39. Invasive Weeds Species from South-central Montana as Potential Feed Additives to Mitigate Enteric Methane Emissions from Beef Cattle: An in Vitro Evaluation.

Abstract Beef cattle production represents the largest agricultural commodity in Montana, relying on grazing systems dominated by fibrous rangeland forages. Diets based on fibrous forages promote greater enteric methane (CH4) emissions. In addition, approximately 10% of Montana’s land is invaded by noxious weeds. Many of these plants contain bioactive secondary metabolites, (e.g., phenolic compounds, tannins) known to reduce ruminal methanogenesis. Despite their widespread presence and potential bioactivity, invasive weeds have received limited attention as potential feed additives or as an alternative CH4 mitigation strategies in northwestern grazing systems. Therefore, this study evaluated the effects of selected invasive weeds on ruminal fermentation, CH4 emissions and in vitro organic matter digestibility (IVOMD). To this aim, 750 mL of ruminal fluid was collected from three Angus cannulated cows (590 ± 18 kg) grazing perennial Bluebunch wheatgrass and meadow hay. Ruminal fluid was diluted with McDougall’s buffer (1:3) and used as inoculant to conduct in vitro batch culture incubations. Meadow hay (0.56 g) was incubated with 0.14 g of ground (1mm) Canada Thistle (Cirsium arvense; CAN), Common Burdock (Arctium minus; BURD), or Curly Dock (Rumex crispus; DOCK) mixed with 50 mL of inoculum in a 125 mL glass bottle, and incubated for 24 hours (39 °C, 60 RPM). In addition, four control bottles (0.70 g of hay; HAY), and four blank bottles (No substrate) were included in each of five independent incubations. After incubation, ruminal pH, ammonia nitrogen (NH3), organic matter digestibility (IVOMD), gas production, CH4 concentration and yield were determined. No differences were detected in gas production (P = 0.83) nor ruminal pH (P = 0.89) after the addition of the evaluated weed species. However, CH4 volume (P < 0.01) and concentration (P < 0.01) were reduced after DOCK addition relative to HAY. In contrast, addition of BURD increased IVOMD by 26% compared to HAY and 43% compared to DOCK (P = 0.01). Addition of DOCK decreased NH3 relative to (P < 0.05). When measured as mmol of CH4 per g of fermented organic matter, BURD and DOCK addition reduced to 25 and 46% CH4 yield respectively relative to HAY, while no differences were detected between HAY and CAN. These results indicate that Curly Dock and Common Burdock added to a can modify ruminal fermentation and reduce CH4 production without significantly compromising ruminal pH and IVOMD. The findings highlight the potential of selected invasive weed species as locally available additives capable of contributing to CH4 mitigation strategies in grazing beef cattle. Further research is warranted to identify the bioactive compounds, changes in ruminal microbiome responsible for these effects and to evaluate their efficacy and safety under in vivo conditions.