This study was conducted to assess the effects of small supplemental doses of a polyphenolic extract from pine bark (PBE) on CH4 output and ruminal fermentation parameters when incubated in batch culture with a high–concentrate diet for 24–h. The data from the dietary substrates supplemented with 0.0, 0.3, 0.6, 0.9, 1.2, 1.5 and 1.8% of PBE were evaluated in a randomized complete block design, and compared using ANOVA followed by Tukey's test and polynomial contrasts. Increasing doses of the PBE caused a linear decrease of the NH3–N concentration (p?