<p>This study examines the role of perennial grasslands in anaerobic digestion (AD) systems, specifically highlighting how the timing of harvest affects economic and environmental outcomes in Lithuania. The research combines economic modeling with Life Cycle Assessment (LCA) to analyse and compare the harvests of perennial grasses during early, medium-early, and late seasons. The methodology involves calculating production costs, revenues, and risk factors for biogas facilities, as well as assessing environmental impacts. The results indicate that harvesting later in the season produces the most favorable outcomes. Economically, late-season harvests lead to higher profits, lower operational risks, and greater stability compared to early or medium-early harvests. Harvesting early in the season is the least effective approach, resulting in the lowest financial returns. Environmentally, late-season harvests are more beneficial because they reduce the potential for global warming, decrease reliance on fossil fuels, and lower ecotoxicity. However, some operational procedures common to all harvest seasons, such as fertilization, soil ploughing, and chopping, have negative impacts. Additionally, transportation and chemical use are significant environmental hotspots. The study concludes that harvest timing is a key management factor for improving both the economic and environmental performance of biogas production. Policymakers are encouraged to support late-season grass harvesting to promote renewable energy, enhance farm profits, and reduce environmental impacts. Future research should focus on new pretreatment methods and crop selection strategies to further boost the efficiency and sustainability of anaerobic digestion systems.</p>