In Vitro Antimethanogenic Potential of Invasive Weed Species from South-Central Montana in Roughage-Based Diets for Beef Cattle

(1) Background: Beef cattle production is essential for Montana’s economy. These operations rely on fibrous diets promoting enteric methane (CH4) emissions, resulting in dietary energy inefficiency and environmental impacts. Concurrently, noxious weeds invade approximately 10% of state rangeland. Yet, some species produce plant secondary metabolites (PSMs) that could be repurposed for CH4 mitigation strategies. (2) Methods: This study evaluated seven invasive weeds from South-Central Montana using in vitro incubations with ruminal fluid from cannulated Angus Cows (590 ± 18 kg). Experiment 1 screened whole plants added at 20% of incubated dry matter (IDM) against a control (hay), then, experiment 2 evaluated ethanolic extracts from leaves and inflorescences from top candidates identified in experiment 1. (3) Results: In the screening phase, common burdock (BURD) and Curly dock (DOCK) modified fermentation dynamics. BURD leaf extract achieved a 25.2% reduction in CH4 yield (mL/g fermented organic matter) without compromising in vitro organic matter digestibility (IVOMD) or ammonia (NH3-N). Conversely, DOCK extracts demonstrated the strongest CH4 volume reduction but at the cost of a dramatic reduction in IVOMD and NH3-N depletion. Bioactivity of PSM remained consistent across anatomical structures. (4) Conclusions: Specific weeds (BURD, DOCK) represent a biochemical repository with potential as a feed additive for roughage-based diets. Leaves from BURD exhibited optimal potential to mitigate CH4 emission without impairing diet digestibility.