Soybean Weed Communities: Relationships with Cultural History and Crop Yield

Cultural history and cropping systems create an environmental heterogeneity that determines Variability in both crop yield and weed communities. Describing relationships between environmental heterogeneity, crop yield, and weed communities may he useful for making agricultural management decisions. We studied the weed communities of soybean [Glycine max (L.) Merr.] fields in the Rolling Pampa, Argentina, to identify floristic and functional (life cycles, origin, and morphotypes) structure in fields with different management practices, and to explore the association between weed species, cultural history, and crop yield. We surveyed 18 and 42 different fields in 1994 and 1995, respectively, in soybean cropped with conventional and notillage practices. In 1994, we recorded presence of weed species and in 1995 we recorded weed presence and abundance, and management information (type of tillage, number of agricultural cycles, grain yield, planting date, and weed control). We used multivariate analysis to examine relationships among variables. We distinguished four communities and eight floristic groups, related to crop yield and tillage system. The presence of Floristic Groups II [Oxalis chrysantha (Kunth) Prog., Sonchus oleraceus L., and Veronica persica Poir.], VII [Carduus acanthoides L., Stellaria media (L.) Vill., Physalis viscosa L., Taraxacum officinale Weber, and Coronopus didymus (L.) Sm.], and IV [Bidens subalternuns DC., Cirsium vulgare (Savi) Ten., and Cyperus sp. L.] may be an indicator of high potential crop yield (low soil degradation), and their absence may he an indicator of low potential crop yield (increasing soil degradation). Functional structure (life cycles, origin, and morphotypes) was similar between communities, indicating that agroecosystems preserve function, despite management practices. The main variables explaining weed distribution in the surveyed region were tillage system, crop yield, planting date, and the number of agricultural cycles.