weight. The animals were adapted to the basal diet for 24-d then introduced to predetermined treatments for 35d. Rumen inoculum was collected weekly from each steer to perform the incubations. Two hundred milligrams of air-dried base diet were incubated for 48-h with a composite rumen inoculum for each treatment over 5 wk. Kinetic analysis of cumulative 48h gas production was performed using Gasfit. Measurements of CH4 were performed via gas chromatography and digested residue was determined post-incubation. Data were analyzed using a random coefficients model. Total gas production was higher for QT0 compared to QT1 and QT3 (P = 0.001), but not different from QT2 (P = 0.554). The fractional rate of gas production was higher for QT2 compared to QT0 (P = 0.011). First and second pool gas production decreased linearly as QT inclusion increased (P = 0.042 and 0.010, respectively). There was no dietary effect in ivNDFD (P = 0.567). However, there was a linear tendency to decrease CH4 production with the addition of QT (P=0.071) likely due to changes in the microbial population.