The evolution of glyphosate-resistant weeds has recently increased dramatically. Six suspected glyphosate-resistant Amaranthus tuberculatus populations were studied to confirm resistance and determine the resistance mechanism. Resistance was confirmed in greenhouse for all six populations with glyphosate resistance factors (R/S) between 5.2 and 7.5. No difference in glyphosate absorption or translocation was observed between resistant and susceptible individuals. No mutation at amino acid positions G101, T102, or P106 was detected in the EPSPS gene coding sequence, the target enzyme of glyphosate. Analysis of EPSPS gene copy number revealed that all glyphosate-resistant populations possessed increased EPSPS gene copy number, and this correlated with increased expression at both RNA and protein levels. EPSPS Vmax and Kcat values were more than doubled in resistant plants, indicating higher levels of catalytically active expressed EPSPS protein. EPSPS gene amplification is the main mechanism contributing to glyphosate resistance in the A. tuberculatus populations analyzed.

:The behaviour and fate of chlorsulfuron in aqueous and soil systems were examined in laboratory studies. Aqueous hydrolysis was pH-dependent and followed pseudo-Ðrst-order degradation kinetics at 25¡C, with faster hydrolysis occurring at pH 5 (half-life 24 days) than at either pH 7 or 9 (half-lives [365 days). Degradation occurred primarily by cleavage of the sulfonylurea bridge to form the major metabolites chlorobenzenesulfonamide (2-chlorobenzenesulfonamide) and triazine amine (4-methoxy-6-methyl-1,3,5-triazin-2-amine). This route is a major degradation pathway in water and soil systems. Aqueous photolysis (corrected for hydrolysis) proceeded much more slowly (half-life 198 days) than aqueous hydrolysis and is not expected to contribute signiÐ-cantly to overall degradation. Hydrolysis in soil thin-layer plates exposed to light (half-life 80 days), however, progressed at a much faster rate than in dark controls (half life 130 days), which suggests that a mechanism other than direct photolysis may have been operative. An aerobic soil metabolism study (25¡C) in a Keyport silt loam soil (pH 6É4, 2É8% OM) showed that degradation was rapid (half-life 20 days). Dissipation in an anaerobic sediment/water system (initial pH of water phase 6É7, Ðnal pH 7É4) progressed much more slowly (half-life [365 days) than in aerobic soil systems. Major degradation products in aerobic soil included the chlorobenzenesulfonamide and triazine amine as in the aqueous hydrolysis study. Neither of these degradation products exhibited phytotoxicity to a variety of crop and weed species in a glasshouse experiment, and both exhibited an acute toxicological proÐle similar to that of chlorsulfuron in a battery of standard tests. Demethylation of the 4-methoxy group on the triazine moiety and subsequent cleavage of the triazine ring is another pathway found in both aqueous solution and soils, though di †erent bonds on the triazine amine appear to be cleaved in the two systems. Hydroxylation of the benzenesulfonamide moiety is a minor degradation pathway found in soils. Two soils amended with 0É1 and 1É0 mg kg~1 chlorsulfuron showed slight stimulation of nitriÐcation. The 1É0 mg kg~1 concentration of chlorsulfuron resulted in minor stimulation and inhibition of 14C-cellulose and 14C-protein degradation, respectively, in the same soils. Batch equilibrium adsorption studies conducted on four soils showed that adsorption was low in this system 13È54). Soil thin-layer (K oc chromatography of chlorsulfuron and its major degradation products demon-(R f \ 0É55È0É86) strated that the chlorobenzenesulfonamide had slightly less mobility and that the (R f \ 0É34È0É68) triazine amine was much less mobile than chlorsulfuron. In an aged column (R f \ 0É035È0É40) leaching study, subsamples of a Fallsington sandy loam 5É6, OM 1É4%) or a Flanagan silt (pH water loam 6É4, OM 4É0%) were treated with chlorsulfuron, aged moist for 30 days in a glass-(pH water house and then placed upon a prewet column of the same soil type prior to initiation of leaching....

In response to changing market dynamics, the discovery of new herbicides has declined significantly over the past few decades and has only seen a modest upsurge in recent years. Nevertheless, the few introductions have proven to be interesting and have brought useful innovation to the market. In addition, herbicide-tolerant or herbicide-resistant crop technologies have allowed the use of existing nonselective herbicides to be extended into crops. An increasing and now major challenge is being posed by the inexorable increase in biotypes of weeds that are resistant to herbicides. This problem is now at a level that threatens future agricultural productivity and needs to be better understood. If herbicides are to remain sustainable, then it is a must that we adopt diversity in crop rotation and herbicide use as well as increase the use of nonchemical measures to control weeds. Nevertheless, despite the difficulties posed by resistant weeds and increased regulatory hurdles, new screening tools promise to provide an upsurge of potential herbicide leads. Our industry urgently needs to supply agriculture with new, effective resistance-breaking herbicides along with strategies to sustain their utility.Only a few companies are significantly pursuing herbicide discovery in the 21st century. Most of these have combined seed and traits businesses, since fees for traits constitute a considerable part of the income of agrochemical companies today. In concert with a review of the historical perspectives of herbicide research (Kraehmer et al., 2014), we provide here a short description of the current major research activities within the remaining 21st century agrochemical companies. After an overview of the chemicals that have entered the market in the 21st century, we provide a brief summary of the current nature of the weed-resistant herbicide problem. We then go on to summarize breedingassisted and transgenic approaches toward the improvement of crop selectivity through the delivery of so-called herbicide-tolerant (HT) or herbicide-resistant crops, and conclude with a discussion of the new herbicide discovery screening tools that have been employed since the year 2000 and prospects for the future. MAJOR CHEMICAL TRENDS AFTER 2000Several new compounds have entered the herbicide market in recent years. Although not representing new modes of action (MoAs), they have increased the number of tools available for farmers to use to control weeds. Even in known and older herbicidal classes, new, interesting, and marketable molecules have been discovered. For example, and perhaps surprising given the relative age of the class of herbicides, new (after 2000) acetolactate synthase (ALS) inhibitors have provided solutions for farmers that can be regarded as real innovations. One of them is mesosulfuron-methyl (Fig. 1), a sulfonylurea herbicide that, when combined with iodosulfuron-methyl sodium, has broad-spectrum postemergence grass weed control at dose rates of 4.5 to 15 g active ingredient (a.i.) ha 21 (Safferling, 2005). Another ...