Weed surveys were performed in commercial no-till glyphosate-tolerant soyabean crops in southern Entre Rios province (Mesopotamic Pampas of Argentina) in 2005 and 2007, during the soyabean grain filling to maturity growth stages. The objectives were to describe the weed communities in fields recently introduced to crop production and to analyse the effect of the new cropping patterns on assemblages. The fields surveyed varied in the length of the no-till period (1-11 years), the previous crop and the soil productivity rating. Weed communities were described in terms of composition, constancy, life forms, morphotypes and (only during 2007) frequency. Tragia geraniifolia, Bidens subalternans, Sida spinosa, and Eryngium horridum were species associated with fields with more than 5 years of no-till glyphosate-tolerant crops. These fields had a significantly higher relative abundance of perennials (52% versus 32%) and of dicotyledons (66% versus 39%) than fields with less than 5 years of no-till. Previous crop and soil productivity affected weed community structure. Six species, five of them annuals, were associated with fields that had high yields and maize as the previous crop. In contrast, perennials and dicotyledons had the highest relative abundance when wheat-soyabean double cropping was the previous crop. The results show that changes in cropping systems acted as filters on functional traits, modifying the previous weed community assemblage. The information may be used to develop integrated crop-weed management strategies, leading to a reduction in the assemblage of highly competitive weed communities.
