Contrasting weed species composition in perennial alfalfas and six annual crops: implications for integrated weed management

Weed communities are most strongly affected by the characteristics and management of the current crop. Crop rotation may thus be used to prevent the repeated selection of particular weed species. While weed communities are frequently compared among annual crops, little is known about the differences between annual and perennial crops that may be included in the rotations. Moreover, nearly all existing studies (17 articles reviewed) are based on local field experiments rather than commercial fields. We compared the weed composition in perennial alfalfas (Medicago sativa) and six annual crops: winter wheat (Triticum aestivum), oilseed rape (Brassica napus), pea (Pisum sativum), sunflower (Helianthus annuus), maize (Zea mays) and sorghum (Sorghum bicolor) using data from 632 commercial fields in western France. Weed species composition showed the strongest dissimilarities between perennial alfalfas and all annual crops, followed by the well-known differences between autumn-and spring/summer-sown annual crops. Indicator Species Analysis showed that most weed species either preferred perennial alfalfas (including Taraxacum officinale, Veronica persica, Crepis spp., Poa trivialis, Silene latifolia, Capsella bursa-pastoris and Picris spp.) or annual crops (including Mercurialis annua, Galium aparine, Fallopia convolvulus, Chenopodium album and Cirsium arvense). Perennial alfalfas thus suppressed many weeds that are widespread (and sometimes problematic) in annual crops while favouring other species. Shifted weed composition and reduced frequency of several noxious weeds suggest that perennial alfalfas may be used as a valuable part of integrated weed management, reducing the need for herbicides and sustaining plant and animal diversity in agricultural landscapes.