South America represents about 12% of the global land area, and Brazil roughly corresponds to 47% of that. The major sustainable agricultural system in South America is based on a no-tillage cropping system, which is a worldwide adopted agricultural conservation system. Societal benefits of conservation systems in agriculture include greater use of conservation tillage, which reduces soil erosion and associated loading of pesticides, nutrients and sediments into the environment. However, overreliance on glyphosate and simpler cropping systems has resulted in the selection of tolerant weed species through weed shifts (WSs) and evolution of herbicide-resistant weed (HRW) biotypes to glyphosate. It is a challenge in South America to design herbicide- and non-herbicide-based strategies that effectively delay and/or manage evolution of HRWs and WSs to weeds tolerant to glyphosate in cropping systems based on recurrent glyphosate application, such as those used with glyphosate-resistant soybeans. The objectives of this paper are (i) to provide an overview of some factors that influence WSs and HRWs to glyphosate in South America, especially in Brazil, Argentina and Paraguay soybean cropped areas; (ii) to discuss the viability of using crop rotation and/or cover crops that might be integrated with forage crops in an economically and environmentally sustainable system; and (iii) to summarize the results of a survey of the perceptions of Brazilian farmers to problems with WSs and HRWs to glyphosate, and the level of adoption of good agricultural practices in order to prevent or manage it.

Imazamox-resistant wheat varieties carry theImi1allele, which confers resistance to the imidazolinone (IMI) herbicide imazamox. This resistance trait allows the selective control of jointed goatgrass, a difficult-to-control winter annual grass weed. Allele movement between IMI-resistant wheat and jointed goatgrass may occur via hybridization and backcross events. Hybrids (F1) of IMI-resistant wheat and jointed goatgrass were identified in 2008 in a commercial wheat field in Eastern Oregon. In 2009 and 2010, surveys were conducted in Eastern Oregon to determine the prevalence of theImi1allele in wheat × jointed goatgrass hybrids. Using polymerase chain reaction assays we detected the presence of theImi1allele. A total of 128 sites were surveyed over the 2 yr. Of 1,548 plants sampled, 1,100 were positive for theImi1alelle and of those, 1,087 were heterozygous and 13 were homozygous for the allele. We assessed hybrid yield components and how these components varied across the sampled sites. The association between the proportion of IMI-resistant hybrids and the area or management practice in the commercial fields was determined. Nonagricultural sites or production of IMI-resistant wheat in consecutive years were two factors associated with a greater proportion of IMI-resistant hybrids. Our results demonstrate that theImi1allele is moving from IMI-resistant wheat to jointed goatgrass, producing resistant hybrids and backcross plants. This is the first report of natural occurrence of IMI-resistant backcross plants in commercial wheat fields. Therefore, it is important to implement field management practices that reduce IMI-resistant hybrid production and to effectively manage nonagricultural areas with jointed goatgrass infestations to prevent introgression of the IMI-resistance allele.

Acetyl-coenzyme A carboxylase (ACCase)–resistant Italian ryegrass is one of the most difficult-to-control weeds in United States wheat-production systems. Seed was collected from a suspected ACCase-resistant Italian ryegrass population in a winter wheat field with a history of ACCase-inhibitor herbicide use. This study investigated cross-resistance patterns in this Italian ryegrass population. Resistance was identified to the commercial dose of the ACCase herbicides pinoxaden, clethodim, sethoxydim, and clodinafop. Partial chloroplastic ACCase sequences revealed aspartate-to-glycine or isoleucine-to-asparagine substitutions at positions 2078 or 2041 in individuals of the resistant population. This is the first report, to our knowledge, of Asp-2078-Gly and Ile-2041-Asn substitutions in ACCase-resistant Italian ryegrass in the United States. Associating the occurrence of resistance alleles with resistance to specific active ingredients provides a better understanding of ACCase cross-resistance in Italian ryegrass and possibly options for its control.

Imidazolinone-resistant (IR) winter wheat allows selective control of jointed goatgrass with the herbicide imazamox. However, the spontaneous hybridization between jointed goatgrass and IR winter wheat threatens the value of the IR technology. The objectives of this study were to determine if F1hybrids collected in a commercial production field under IR winter wheat–fallow rotation in Oregon and their first-backcross progeny (BC1) carried theImi1gene and were resistant to imazamox, and to analyze the parentage of F1and BC1plants. The average seed set of the F1spikes was 3.3%, and the average germination of BC1seed was 52%. All F1and BC1plants tested carriedImi1. Jointed goatgrass plant mortality was 100% when treated with imazamox at 0.053 kg ai ha−1, compared to 0% for IR winter wheat and BC1progeny. All F1plants had jointed goatgrass as the maternal parent; whereas, most BC1plants (85.7%) were produced with IR winter wheat as the paternal backcross parent. Although the backcrossing of F1hybrids with jointed goatgrass is very low, it demonstrates the potential for introgression ofImi1from IR winter wheat into jointed goatgrass under natural field conditions.

The effects of mesotrione, S-metolachlor, and terbuthylazine, applied in mixture, on soil biodegradation remain insufficiently researched. However, herbicide mixtures have been a common practice in agricultural systems in the last years. Understanding the fate of soil-applied herbicides may help on planning weed management tactics towards more sustainable and efficient weed control. Therefore, this study evaluated the fate of mesotrione alone and in mixture with S-metolachlor and terbuthylazine when applied to two contrasting arable Brazilian soils. Mineralization and degradation experiments were conducted using C-mesotrione alone or in mixture. From the 49-day laboratory incubation data, increased mineralization half-life of mesotrione was observed for the mixture of herbicides, ranging from a 4-day increase for the sandy loam soil to a 1-day increase in the sandy clay texture soils. Mesotrione degradation rate had a twofold increase in the sandy loam compared to the sandy clay soil. Two metabolites can be identified from mesotrione degradation, 4-methyl-sulfonyl-2-nitrobenzoic acid (MNBA) and 2-amino-4-methylsulfonyl benzoic acid (AMBA). Indices for the score of ubiquity in groundwater indicated mesotrione possesses leaching potential for both soils. Applying mesotrione alone or in mixture did not influence the amount of bound residues from mesotrione. However, mesotrione degradation rate was influenced by soil texture regardless if applied alone or in mixture. Mesotrione biotransformation was relatively quick, indicating that this herbicide has low persistence and, consequently, low residual effect on crops and weeds when present in similar soils to this present study.

Herbicide-resistant Echinochloa species are among the most problematic weeds in agricultural crops globally. Recurring herbicide selection pressure in the absence of diverse management practices has resulted in greater than 20% of sampled Echinochloa populations from rice (Oryza sativa L.) fields demonstrating multiple resistance to herbicides in Arkansas, USA. We assessed the resistance profile and potential mechanisms of resistance in a multiple herbicide–resistant junglerice [Echinochloa colona (L.) Link] (ECO-R) population. Whole-plant and laboratory bioassays were conducted to identify the potential mechanisms of non–target site resistance in this population. ECO-R was highly resistant to propanil (>37,800 g ha−1) and quinclorac (>17,920 g ha−1) and had elevated tolerance to cyhalofop (R/S = 1.9) and glufosinate (R/S = 1.2) compared to the susceptible standard. The addition of glufosinate (590 g ha−1) to cyhalofop (314 g ha−1), propanil (4,500 g ha−1), or quinclorac (560 g ha−1) controlled ECO-R 100%. However, cyhalofop applied with propanil (48% control) or quinclorac (15% control) was antagonistic. The application of the known metabolic enzyme inhibitors malathion, carbaryl, and piperonyl butoxide increased control of ECO-R with propanil (>75%) but not with other herbicides. Neither absorption nor translocation of [14C]cyhalofop or propanil was different between ECO-R and ECO-S. [14C]Quinclorac absorption was also similar between ECO-R and ECO-S; however, translocation of quinclorac into tissues above the treated leaf of ECO-R was >20% higher than that in ECO-S. The abundance of metabolites was higher (∼10%) in the treated leaves of ECO-R than in ECO-S beginning 48 h after treatment. The activity of β-cyanoalanine synthase, which detoxifies hydrogen cyanide, was not different between ECO-R and ECO-S following quinclorac treatment. Resistance to propanil was due to herbicide detoxification by metabolic enzymes. Resistance to quinclorac was due to a detoxification mechanism yet to be understood. The reduction in sensitivity to cyhalofop and glufosinate might be a secondary effect of the mechanisms conferring high resistance to propanil and quinclorac.

-In Brazil, the "Pesticide Act" (Act no. 7,802/89) has introduced new criteria related to the environment, public health and agronomic performance in the analyses of pesticide-related activities. Likewise, radioisotopes are used for environmental behavior and in planta studies, since they provide some advantages in comparison to chemical measures, including greater sensitivity, stepwise description of a particular element in a metabolic system, and pesticide position and detection through X-ray films and/or radio image (in plants) and liquid scintillation (in plants and soil), respectively. This review describes methodologies related to radioisotope utilization in studies on herbicide absorption, translocation and metabolism in plants, as well as in studies on herbicide biodegradation, mineralization, leaching and sorption-desorption on the soil. The step-by-step of the described methodologies is based on the guidelines that were established, mostly by the Organization for Economic Co-operation and Development (OECD) and the Environmental Protection Agency (EPA). On this review, methodological information on soil and plant studies, using radioisotopes, is available to Brazilian researchers. Thus, the objective of this review is to stimulate the conduction of further studies that use the methodologies described herein. Keywords

-The weed Borreria densiflora is a management issue in soybean and sugarcane crops from North and Northeastern Brazil. Knowledge upon chemical control of B. densiflora contributes to the integrated management of this weed species, especially when active ingredient options become reduced due to the selection of herbicide resistant or tolerant weed species. Experiments in pre-and post-emergence of B. densiflora were conducted in greenhouse, in a randomized block design and four replications. In pre-emergence, the doseresponse curve methodology was used and 7 herbicides were tested. In post-emergence, 9 herbicides at the recommended rate and 4 herbicide mixtures were tested. For pre and post-emergence conditions, evaluations were conducted at 60 and 21 days after treatment (DAT), respectively, and the variables analyzed were weed control and dry weight (%). The results showed options of pre-emergent herbicides that can be used for controlling B. densiflora, especially in sugarcane, where chemical weed control is mainly based on preemergent applications. In the current glyphosate resistance scenario, one should consider the use of pre-emergent herbicides within an integrated management of B. densiflora. For satisfactory post-emergence control, B. densiflora plants should be sprayed at the phenological stage of up to three pairs of leaves. Herbicide mixtures have been and will continue to be an important tool in chemical weed management, broadening the spectrum of weed control, while diversifying herbicide mechanisms of action, which helps to prevent or delay the appearance of herbicide resistance.Keywords: vassourinha-de-botão, chemical control, dose-response, herbicide mixtures, integrated weed management. Palavras-chave: vassourinha-de-botão, controle químico, dose-resposta, mistura de herbicidas, manejo integrado de plantas daninhas. RESUMO -A planta daninhaMARTINS, B