Eight ruminally cannulated Angus cross heifers (518 ± 33.4 kg) were used in a replicated 4 × 4 Latin square to examine the impact of a sugarcane polyphenol extract (SPE) on feed intake, rumen fermentation, apparent total tract digestibility, methane (CH₄) production, and the rumen microbiome. Heifers were offered a corn silage-based diet with either 0%, 0.3%, 0.6%, or 1.2% of SPE replacing barley grain. Rumen fluid was analysed for volatile fatty acids, protozoa counts, and bacterial diversity. Rumen pH data loggers were used for continuous measurement of ruminal pH and heifers were housed individually to measure apparent total tract digestibility. Cattle were placed into respiration chambers to measure CH₄ production. Data were analysed using the mixed model procedure of SAS with the fixed effect of treatment, and the random effects of block, heifer nested within block, and period. Supplementation of SPE increased absolute ruminal pH (P ≤ 0.01) and its variability (P = 0.05); however, there was no impact on intake, rumen fermentation or bacterial diversity (P ≥ 0.11). The results of this study suggest that SPE does not influence CH₄ production in heifers fed high-forage diets.