Livestock production raises significant environmental concerns, necessitating the development of sustainable feeding strategies based on non-conventional forages, such as locally available vegetation. This study evaluated the effects of a pelleted concentrate containing 10% Acacia farnesiana leaves as a dietary supplement on in vitro ruminal fermentation. Four experimental diets were formulated with increasing levels of the concentrate (0%, 25%, 50%, and 75%). Analyses were performed in triplicate and included chemical composition, in vitro gas and methane production, fermentation kinetics, ammonia nitrogen concentration (N–NH₃), in vitro dry matter digestibility (IVDMD), and metabolizable energy (ME) estimation. The results revealed no significant differences (p > 0.05) in most gas production kinetic parameters, overall fermentation patterns, or metabolizable energy. In contrast, a significant increase (p < 0.05) in secondary metabolite concentrations was detected. While methane production remained unaltered (p > 0.05), a significant linear reduction was observed for IVDMD, the lag phase (L), and N–NH₃ concentration (p = 0.0064, p = 0.0036, and p < 0.0001, respectively). These findings suggest that A. farnesiana can be incorporated into ruminant concentrates without increasing methane emissions. However, in vivo trials and mechanistic studies are required to validate and further elucidate these results.