Weed Biomass Responses to Tillage Systems in a Long-Term Mediterranean Cereal–Legume Experiment

In Mediterranean dryland agroecosystems, conservation tillage is increasingly adopted, yet its long-term effects on weed biomass within cereal–legume rotations remain insufficiently quantified. This study evaluated the effects of conventional tillage (CT), minimum tillage (MT), and no-tillage (NT) on aboveground weed biomass over seven years (2011–2017) within a long-term (established in 1985) cereal–legume rotation experiment in central Spain. Weed biomass was sampled annually prior to herbicide application and analyzed using linear mixed-effects models to assess the effects of crop type, tillage system, year, and their interactions. A total of 36 weed species were recorded, with annual broadleaf species accounting for 94% of total biomass. Mean weed biomass was greater in legume phases than in wheat, and tillage system significantly affected biomass. Minimum tillage resulted in higher weed biomass than CT or NT, particularly in legume crops. A significant crop type × tillage interaction indicated that tillage effects on weed biomass were crop-dependent, with stronger differences among tillage systems in legumes than in wheat. These results demonstrate that weed biomass responses to tillage cannot be generalized across crops, highlighting the importance of considering crop–tillage combinations when designing weed management strategies in Mediterranean cereal–legume rotations.