Perennial lucerne affects weed community trajectories in grain crop rotations

Meiss H, Médiène S, Waldhardt R, Caneill J, Bretagnolle V, Reboud X & Munier‐Jolain N (2010). Perennial lucerne affects weed community trajectories in grain crop rotations. Weed Research50, 331–340. Summary Complex crop rotations may be beneficial for weed management. We analysed how pluriannual forage crops may affect weed composition during cereal‐based crop rotations. Using a space‐for‐time‐substitution design, we compared weed composition and diversity before, during and after perennial crops. We surveyed four groups of fields: (a) winter wheat (Triticum aestivum L.) following annual crops, (b) 1‐year old lucerne (Medicago sativa L.) following annual crops, (c) 2–6 years old lucerne and (d) winter wheat following pluriannual lucerne in western France (420 fields in total). Weed composition varied among the four groups, suggesting a cyclic trajectory corresponding to the phases of the crop rotation. Indicator Species Analysis showed that these differences were due to at least 40 species, including the most common weeds. A functional group analysis showed that perennial lucerne crops shifted the communities away from several problematic weeds, especially annual broad‐leaved species with an upright or climbing morphology. This effect was also visible in the wheat following lucerne. Other species (including perennials, annuals with rosettes and some grasses) benefited from the particular growth conditions in lucerne but decreased in the following wheat. The diversification of arable crop rotations with perennial crops may thus be useful for Integrated Weed Management, reducing the need for herbicides. Other species less harmful to annual crops were favoured, resulting in increased floristic diversity.