During 2004 to 2008, weed surveys were conducted in 373 wheat fields of two different cropped areas (southwest [SW] and southeast [SE]) of the southern region of Buenos Aires Province of Argentina where different weed communities were expected because of changes in cropping practices over time, including tillage, crop sequence, fertilizers, and herbicides applied. Weed communities differed between regions, with greater numbers of native species for the SW. Weed community diversity was also greater for the SW region, probably due to the more diverse land use that resulted in greater landscape heterogeneity. Rush skeletonweed, sand rocket, yellow starthistle and turnipseed occurred at higher constancy (proportion of fields in which a given species is present) in the SW region, whereas common chickweed, false bishop's weed, corn speedwell, and common lambsquarters were present more frequently in the SE region. Compared with the 1982 survey, constancy of weeds increased, but those species with high constancy in 1982 were also with high constancy in the recent surveys. Diversity (species richness) was greater in conventional than in a no-tillage system. The constancy of Italian ryegrass, sand rocket, and yellow starthistle was lower under no-till than conventional tillage. Surveys allow identification of changes in weed community related to different agricultural systems. Rotation of crops and livestock avoid the homogenization of the environment at the landscape level. Management strategies will be necessary to prevent the increase of weeds populations' size, preserving plant diversity and the properties of the agroecosystem.

Wild oat (Avenafatua L.) is a major weed in barley (Hordeum vulgare L.) in Argentina. During 1993During , 1994During , 1995During , and 1996, demographic studies of wild oat in barley crops were conducted to assess the effect of herbicide treatment, crop, and sowing rate on the demo· graphic processes of seedling establishment, survival rate, reproduc· tive performance, preharvest seed dispersal rate, and fate of seed in the soil during the fallow period. The herbicide was diclofop-methyl: methyl 2-[4-(2,4-dichlorophenoxy)phenoxylpropanoic acid. Increasing barley sowing rate from 160 plants m-2 to 280 plants m-2 effectively reduced the amount of wild oat seeds that entered the soil seed bank by 50% through lowering fecundity. Diclofop-methyl reduced the number of seeds that entered the soil seed bank sevenfold (1050 vs. 140 seeds m-2 ) through a reduction in both seedling survivor rate and reproductive output. In addition, the diclofop-methyl treatment delayed the onset of seed maturity and reduced the dormancy level of the seeds produced by the surviving plants. Half the number of wild oat seeds entered the seed bank in a barley than in a wheat (Triticum aestivum L.) crop. This was through a lower wild oat reproductive output and a higher rate of seed dispersal related to the later harvest time of wheat relative to barley. These data indicate that integrated control of wild oat should include herbicide treatment, crop selection, and sowing rate.

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Wild oat is the most serious grass weed in Argentine barley crops and its control has concentrated on herbicide strategies. Increasing crop density could be an effective strategy to reduce the effect of wild oat on barley yield. However, limited research has been conducted to evaluate the effect of crop density on the competitive balance between barley and spontaneous populations of wild oat. A field experiment was conducted in 1992, 1993, and 1999, to study the effect of spontaneous populations of wild oat on barley sown at densities of 160, 220, and 280 plants/m2. Wild oat density averaged 84 plants/m2. Wild oat biomass increased linearly with weed density in all treatments but was reduced by increasing barley seeding rates. Barley biomass and yield were not affected by wild oat at high crop sowing densities, but for the low and medium barley densities, yield loss was almost 25% when 70 wild oat plants/m2 were established. Barley yield loss was mostly related to competition from the early emerged wild oat. The relationship between yield losses and wild oat density was equally significant when the whole population or only early emerged individuals of the weed were considered.

There are many concerns about the effects of repeated use of glyphosate in glyphosate-resistant (GR) crops, including two that are seemingly contradictory. These are (1) weed escapes and (2) loss of weed diversity. Weeds that escape glyphosate treatment represent species that likely will become troublesome and difficult to control in the future, and identifying these future problems may allow more effective management. In contrast, complete weed control directly reduces the weed component of agroecosystem biodiversity and may lower other components indirectly (e.g., weed-dependent granivores). During 2001 and 2002 effects of glyphosate and conventional weed control treatments on weed community composition and GR soybean yields were studied. Field studies were conducted along a north–south transect of sites spanning a distance of 1600 km from Minnesota to Louisiana. Low-intensity use (single application yr−1) of glyphosate allowed more escapes and maintained higher weed diversity than high-intensity use (two applications yr−1) of glyphosate, and it was equivalent to or even higher than diversity in non-GR systems. Although the same weeds escaped from low- and high-intensity glyphosate treatments, frequency of escapes was higher with less intensive use. These results suggest that limited use of glyphosate would not have profound effects on weed diversity. In addition, crop yield did not differ between GR and non-GR treatments at high latitudes, but below 40° N latitude, with a longer cropping season, yields with low-intensity glyphosate use decreased by about 2% per degree latitude because of competition from escaped weeds.