LAUSR

Abstract Extensive grain crop production systems in the Flat inland Pampas mainly include soybean, double-crop wheat-soybean and maize in rotation. Due to difficult-to-control weed problems, farmers are tending to intensify the rotations in their fields by increasing the number of double crops or by including cover crops before the main crop. Land use intensification may be characterized using the intensification sequence index (ISI), which is the number of crops per year considering all crops sown in a particular period; i.e. the average number of crops sown in a time unit. To determine how agricultural intensification and crop sequences may modify weed communities, 31 soybean fields of commercial farms located in the Flat Pampa of Argentina were surveyed from 2012 to 2014. Frequency of individual weeds within the fields was determined and various statistical methods were used to evaluate changes in weed community composition or function due to intensification (ISI level). A total of fifty-three species, mostly therophytes (28 species), were recorded in soybean crop fields at harvest. Three weed communities were identified, which were related to the ISI level of the fields, and also to the number of years continuously sown with grain crops (i.e. number of years since the last pasture). Weed community under intensified fields was characterized by low species richness (p