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Glufosinate is a key herbicide to manage glyphosate-resistant weeds mainly because it is a broad-spectrum herbicide, and transgenic glufosinate-resistant crops are available. Although glufosinate use has increased exponentially over the past decade, the treated area with this herbicide is far less than that with glyphosate. This is because glufosinate often provides inconsistent performance in the field, which is attributed to several factors including environmental conditions, application technology, and weed species. Glufosinate is also highly hydrophilic and does not translocate well in plants, generally providing poor control of grasses and perennial species. In the soil, glufosinate is rapidly degraded by microorganisms, leaving no residual activity. While there have been concerns regarding glufosinate toxicology, its proper use can be considered safe. Glufosinate is a fast-acting herbicide that was first discovered as a natural product, and is the only herbicide presently targeting glutamine synthetase. The mode of action of glufosinate has been controversial, and the causes for the rapid phytotoxicity have often been attributed to ammonia accumulation. Recent studies indicate that the contact activity of glufosinate results from the accumulation of reactive oxygen species and subsequent lipid peroxidation. Glufosinate disrupts both photorespiration and the light reactions of photosynthesis, leading to photoreduction of molecular oxygen, which generates reactive oxygen species. The new understanding of the mode of action provided new ideas to improve the herbicidal activity of glufosinate. Finally, a very few weed species have evolved glufosinate resistance in the field, and the resistance mechanisms are generally not well understood requiring further investigation. © 2020 Society of Chemical Industry.

BACKGROUND Indaziflam is an alkylazine herbicide used to control annual bluegrass (Poa annua L.). Several locations in the southeastern USA reported poor annual bluegrass control following treatment with indaziflam during autumn 2015. A series of controlled environment experiments were conducted to confirm putative resistance to indaziflam in annual bluegrass collected from these field locations. RESULTS Indaziflam (25 g ha-1 ) effectively controlled all putative-resistant annual bluegrass collections when applied preemergence (PRE), but was ineffective when applied early-postemergence (

Background Amaranthus palmeri S. Wats is among the most problematic annual broadleaf weed species in the USA, including in Kansas. In late summer 2015, seeds of an A. palmeri population (MHR) that had survived field-use rates of 2,4-D were collected from Barton County, KS, USA. The main objectives were to: (i) confirm and characterize 2,4-D resistance in a MHR population; (ii) characterize the resistance profile of the MHR population in relation to a multiple herbicide-susceptible (MHS) population to glyphosate, chlorsulfuron, atrazine, mesotrione, fomesafen; and (iii) determine the effectiveness of alternative POST burndown herbicides for controlling MHR population. Results The MHR population had 3.2-fold resistance to 2,4-D. In addition, the MHR population also exhibited multiple resistance to glyphosate (11.8-fold), chlorsulfuron (5.0-fold), atrazine (14.4-fold), and mesotrione (13.4-fold). Furthermore, the MHR population also showed reduced sensitivity to fomesafen (2.3-fold). In a separate study, dicamba with glyphosate, atrazine or fluroxypyr + 2,4-D, and paraquat alone or with atrazine, metribuzin, saflufenacil or 2,4-D provided ≥ 99% injury to the MHR population. Similarly, saflufenacil alone or with atrazine, metribuzin or 2,4-D, and glufosinate alone or with glyphosate + 2,4-D, and glyphosate + dicamba, and a premix of bicyclopyrone + atrazine + mesotrione + S-metolachlor also effectively controlled the MHR population. Conclusion This research confirms the first global case of an A. palmeri population from Kansas with multiple resistance to 2,4-D, glyphosate, chlorsulfuron, atrazine and mesotrione, and reduced sensitivity to fomesafen. Dicamba, glufosinate, paraquat, and saflufenacil alone or in tank-mixtures with PRE herbicides effectively controlled this MHR population. © 2019 Society of Chemical Industry.

Background Conventional crossing of soft wheat cultivars resistant to imazamox and glufosinate resulted in two (Rados and Helter) lines resistant to both herbicides. Stacked traits conferring this dual herbicide resistance in these lines, compared with a susceptible (S) cultivar, were characterized. Results Rados and Helter lines were ∼ 18-fold more resistant (R) to glufosinate, and between 15.1 and 19.8-fold more resistant to imazamox than the S cultivar. Resistance to glufosinate and imazamox decreased up to 12% and 50%, respectively, when the herbicides were applied sequentially. The basal activities of the acetolactate and glutamine synthases were similar between R and S plants. Rados and Helter lines were 11.7- and 17.7-fold more resistant to imazamox than the S cultivar, due to the Ser653-Asn mutation in their imi-ALS genes. R lines, susceptible to glufosinate at the target site level, showed lower ammonia accumulation evidencing the activity of the phosphinothricin acetyl transferase. Absorption and translocation patterns for 14 C-imazamox and 14 C-glufosinate were similar between R and S cultivars and so do not contribute to resistance. Conclusion Stacked traits conferring dual herbicide resistance to the lines Rados and Helter come from the resistant parents. These R lines are potential tools for weed management in wheat production, mainly via herbicide rotation. © 2018 Society of Chemical Industry.

BACKGROUND Because herbicide resistance evolves in very large populations over periods of many years, modelling is an important tool for investigating the dynamics of the problem. The Diversity model tracks the simultaneous evolution of resistance to multiple herbicides, using multiple genetic pathways, in several weed species at once. Tracking multiple species and simultaneous resistances is an important development in resistance modelling. We used the Diversity model to test weed management strategies for new cropping cotton varieties with multiple herbicide tolerances (‘triple-stacked’ varieties), in an Australian context. RESULTS The diversity required for long-term control of three key weeds in Australian cotton goes beyond using three herbicides, especially where there is already a substantial background of existing resistance to one or more of these herbicides. Assuming some glyphosate resistance is already present, simulations showed that glyphosate-resistant summer grass populations reach 20 000 seeds m−2 within 12 years using the triple-stack herbicides (glyphosate, glufosinate and dicamba) and a minimum of other tactics. Adding three pre-emergent modes of action plus cultivation to the system effectively controls glyphosate-resistant grasses for over 30 years. In conditions where resistance genes are as frequent as 1 in 100, however, highly fecund weeds such as Conyza bonariensis are hard to control beyond 15 years even with very highly diverse management. CONCLUSIONS Stacked herbicide tolerances in new crop varieties offers potential for increased herbicide diversity, but existing glyphosate-resistant weed populations need substantial extra management beyond what a glyphosate/glufosinate/dicamba resistance stack provides. More diverse systems can provide robust management over 30 years in the absence of very high levels of background resistance to other herbicides. © 2018 Society of Chemical Industry

Background Pesticide applications using a specific droplet size and carrier volume could maximize herbicide efficacy while mitigating particle drift in a precise and efficient manner. The objectives of this study were to investigate the influence of spray droplet size and carrier volume on dicamba and glufosinate efficacy, and to determine the plausibility of droplet-size based site-specific weed management strategies. Results Generally, across herbicides and carrier volumes, as droplet size increased, weed control decreased. Increased carrier volume (187 L ha-1 ) buffered this droplet size effect, thus greater droplet sizes could be used to mitigate drift potential while maintaining sufficient levels of weed control. To mitigate drift potential and achieve satisfactory weed control (≥ 90% of maximum observed control), a 900 µm (Ultra Coarse) droplet size paired with 187 L ha-1 carrier volume is recommended for dicamba applications and a 605 µm (Extremely Coarse) droplet size across carrier volumes is recommended for glufosinate applications. Although general droplet size recommendations were created, optimum droplet sizes for weed control varied significantly across site-years. Conclusion Convoluted interactions occur between droplet size, carrier volume, and other application parameters. Recommendations for optimizing herbicide applications based on droplet size should be based on a site-specific management approach to better account for these interactions. © 2018 Society of Chemical Industry.

BACKGROUND Bromus tectorum L. is one of the most troublesome grass weed species in cropland and non-cropland areas of the northwestern USA. In summer 2016, a B. tectroum accession (R) that survived imazamox at the field-use rate (44 g ha-1 ) in an imidazolinone-tolerant (IMI-tolerant or Clearfield™) winter wheat field was collected from a wheat field in Carter County, MT, USA. The aim of this study was to determine the resistance profile of the B. tectroum R accession to imazamox and other ALS inhibitors, and investigate the mechanism of resistance to imazamox. RESULTS The R B. tectorum accession had a high-level resistance (110.1-fold) to imazamox (IMI) and low to moderate-levels cross-resistance to pyroxsulam (TP) (4.6-fold) and propoxycarbazone (SCT) (13.9-fold). The R accession was susceptible to sulfosulfuron (SU) and quizalofop and clethodim (ACCase inhibitors), paraquat (PS I inhibitor), glyphosate (EPSPS inhibitor) and glufosinate (GS inhibitor). Sequence analysis of the ALS gene revealed a single, target-site Ser653Asn mutation in R plants. Pretreatment of malathion followed by imazamox at 44 or 88 g ha-1 did not reverse the resistance phenotype. CONCLUSION This is the first report of evolution of cross-resistance to ALS-inhibiting herbicides in B. tectorum. A single-point mutation, Ser653Asn, was identified, conferring the high-level resistance to imazamox. © 2017 Society of Chemical Industry.

Glyphosate and glyphosate-resistant crops had a revolutionary impact on weed management practices, but the epidemic of glyphosate-resistant (GR) weeds is rapidly decreasing the value of these technologies. In areas that fully adopted glyphosate and GR crops, GR weeds evolved and glyphosate and glyphosate traits now must be combined with other technologies. The chemical company solution is to combine glyphosate with other chemicals, and the seed company solution is to combine glyphosate resistance with other traits. Unfortunately, companies have not discovered a new commercial herbicide mode-of-action for over 30 years and have already developed or are developing traits for all existing herbicide types with high utility. Glyphosate mixtures and glyphosate trait combinations will be the mainstays of weed management for many growers, but are not going to be enough to keep up with the capacity of weeds to evolve resistance. Glufosinate, auxin, HPPD-inhibiting and other herbicide traits, even when combined with glyphosate resistance, are incremental and temporary solutions. Herbicide and seed businesses are not going to be able to support what critics call the chemical and transgenic treadmills for much longer. The long time without the discovery of a new herbicide mode-of-action and the epidemic of resistant weeds is forcing many growers to spend much more to manage weeds and creating a worst of times, best of times predicament for the crop protection and seed industry. © 2016 Society of Chemical Industry.

BACKGROUND In the Mediterranean area, Lolium species have evolved resistance to glyphosate after decades of continual use without other alternative chemicals in perennial crops (olive, citrus and vineyards). In recent years, oxyfluorfen alone or mixed with glyphosate and glufosinate has been introduced as a chemical option to control dicot and grass weeds. RESULTS Dose-response studies confirmed that three glyphosate-resistant Lolium weed species (L. rigidum, L. perenne, L. multiflorum) collected from perennial crops in the Iberian Peninsula have also evolved resistance to glufosinate and oxyfluorfen herbicides, despite their recent introduction. Based on the LD50 resistance parameter, the resistance factor was similar among Lolium species and ranged from 14- to 21-fold and from ten- to 12-fold for oxyfluorfen and glufosinate respectively. Similarly, about 14-fold resistance to both oxyfluorfen and glufosinate was estimated on average for the three Lolium species when growth reduction (GR50) was assessed. This study identified oxyfluorfen resistance in a grass species for the first time. CONCLUSION A major threat to sustainability of perennial crops in the Iberian Peninsula is evident, as multiple resistance to non-selective glyphosate, glufosinate and oxyfluorfen herbicides has evolved in L. rigidum, L. perenne and L. multiflorum weeds. © 2016 Society of Chemical Industry

BACKGROUND The widespread occurrence of ALS inhibitor‐ and glyphosate‐resistant Amaranthus palmeri has led to increasing use of protoporphyrinogen oxidase (PPO)‐inhibiting herbicides in cotton and soybean. Studies were conducted to confirm resistance to fomesafen (a PPO inhibitor), determine the resistance frequency, examine the resistance profile to other foliar‐applied herbicides and investigate the resistance mechanism of resistant plants in a population collected in 2011 (AR11‐LAW B) and its progenies from two cycles of fomesafen selection (C1 and C2). RESULTS The frequency of fomesafen‐resistant plants increased from 5% in the original AR11‐LAW‐B to 17% in the C2 population. The amounts of fomesafen that caused 50% growth reduction were 6‐, 13‐ and 21‐fold greater in AR11‐LAW‐B, C1 and C2 populations, respectively, than in the sensitive ecotype. The AR11‐LAW‐B population was sensitive to atrazine, dicamba, glufosinate, glyphosate and mesotrione but resistant to ALS‐inhibiting herbicides pyrithiobac and trifloxysulfuron. Fomesafen survivors from C1 and C2 populations tested positive for the PPO glycine 210 deletion previously reported in waterhemp (Amaranthus tuberculatus). CONCLUSION These studies confirmed that Palmer amaranth in Arkansas has evolved resistance to foliar‐applied PPO‐inhibiting herbicide. © 2016 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Herbicide-resistant crops have had a profound impact on weed management. Most of the impact has been by glyphosate-resistant maize, cotton, soybean and canola. Significant economic savings, yield increases and more efficacious and simplified weed management have resulted in widespread adoption of the technology. Initially, glyphosate-resistant crops enabled significantly reduced tillage and reduced the environmental impact of weed management. Continuous use of glyphosate with glyphosate-resistant crops over broad areas facilitated the evolution of glyphosate-resistant weeds, which have resulted in increases in the use of tillage and other herbicides with glyphosate, reducing some of the initial environmental benefits of glyphosate-resistant crops. Transgenic crops with resistance to auxinic herbicides, as well as to herbicides that inhibit acetolactate synthase, acetyl-CoA carboxylase and hydroxyphenylpyruvate dioxygenase, stacked with glyphosate and/or glufosinate resistance, will become available in the next few years. These technologies will provide additional weed management options for farmers, but will not have all of the positive effects (reduced cost, simplified weed management, lowered environmental impact and reduced tillage) that glyphosate-resistant crops had initially. In the more distant future, other herbicide-resistant crops (including non-transgenic ones), herbicides with new modes of action and technologies that are currently in their infancy (e.g. bioherbicides, sprayable herbicidal RNAi and/or robotic weeding) may affect the role of transgenic, herbicide-resistant crops in weed management. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.

BACKGROUND An Eleusine indica population has evolved resistance to glufosinate, a major post-emergence herbicide of global agriculture. This population was analysed for target-site (glutamine synthetase) and non-target-site (glufosinate uptake, translocation and metabolism) resistance mechanisms. RESULTS Glutamine synthetase (GS) activity extracted from susceptible (S) and resistant (R*) plants was equally sensitive to glufosinate inhibition, with IC50 values of 0.85 mM and 0.99 mM, respectively. The extractable GS activity was also similar in S and R* samples. Foliar uptake of [14C]-glufosinate did not differ in S and R* plants, nor did glufosinate net uptake in leaf discs. Translocation of [14C]-glufosinate into untreated shoots and roots was also similar in both populations, with 44 to 47% of the herbicide translocated out from the treated leaf 24 h after treatment. The HPLC and LC-MS analysis of glufosinate metabolism revealed no major metabolites in S or R* leaf tissue. CONCLUSIONS Glufosinate resistance in this resistant population is not due to an insensitive GS, or increased activity, or altered glufosinate uptake and translocation, or enhanced glufosinate metabolism. Thus, target-site resistance is likely excluded and the exact resistance mechanism(s) remain to be determined.

BACKGROUND: Studies were conducted to elucidate the mechanism of glufosinate resistance in an Italian ryegrass population. RESULTS: Glufosinate rates required to reduce growth by 50% (GR50) were 0.15 and 0.18 kg AI ha−1 for two susceptible populations C1 and C2 respectively, and 0.45 kg AI ha−1 for the resistant population MG, resulting in a resistance index of 2.8. Ammonia accumulation after glufosinate treatment was on average 1.5 times less for the resistant population than for the susceptible populations. The glufosinate concentrations (µM) required to reduce the glutamine synthetase (GS) enzyme activity by 50% (I50) were 31 and 137 for C1 and C2 respectively, and 2432 for the resistant population MG. One amino acid substitution in the plastidic GS2 gene, aspartic acid for asparagine at position 171, was identified in the resistant population. CONCLUSIONS: This is the first report of glufosinate resistance in a weed species that involves an altered target site. Copyright © 2012 Society of Chemical Industry

BACKGROUND: Studies of hybrid fitness, of which agronomic performance may be an indicator, can help in evaluating the potential for introgression of a transgene from a transgenic crop to wild relatives. The objective of this study was to assess the agronomic performance of reciprocal hybrids between two transgenic glufosinate-resistant rice lines, Y0003 and 99-t, and two weedy rice accessions, WR1 and WR2, in the greenhouse. RESULTS: F1 hybrids displayed heterosis in height, flag leaf area and number of spikelets per panicle. The agronomic performance of F1 between WR1 and Y0003 was not affected by crossing direction. The tiller and panicle numbers of F1 individuals were higher than their F2 counterparts. However, these traits did not change significantly from the F2 to the F3 generation or in hybrids with weedy rice as maternal or paternal plants. For all hybrids, the in vitro germination rates of fresh pollen were similar and significantly lower than those of their parents, seed sets were similar to or of lower value than those of weedy rice parents and seed shattering characteristics were partially suppressed, but the survival of hybrids over winter in the field was similar to that of weedy rice parents. All F1, F2 and F3 hybrids had similar composite agronomic performance to weedy rice parents. CONCLUSION: There was no significant decrease in the composite agronomic performance of any of the hybrids compared with weedy rice. This implies that gene flow from transgenic cultivated rice to weedy rice could occur under natural conditions. Copyright © 2011 Society of Chemical Industry

BACKGROUND: In the present study the principle of energy dispersive X-ray microanalysis (EDX), i.e. the detection of elements based on their characteristic X-rays, was used to localise and quantify organic and inorganic pesticides on enzymatically isolated fruit cuticles. Pesticides could be discriminated from the plant surface because of their distinctive elemental composition. RESULTS: Findings confirm the close relation between net intensity (NI) and area covered by the active ingredient (AI area). Using wide and narrow concentration ranges of glyphosate and glufosinate, respectively, results showed that quantification of AI requires the selection of appropriate regression equations while considering NI, peak-to-background (P/B) ratio, and AI area. The use of selected internal standards (ISs) such as Ca(NO3)2 improved the accuracy of the quantification slightly but led to the formation of particular, non-typical microstructured deposits. The suitability of SEM-EDX as a general technique to quantify pesticides was evaluated additionally on 14 agrochemicals applied at diluted or regular concentration. CONCLUSION: Among the pesticides tested, spatial localisation and quantification of AI amount could be done for inorganic copper and sulfur as well for the organic agrochemicals glyphosate, glufosinate, bromoxynil and mancozeb. Copyright © 2009 Society of Chemical Industry

BACKGROUND: The possibility of gene flow from transgenic crops to wild relatives may be affected by reproductive capacity between them. The potential gene flow from two transgenic rice lines containing the bar gene to five accessions of weedy rice (WR1–WR5) was determined through examination of reproductive compatibility under controlled pollination. RESULTS: The pollen grain germination of two transgenic rice lines on the stigma of all weedy rice, rice pollen tube growth down the style and entry into the weedy rice ovary were similar to self-pollination in weedy rice. However, delayed double fertilisation and embryo abortion in crosses between WR2 and Y0003 were observed. Seed sets between transgenic rice lines and weedy rice varied from 8 to 76%. Although repeated pollination increased seed set significantly, the rank of the seed set between the weedy rice accessions and rice lines was not changed. The germination rates of F1 hybrids were similar or greater compared with respective females. All F1 plants expressed glufosinate resistance in the presence of glufosinate selection pressure. CONCLUSIONS: The frequency of gene flow between different weedy rice accessions and transgenic herbicide-resistant rice may differ owing to different reproductive compatibility. This result suggests that, when wild relatives are selected as experimental materials for assessing the gene flow of transgenic rice, it is necessary to address the compatibility between transgenic rice and wild relatives. Copyright © 2009 Society of Chemical Industry

BACKGROUND: Herbicide-resistant crop technology could provide new management strategies for the control of parasitic plants. Three herbicide-resistant oilseed rape (Brassica napus L.) genotypes were used to examine the response of attached Cuscuta campestris Yuncker to glyphosate, imazamox and glufosinate. Cuscata campestris was allowed to establish on all oilseed rape genotypes before herbicides were applied. RESULTS: Unattached seedlings of C. campestris, C. subinclusa Durand & Hilg. and C. gronovii Willd. were resistant to imazamox and glyphosate and sensitive to glufosinate, indicating that resistance initially discovered in C. campestris is universal to all Cuscuta species. Glufosinate applied to C. campestris attached to glufosinate-resistant oilseed rape had little impact on the parasite, while imazamox completely inhibited C. campestris growth on the imidazolinone-resistant host. The growth of C. campestris on glyphosate-resistant host was initially inhibited by glyphosate, but the parasite recovered and resumed growth within 3–4 weeks. CONCLUSION: The ability of C. campestris to recover was related to the quality of interaction between the host and parasite and to the resistance mechanism of the host. The parasite was less likely to recover when it had low compatibility with the host, indicating that parasite-resistant crops coupled with herbicide resistance could be highly effective in controlling Cuscuta. Published 2009 by John Wiley & Sons, Ltd.

Transgenic herbicide-resistant rice is needed to control weeds that have evolved herbicide resistance, as well as for the weedy (feral, red) rice problem, which has been exacerbated by shifting to direct seeding throughout the world-firstly in Europe and the Americas, and now in Asia, as well as in parts of Africa. Transplanting had been the major method of weedy rice control. Experience with imidazolinone-resistant rice shows that gene flow to weedy rice is rapid, negating the utility of the technology. Transgenic technologies are available that can contain herbicide resistance within the crop (cleistogamy, male sterility, targeting to chloroplast genome, etc.), but such technologies are leaky. Mitigation technologies tandemly couple (genetically link) the gene of choice (herbicide resistance) with mitigation genes that are neutral or good for the crop, but render hybrids with weedy rice and their offspring unfit to compete. Mitigation genes confer traits such as non-shattering, dwarfism, no secondary dormancy and herbicide sensitivity. It is proposed to use glyphosate and glufosinate resistances separately as genes of choice, and glufosinate, glyphosate and bentazone susceptibilities as mitigating genes, with a six-season rotation where each stage kills transgenic crop volunteers and transgenic crop x weed hybrids from the previous season.

BACKGROUND: Glufosinate-resistant soybean cells were achieved through direct selection of diploid cells in the suspension culture. Here, the mutations in the glutamine synthetase (GS) gene are described to understand the evidence pointing to the functional role of the GS gene in the herbicide sensitivity of the mutant cells. RESULTS: Based on the I50 values, dose–response experiments at the cell level showed that the resistance ratio of the resistant cell was 50-fold, whereas the in vitro inhibition of GS activity required a 4.56-fold greater concentration of glufosinate in the resistant cell than in the untreated control. Comparison of the nucleotide sequences identified nine point differences in the GS gene between the resistant and untreated cells, leading to eight amino acid substitutions in the deduced polypeptide sequence. Northern hybridization of the GS mRNA showed that the accumulation of GS gene mRNA transcript in resistant cells was higher than that in the untreated cells. CONCLUSION: Changes in sensitivity to glufosinate have been related to mutations at the binding site of the herbicide on the glutamine synthetase. His249 is one of the residues implicated in the binding domain for the substrate and inhibitor, and hence the exchange of this residue with tyrosine plays a role in lowering the sensitivity of the mutated enzyme. Copyright © 2008 Society of Chemical Industry

Greenhouse studies were conducted to evaluate the sensitivity of three commercial cultivars, eight experimental cultivars and common bermudagrass to clethodim, glufosinate and glyphosate. Each herbicide was applied at eight doses. Data were regressed on herbicide dose using a log-logistic curve (R2 = 0.56-0.95 for clethodim, R2 = 0.60-0.94 for glufosinate, and R2 = 0.70-0.96 for glyphosate). The herbicide rate that elicited a 50% plant response (I50) in the bermudagrass cultivars ranged from 0.04 to 0.19 kg ha(-1) clethodim, 0.19 to 1.33 kg ha(-1) glufosinate and 0.34 to 1.14 kg ha(-1) glyphosate. Relative to other cultivars, common bermudagrass was intermediate in its response to clethodim and among the most tolerant cultivars to glufosinate and glyphosate. TifSport was relatively tolerant to clethodim and glufosinate compared with other cultivars, but relatively sensitive to glyphosate. One cultivar, 94-437, was consistently among the most sensitive cultivars to each of the herbicides. While there were differential herbicide tolerances among the tested bermudagrass cultivars, there did not appear to be any naturally occurring herbicide resistance that could be commercially utilized. However, research indicated that breeding efforts should target herbicide resistance that is at least four times the registered use rate. Also, TifSport and Tifway have been identified as suitable representatives of triploid hybrid bermudagrass cultivars to be used to evaluate the success of turfgrass renovation programs.

Glyphosate and glufosinate-ammonium herbicide tolerance traits were combined into both winter and spring lines of Brassica napus L. This allowed the study of possible interactions between these transgenes in two genetic backgrounds when treated with a variety of herbicides. Selective herbicides that are commonly used within Brassica crops showed no adverse effects on the transgenic plants or their null controls. Lines containing both glyphosate and glufosinate transgenes remained tolerant to their respective herbicides, regardless of the presence of the second tolerance transgene. Lines containing only a single transgene retained tolerance to the encoded trait and did not show cross-tolerance to the second. Null lines were killed by either herbicide. All plant lines, regardless of their transgene content, were found to be equally susceptible to three herbicides (paraquat, metsulfuron-methyl and mecoprop), commonly used to remove volunteer B napus from succeeding crops and set-a-side land. © 2002 Society of Chemical Industry

In the intensely farmed corn-growing regions of the mid-western USA, surface waters have often been contaminated by herbicides, principally as a result of rainfall runoff occurring shortly after application of these to corn and other crops. In some vulnerable watersheds, water quality criteria for chronic human exposure through drinking water are occasionally exceeded. We selected three settings representative of vulnerable corn-region watersheds, and used the PRZM-EXAMS model with the Index Reservoir scenario to predict corn herbicide concentrations in the reservoirs as a function of herbicide properties and use pattern, site characteristics and weather in the watersheds. We compared herbicide application scenarios, including broadcast surface pre-plant atrazine and alachlor applications with a glyphosate pre-plant application, scenarios in which losses of herbicides were mitigated by incorporation or banding, and scenarios in which only glyphosate or glufosinate post-emergent herbicides were used with corn genetically modified to be resistant to them. In the absence of drift, in almost all years a single runoff event dominates the input into the reservoir. As a result, annual average pesticide concentrations are highly correlated with annual maximum daily values. The modeled concentrations were generally higher than those derived from monitoring data, even for no-drift model scenarios. Because of their lower post-emergent application rates and greater soil sorptivity, glyphosate and glufosinate loads in runoff were generally one-fifth to one-tenth those of atrazine and alachlor. These model results indicate that the replacement of pre-emergent corn herbicides with the post-emergent herbicides allowed by genetic modification of crops would dramatically reduce herbicide concentrations in vulnerable watersheds. Given the significantly lower chronic mammalian toxicity of these compounds, and their vulnerability to breakdown in the drinking water treatment process, risks to human populations through drinking water would also be reduced. © 2001 Society of Chemical Industry

The herbicide glufosinate-ammonium (GLA) is a competitive inhibitor of glutamine synthetase (GS), an enzyme converting glutamate to glutamine in both plants and animals. Because GS is essential for ammonia detoxification in plants, GLA treatment disrupts photorespiration by causing a build-up of ammonia and a loss of glutamine in plant tissues. This study reports that GLA applied to leaf surfaces is also toxic to 5th-instar caterpillars of the skipper butterfly Calpodes ethlius (LD50 = 400 mg kg-1). After ingesting GLA, caterpillars stopped feeding and became dehydrated through a loss of rectal function. Caterpillars showed symptoms of neurotoxicity, such as proleg tremors, body convulsions and complete paralysis before death. Incubation of several tissues isolated from normal feeding-stage caterpillars with the GS substrates glutamate and ammonium showed that GLA inhibited GS activity in vitro. Within 24 h of ingesting GLA, caterpillars had a greatly reduced glutamine content and the ammonium ion levels had more than doubled. Injection of ammonium chloride into non-GLA-treated caterpillars had no deleterious effect, suggesting that glutamine depletion, and not a rise in body ammonium, was the primary cause of GLA toxicity following GS inhibition. This was supported by the observation that the onset of the symptoms of GLA poisoning could be postponed by giving GLA-fed caterpillars several subsequent daily injections of glutamine. The effective GLA dose fed to 5th-instar caterpillars in this study was comparable to the amount that might realistically by acquired from feeding on GLA-treated crops.

Current herbicide and herbicide trait practices are changing in response to the rapid spread of glyphosate-resistant weeds. Growers urgently needed glyphosate when glyphosate-resistant crops became available because weeds were becoming widely resistant to most commonly used selective herbicides, making weed management too complex and time consuming for large farm operations. Glyphosate made weed management easy and efficient by controlling all emerged weeds at a wide range of application timings. However, the intensive use of glyphosate over wide areas and concomitant decline in the use of other herbicides led eventually to the widespread evolution of weeds resistant to glyphosate. Today, weeds that are resistant to glyphosate and other herbicide types are threatening current crop production practices. Unfortunately, all commercial herbicide modes of action are over 20 years old and have resistant weed problems. The severity of the problem has prompted the renewal of efforts to discover new weed management technologies. One technology will be a new generation of crops with resistance to glyphosate, glufosinate and other existing herbicide modes of action. Other technologies will include new chemical, biological, cultural and mechanical methods for weed management. From the onset of commercialization, growers must now preserve the utility of new technologies by integrating their use with other weed management technologies in diverse and sustainable systems. © 2014 Society of Chemical Industry

The fate of five herbicides (glyphosate, glufosinate-ammonium, phenmedipham, ethofumesate and metamitron) was studied in two Finnish sugar beet fields for 26 months. Soil types were sandy loam and clay. Two different herbicide-tolerant sugar beet cultivars and three different herbicide application schedules were used. Meteorological data were collected throughout the study and soil properties were thoroughly analysed. An extensive data set of herbicide residue concentrations in soil was collected. Five different soil depths were sampled. The study was carried out using common Finnish agricultural practices and represents typical sugar beet cultivation conditions in Finland. The overall observed order of persistence was ethofumesate > glyphosate > phenmedipham > metamitron > glufosinate-ammonium. Only ethofumesate and glyphosate persisted until the subsequent spring. Seasonal variation in herbicide dissipation was very high and dissipation ceased almost completely during winter. During the 2 year experiment no indication of potential groundwater pollution risk was obtained, but herbicides may cause surface water pollution.

Herbicide-tolerant (HT) varieties of corn, soybean, canola (oilseed rape) and cotton have been grown since the mid-1990s, and have been widely adopted by farmers in several countries. HT genes have been inserted into or selected for in many other species, including almost all major crop species in the world and many minor crops and ornamental species. In some cases this has been done specifically to introduce the HT trait, whereas in others the HT gene has been used as a selectable marker (eg the bar gene, conferring glufosinate-ammonium tolerance). However, in very few cases have the HT crops generated been commercialized, despite the weed-control advantages that many such crops would offer. There are several reasons for this, including the high research and development costs associated with developing the new HT crop cultivars, the high cost of obtaining regulatory clearance for the HT crops or products derived from them, international trade issues relating to genetically modified crops, and issues surrounding the expanded herbicide registration for the new use and the potential impact of this on the existing registration status of the herbicide. New HT crops are unlikely to be developed unless they offer a sufficiently large advantage to farmers and a substantial assured market to justify the associated development and regulatory costs.

The aim of the present study was to determine whether post-emergence application of glufosinate to transgenic crops could lead to an increase in residues or to the formation of new, hitherto unknown metabolites. Transgenic oilseed rape and maize plants were treated separately with L-glufosinate, D-glufosinate or the racemic mixture. Whereas about 90% of the applied D-glufosinate was washed off by rain and only 5-6% was metabolised, 13-35% of the applied L-glufosinate remained in the form of metabolites and unchanged herbicide in both transgenic maize and oilseed rape. The main metabolite was N-acetyl-L-glufosinate with total residues of 91% in oilseed rape and 67% in maize, together with small amounts, of 5% in oilseed rape and 28% in maize, of different methylphosphinyl fatty acids. These metabolites were probably formed from L-glufosinate by deamination and subsequent decarboxylation. The residues were distributed in all fractions of the plants, with the highest contents in treated leaves and the lowest in the grains (0.07-0.3% in maize and 0.4-0.6% in oilseed rape). There was no indication of an accumulation of total residues or of residue levels above the official tolerances for glufosinate.

A biotype of Aster squamatus (Sprengel) Hieronymus with suspected resistance to the ALS-inhibiting herbicide imazapyr was detected in a chicken farm in the province of Seville, Spain, which had been treated once a year with imazapyr for 10 years. Resistance to imazapyr in this biotype was studied using dose–response experiments, absorption and translocation assays, metabolism studies and ALS activity assays. The rate of imazapyr required to inhibit A squamatus growth by 50% (ED50) was 15 times higher for the R (resistant) than for the S (susceptible) biotype. Cross-resistance existed for the ALS-inhibitors imazamox, imazethapyr, amidosulfuron, nicosulfuron, rimsulfuron, triasulfuron and tribenuron, but not for bensulfuron. Control of A squamatus using alternative herbicides was poor with clopyralid, intermediate with quinclorac, amitrole and MCPA, and excellent with 2,4-D, glufosinate and glyphosate. Absorption of [14C]imazapyr increased over time for both the R and S biotypes, and translocation from the treated leaf to shoots and roots was similar in both biotypes, with most of the radioactivity remaining in the treated leaf. No metabolites of imazapyr were detected in either biotype. Sensitivity of the ALS enzyme (target site) to imazapyr was lower for the R biotype (I50(R) = 4.28 × I50 (S)). The mechanism of imazapyr resistance in this R biotype appears to be an altered ALS conferring decreased sensitivity to imazapyr at the whole-plant level. Copyright © 2003 Society of Chemical Industry

The metabolism of the herbicide glufosinate-ammonium was investigated in heterotrophic cell suspension and callus cultures of transgenic (bar-gene) and non-transgenic sugarbeet (Beta vulgaris). Similar studies were performed with suspensions of carrot (Daucus carota), purple foxglove (Digitalis purpurea) and thorn apple (Datura stramonium). 14C-labelled chemicals were the (racemic) glufosinate, L-glufosinate, and D-glufosinate, as well as the metabolites N-acetyl L-glufosinate and 3-(hydroxymethylphosphinyl)propionic acid (MPP). Cellular absorption was generally low, but depended noticeably on plant species, substance and enantiomer. Portions of non-extractable residues ranged from 0.1% to 1.2% of applied 14C. Amounts of soluble metabolites resulting from glufosinate or L-glufosinate were between 0.0% and 26.7% of absorbed 14C in non-transgenic cultures and 28.2% and 59.9% in transgenic sugarbeet. D-Glufosinate, MPP and N-acetyl L-glufosinate proved to be stable. The main metabolite in transgenic sugarbeet was N-acetyl L-glufosinate, besides traces of MPP and 4-(hydroxymethylphosphinyl)butanoic acid (MPB). In non-transgenic sugarbeet, glufosinate was transformed to a limited extent to MPP and trace amounts of MPB. In carrot, D stramonium and D purpurea, MPP was also the main product; MPB was identified as a further trace metabolite in D stramonium and D purpurea.