Resistance of Amaranthus retroflexus to acetolactate synthase inhibitor herbicides in Brazil

When in competition with cotton, Amaranthus retroflexus can cause high yield losses. Due to the limited availability of selective herbicides registered for post emergence control of this weed, the same herbicides have been used repeated times over the last few years, which may have selected resistant biotypes. Biotypes of A. retroflexus collected from the main areas of cotton cultivation in Brazil were submitted to dose-response trials, by applying the herbicides trifloxysulfuron-sodium and pyrithiobac-sodium in doses equivalent to 0, ��, ��, 1, 2 and 4 times the recommended rates. Resistance to ALS inhibitors was confirmed in biotypes of A. retroflexus. Biotype MS 2 from Mato Grosso do Sul, was cross-resistant to both trifloxysulfuron-sodium and pyrithiobac-sodium, while biotype MS 1 was resistant to trifloxysulfuron-sodium only. Likewise, singular and cross resistance was also confirmed in biotypes from Goi��s (GO 3, GO 4 and GO 6), in relation to trifloxysulfuron��sodium and pyrithiobac-sodium. One biotype from Mato Grosso (MT 13) was not resistant to any of the ALS inhibitors evaluated in this work.

First report of Amaranthus viridis resistance to herbicides

Due to the limited number of herbicides registered for the control of dicot weeds in cotton crops, ALS inhibitors have been used on an intensive and recurrent basis. Given that, cases of poor weed control have been described after application of these herbicides in the main cotton producing areas in Brazil, the purpose of the present work was to evaluate the occurrence of resistance to ALS herbicides in Amaranthus viridis biotypes from those areas. Dose-response curves were prepared after pre-emergence applications of trifloxysulfuron-sodium (0; 1.8; 3.7; 7.5; 15 and 30 g ha-1) and pyrithiobac-sodium (0; 35; 70; 140; 280 and 560 g ha-1), equivalent to 0, ��, ��, 1, 2 and 4 times the recommended commercial rates. The selection of trifloxysulfuron-sodium resistant biotypes of A. viridis was confirmed in samples from Bahia (BA 7, BA 8, BA 9 and BA 11). However, no resistance to pyrithiobac-sodium was found for biotypes either from Bahia or from Mato Grosso do Sul.

Selectivity of chemical weed control systems in conventional cotton

Cotton is highly susceptible to the interference imposed by weed community, being therefore essential to adopt control measures ensuring the crop yield. Herbicides are the primary method of weed control in large-scale areas of production, and usually more than one herbicide application is necessary due to the extensive crop cycle. This study aimed to evaluate the selectivity of different chemical weed control systems for conventional cotton. The experiment took place in the field in a randomized block design, with twenty nine treatments and four replications in a split plot layout (adjacent double check). Results showed that triple mixtures in pre-emergence increased the chance of observing reductions in the cotton yield. To avoid reductions in crop yield, users should proceed to a maximum mixture of two herbicides in pre-emergence, followed by S-metolachlor over the top, followed by one post-emergence mixture application of pyrithiobac-sodium + trifloxysulfuron-sodium.

Herbicide detection in groundwater in C��rrego Rico-SP watershed

Due to the large amount of pesticides applied in agriculture, mainly herbicides, there is a growing concern about a possible environmental contamination with these products, including water bodies. Given the above, the aim of the present work was to detect and quantify herbicides through multiresidue analysis in water samples collected in semi-artesian wells and springs in a rural area of the city of Jaboticabal (SP). Samples were collected from 32 wells and 13 water springs, in three different seasons: October 2010, February 2011 and May 2011. Additionally, samples at a residence in the urban area were also collected. Analysis using high performance liquid chromatography coupled to mass spectrometry was performed and herbicides ametryn, amicarbazone, clomazone, diclosulan, diuron, hexazinone, imazapic, imazapyr, isoxaflutole, S-metolachlor, sulfentrazone, sulfometuron-methyl, and tebuthiuron were evaluated. In semi-artesian wells, an incresed quantity of herbicides was found in comparison with the water springs. Among the tested herbicides, hexazinone, imazapyr and sulfentrazone were detected in measurable amounts in accordance with the analytical method applied, while clomazone was the most common herbicide being detected in more than 60% of the samples. Ametryn, diuron and amicarbazone herbicides were also detected. Diclosulan, imazapic, isoxaflutole, S-metolachlor, sulfometuron-methyl, and tebuthiuron were not detected in any sample. Inappropriate use of these products without prior knowledge of the behavior of the soil can lead to groundwaters and water springs contamination, thus an ongoing monitoring of this resource becomes very important.

SELECTION OF HERBICIDES TARGETING THE USE IN CROP SYSTEMS CULTIVATED WITH SHOWY CROTALARIA

ABSTRACT The increase in the area planted with Crotalaria spectabilishas occurred by several factors, highlighting the potential to reduce the nematodes, nitrogen fixation and the high production of biomass. By becoming a species sown as a crop, it is necessary to control the weeds that coexist with showy crotalaria. This change in the use of this crop creates the possibility of this specie becoming a weed. The aim of this study was to assess the potential use of herbicides applied in preemergence and postemergence of C.spectabilisfor different purposes (control of volunteer and selectivity plants). Three experiments were installed in a greenhouse (two with herbicides applied in preemergence - in soils with distinct textural categories; and one experiment with herbicides applied in postemergence). The results of the experiments with herbicides applied in preemergence showed that: amicarbazone, atrazine, diuron, metribuzin, prometryn, fomesafen and sulfentrazone showed effectiveness for control of C.spectabilis in clayey soil. Besides these, flumioxazin and isoxaflutole also showed potential to be used in the control of showy crotalaria in soils with loam texture. In relation to the postemergence herbicides, atrazine, diuron, prometryn, flumioxazin, fomesafen, lactofen, saflufenacil, amonio-glufosinate and glyphosate can be used aiming the chemical control of C.spectabilis. Herbicides chlorimuron-ethyl, diclosulan, imazethapyr, pyrithiobac-sodium, trifloxysulfuron-sodium, clomazone, pendimethalin, S-metolachlor and trifluralin applied in preemergence, and imazethapyr, pyrithiobac-sodium, flumiclorac, bentazon and clethodim applied in postemergence caused low levels of injury to C.spectabilis plants, making necessary the development of new searches to ensure the selectivity of these products.