Annual bluegrass (Poa annua L.) is a problematic annual weed in established turf where the intensive use of herbicides has resulted in the evolution of herbicide resistance. In 2017, 31 populations of P. annua suspected to be resistant to herbicides commonly used to control this weed in turf were collected from golf courses across southeastern Australia to check the resistance status to different herbicide groups. All populations were found to be resistant to multiple turf herbicides. Dose–response experiments confirmed resistance to propyzamide, simazine, rimsulfuron, foramsulfuron, endothall, and pinoxaden. Levels of resistance to rimsulfuron (>56-fold), foramsulfuron (>19-fold), endothall (>7-fold), and pinoxaden (>4.3-fold) compared with the susceptible population were high, but levels of resistance to propyzamide (>2-fold) and simazine (>2-fold) were lower. Considerable variation in resistance to endothall and pinoxaden was observed among the populations of P. annua. Target-site resistance was confirmed for acetolactate synthase and acetyl-CoA carboxylase inhibitors, but not for photosystem II and microtubule assembly inhibitors. This study documented the extensive resistance to herbicides in P. annua from turf in Australia. Three of the populations investigated exhibited multiple resistance to herbicides from five mechanisms of action. The identification of multiple-resistant P. annua on several golf courses is a serious concern for turf managers.

Paraquat was the most successful nonselective herbicide in Korea due to its rapid herbicidal activity. However, its high mammalian toxicity, frequent self-poisoning incidents, and a lack of effective antidotes led to a paraquat ban in Korea in 2012. Therefore, this review was conducted to revisit the toxicological profile of paraquat and to investigate the impacts of the paraquat ban on human health and agriculture in Korea. A review of toxicological information reconfirmed that paraquat is highly acutely toxic to humans, and ingestion, inhalation, or dermal administration of the herbicide can cause severe clinical signs and inevitably lead to death by respiratory failure. In Korea, the paraquat ban immediately decreased the suicide rate due to pesticides (mainly paraquat) by 46.1%, resulting in a 10% decrease of the total suicide rate. However, this also led to an increase in suicide attempts with other poisons such as carbon monoxide, suggesting that suicide attempts and rates of suicide by poisoning depend on not only the toxicity of the poison but also the accessibility of the poisoning agents. In agriculture, paraquat was quickly replaced by other nonselective herbicides such as glufosinate and glyphosate. Thus, the paraquat ban did not have a significant impact on agricultural practices but influenced the nonselective herbicide market; the use of glufosinate was higher than use of glyphosate due to glufosinate’s rapid herbicidal activity, which is similar to that of paraquat. Though the paraquat ban can be considered as a national strategy to lower suicide rates, the increase in suicide attempts with other poisons suggests that multilateral efforts are required for not only keeping suicidal agents away from people but also minimizing motives for suicide.

Rapid growth of Palmer amaranth (Amaranthus palmeri S. Watson) poses a challenge for timely management of this weed. Dose–response studies were conducted in 2017 and 2018 under field and greenhouse conditions near Garden City and Manhattan, KS, respectively, to evaluate the efficacy of dicamba to control ≤10-, 15-cm, and 30-cm-tall A. palmeri, which mimics three herbicide application timings: on-time application (Day 0) and 1- (Day 1) and 4-d (Day 4) delays. Visual injury rating and reduction in shoot biomass (% of nontreated), and mortality were assessed at 4 wk after treatment using a three- and four-parameter log-logistic model in R. Increasing dicamba doses increased A. palmeri control regardless of plant height in both the field and greenhouse studies. The results suggest that delaying application 1 (15 cm) and 4 d (30 cm) resulted in 2- and 27-fold increases in the effective dose of dicamba on A. palmeri, respectively, under field conditions. However, in the greenhouse, for the same level of A. palmeri control, more than 1- and 2-fold increases in dicamba dose, respectively, were required. Similarly, the effective dose of dicamba required for 50% reduction in A. palmeri shoot biomass (GR50) increased more than 4- and 8-fold or more than 1- and 2-fold when dicamba application was delayed by 1 (15 cm) and 4 d (30 cm), in the field or in the greenhouse, respectively. To understand the basis of increased efficacy of dicamba in controlling early growth stages of A. palmeri, dicamba absorption and translocation studies were conducted. Results indicate a significant reduction in dicamba absorption (7%) and translocation (15%) with increase in A. palmeri height. Therefore, increased absorption and translocation of dicamba results in increased efficacy in improving A. palmeri control at early growth stages.

Broomrapes (Orobanche and Phelipanche species, Orobanchaceae) are obligate root parasites of dicotyledonous plants. This taxonomic group includes seven weedy parasites of agricultural crops that damage vegetables, sunflower (Helianthus annuus L.), and legumes. Processing-tomato (Solanum lycopersicum L.) fields in Israel have been recently found infested with a new broomrape, first identified as nodding broomrape (Orobanche cernua Loefl.) based on its host. However, its morphology resembled the closely related sunflower broomrape (Orobanche cumana Wallr.), an obligate parasite of sunflower. The new race (CUCE) parasitized sunflower, tomato, and tobacco (Nicotiana tabacum L.) in vitro, in a polyethylene bag system and in pots. Its seeds germinated in response to strigolactones (orobanchol, 5-deoxystrigol, 2′-epiorobanchol, and GR24) and dehydrocostus lactone (DCL), whereas O. cumana seeds responded only to DCL and GR24, and O. cernua only to strigolactones. Based on morphological similarities with O. cumana, shared molecular markers with O. cumana, ability to parasitize sunflower and respond to sunflower-germination stimulants, it was concluded that CUCE is a new race of O. cumana, with a host range expanding to Solanaceae crops. While being an important noxious weed of sunflower, this new O. cumana race is currently spreading and posing a threat to processing tomato in Israel. This finding is an alarming indication that broomrapes can shift host range and that similar new races of O. cumana could potentially appear in other countries.

Annual sedge (Cyperus compressus L.) populations with resistance to halosulfuron were identified in turfgrass at two new locations in Georgia. Research was conducted to evaluate (1) resistance levels to two acetolactate synthase (ALS) inhibitors, (2) ALS enzyme susceptibility, (3) genetic differences associated with resistance, and (4) differential levels of ALS gene expression in these biotypes. In dose–response experiments, the biotypes were >160 times resistant to halosulfuron but only 12 times more resistant to imazaquin compared with a susceptible biotype. In vitro enzyme assays indicated that resistant (R) biotypes required 6.1-fold greater concentrations of imazaquin to reduce ALS activity 50% compared with the susceptible (S) biotype. Both R biotypes had a similar Pro-197-Ser amino acid substitution in the ALS gene that confers resistance to sulfonylureas. Compared with the S biotype, R biotypes had 4.4 times higher ALS gene expression than the S biotype. No differences in gene copy number were found between any biotypes for the ALS gene. Overall, ALS-resistant C. compressus selected by halosulfuron use in turfgrass may be exhibiting partial susceptibility to imazaquin but complete resistance to sulfonylureas. Differential levels of susceptibility to ALS inhibitors for these biotypes are associated with the Pro-197-Ser substitution and enhanced expression of the ALS gene.

Common seepweed [Suaeda glauca (Bunge) Bunge] is a common salt-tolerant weed species distributed across the agricultural regions of northern China. It produces dimorphic seeds with different phenotypic characteristics and seed sizes. However, there is no information regarding the germination biology of these dimorphic seeds. Studies were conducted to evaluate the effects of ecological factors such as temperature, light, pH, osmotic stress, salt concentration, and planting depth on seed germination and seedling emergence. The results showed that brown seeds were nondormant, whereas black seeds had an intermediate physiological dormancy. The germination percentage of brown seeds was more than 80% at all temperature regimes and light conditions, but the optimum germination occurred at the cold thermoperiod of 20/10 C. In contrast, less than 6% of black seeds germinated at all temperature regimes and light conditions. Eight weeks of cold stratification did not break the dormancy of black seeds, whereas low concentrations of gibberellic acid (0.1 and 1.0 mM) significantly increased seed germination. Removal of the testa of black seeds also promoted germination and produced normal seedlings. Brown seeds showed moderate tolerance to salt stress, with 16% germination percentage at a salt concentration of 600 mM NaCl. The germination of brown seeds was 38% at an osmotic potential stress of −0.8 MPa; above that, no germination was obtained. Brown seeds germinated well in a wide pH range (4 to 10), with a germination percentage higher than 95%. Seedling emergence percentage was higher than 90% at burial depths of 0 to 2 cm, while germination percentage severely decreased for brown seeds with burial depths >2 cm, indicating that shallow tillage could be an effective measure to minimize seed germination. Information gathered from this study will help to develop an effective protocols for controlling S. glauca.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv] is the foremost weed in rice (Oryza sativa L.) systems, and its control is crucial to successful rice production. Quinclorac, a synthetic auxin herbicide, has been used effectively to manage E. crus-galli. However, occurrences of quinclorac-resistant genotypes are frequently reported, and its resistance evolution has led to questions about the continued utility of quinclorac for grass control. Identification of the resistance mechanism(s) of resistant genotypes will facilitate development of integrated weed management strategies that sustain quinclorac use for management of E. crus-galli. We evaluated the responses to quinclorac of two contrasting genotypes: E7 (resistant, R) and LM04 (susceptible, S). Quinclorac induced ethylene and cyanide biosynthesis in the S-genotype. Both genotypes responded similarly to an increasing application of exogenous 1-carboxylic acid aminocyclopropane (ACC) and potassium cyanide, and their growth was inhibited at higher doses. The key mechanism for cyanide (HCN) detoxification in plants, β-cyanoalanine synthase (β-CAS) activity, was evaluated in both genotypes, and no significant difference was observed in the basal activity. However, quinclorac significantly induced β-CAS–like activity in the S-genotype, which is consistent with the increased synthesis of ethylene and cyanide. This work suggests that the resistance to quinclorac of the E7 R-genotype is likely related to an alteration in the auxin signal transduction pathway, causing a lower stimulation of ACC synthase and, therefore, limited synthesis of ethylene and HCN after quinclorac treatment.

The transgenic Liberty Link® (LL) soybean is tolerant to glufosinate, conferred by the enzyme phosphinothricin acetyltransferase (PAT), which is encoded by the pat gene from Streptomyces viridochromogenes. Because symptoms of injury can be observed in soybean [Glycine max (L.) Merr.] plants in some situations, this study evaluated the effects of rates of glufosinate on agronomic performance; quality of LL soybean seeds; and the ammonia, glufosinate, and N-acetyl-l-glufosinate concentration (NAG) in soybeans with and without the pat gene after application of increasing glufosinate rates. Field and greenhouse experiments were conducted; the first evaluated the selectivity of glufosinate in LL soybeans, and the second evaluated the metabolic changes in soybeans with (LL) and without (RR2) the pat gene, after application of glufosinate. For fieldwork, application of glufosinate at rates up to four times the maximum recommended caused initial injury symptoms (up to 38.5%) in LL soybean plants. However, no negative effect was found on seed quality and agronomic performance of LL plants, including yield. This shows the selectivity of glufosinate promoted by pat gene insertion for application in POST (V4), in LL soybean. For the greenhouse experiment, it was concluded that the LL soybean plants presented high glufosinate metabolism, lower ammonia concentration, and no reduction in dry matter, in comparison with RR2 soybean, after application of high rates of glufosinate.

Glyphosate is a popular herbicide used to control goosegrass [Eleusine indica (L.) Gaertn.], one of the most troublesome weeds in cotton (Gossypium hirsutum L.) fields. However, high selection pressure has led to some populations being difficult to control in cotton fields in China. In this study, levels of glyphosate resistance were quantified and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) amplification was confirmed. In addition, distribution of the EPSPS gene among the chromosomes was also investigated using fluorescence in situ hybridization (FISH) methodology. One population (AHCZ) was confirmed to be glyphosate resistant with a resistance index of 3.4 and significantly reduced shikimate accumulation compared with the susceptible population. All examined AHCZ individuals exhibited no mutations in the EPSPS gene. Expression and copy numbers of EPSPS in the AHCZ population were 5.7 and 15.4 times higher, respectively, than in the susceptible population. A positive correlation was identified between signal intensities of primary anti-EPSPS antibody and copy numbers of the EPSPS protein, as indicated by immunoblot analysis. FISH results revealed that, in mitotic metaphase chromosomes, signals were observed dispersed across two sets of homologous chromosomes in a resistant individual (copy number = 31), while in susceptible individuals, signals were only partially detected in one set of homologous chromosomes. In interphase nuclei, EPSPS signals were brightest in resistant individuals compared with susceptible individuals. In conclusion, one E. indica population from a cotton field in the Anhui Province has evolved resistance to glyphosate, and EPSPS gene amplification was confirmed as the resistance mechanism.

The glyphosate controversy before the renewal of the authorization of glyphosate in the European Union (EU) once again turned the spotlight on pesticide regulation in the EU. In the EU, pesticides are attracting more public attention than in other parts of the world, and many nongovernmental organizations specifically target pesticide regulation, trying to influence politicians and other decision makers. Following an overview of the EU pesticide legislation and the impact hitherto on EU agriculture, this paper outlines the glyphosate controversy and presents the outcome of desk studies conducted in Germany, the United Kingdom, France, and Sweden on the potential effects of a glyphosate ban on agricultural productivity and farm income. All studies concluded that the loss of income depends very much on farm type and cropping practice, but they all reached the conclusion that particularly no-tillage farming/conservation agriculture will be facing severe problems without glyphosate to control weeds and terminate cover crops. No-tillage/conservation agriculture is viewed as an effective strategy to prevent soil erosion and loss of nutrients, which could become larger problems without glyphosate. Other issues highlighted in the studies were the impact on resistance management, as glyphosate is largely seen as a “herbicide-resistance breaker.” Without glyphosate, fundamental changes in farming practices in the EU are required, and it is hard to imagine that they will come without a cost, at least in the short term.

In Australia, glyphosate is widely used in glyphosate-tolerant crops and fallows to control weeds such as common sowthistle (Sonchus oleraceus L.). It has been hypothesized that glyphosate at sublethal doses, as a consequence of herbicide drift, may have a stimulatory effect on S. oleraceus growth. In 2017, pot trials were conducted to evaluate the effect of low doses of glyphosate on growth and seed production of this weed at the Weed Science Screenhouse Facility at the University of Queensland, Australia. At the 4- to 5-leaf stage (3-wk-old rosette), plants were treated with low doses of glyphosate (0 [control], 5, 10, 20, 40, 80, and 800 g ae ha−1), and their responses were recorded until plant maturity. The study was repeated after completion of the first experimental run. An additional glyphosate dose (2.5 g ha−1) was added in the second run. The low doses of glyphosate (<40 g ha−1) caused a significant increase in S. oleraceus plant height and number of leaves compared with the no-glyphosate treatment. The highest stimulatory effect was observed at 5 g ha−1. At 5 g ha−1 glyphosate, S. oleraceus seed production increased by 154% and 101% in the first and second experimental runs, respectively, compared with the no-glyphosate treatment. The results of this study suggest that the sublethal doses of glyphosate produced hormetic effects on growth and seed production of S. oleraceus that changed the dynamics of weed–crop competition.

Hazelnut (Corylus avellana L.) is a native shrub on the steep slopes of the mountains and on the plains in the Black Sea coast of Turkey. Turkey is the world’s largest hazelnut producer and exporter, followed by Italy, Spain, the United States, and Greece. Within the scope of this project, a number of surveys were conducted in Turkey to understand the impact of a glyphosate ban on hazelnut production and the economy. Sixty farm surveys were conducted within the Black Sea region, and those data were used as primary information sources. Databases of institutions, theses, academic reports, and published articles were used as secondary data sources to determine the possible effects of a glyphosate ban on Turkish hazelnut production and economy. One of the most important findings of the study was that agricultural business and employment sustained by hazelnut production constitute a significant part of the rural economy. Tillage and mechanical strategies remain as the most sustainable alternative methods for controlling weeds. A potential glyphosate restriction may increase the demand for labor due to a higher need for mechanical strategies. The cost of these alternative methods are 80% more compared with glyphosate weed control systems. The benefit–cost model predicted that, in the case of no glyphosate use, total hazelnut production would decrease by 12% to 21% due to inefficient weed control. A glyphosate ban would result in a reduction in Turkish gross domestic product. Yearly, an average of US$2 billion in revenue is obtained from hazelnut exports, and this number corresponds to 1.37% of Turkey’s annual export value. The glyphosate ban would cause a US$240 to US$420 million loss in hazelnut export value and reduce production by 66 to 115 million kg. Global demand for hazelnut is believed to be on the increase, and world production depends largely on Turkey.

Blackgrass (Alopecurus myosuroides Huds.) and silky windgrass [Apera spica-venti (L.) P. Beauv.] are becoming a significant problem in Europe. Due to the development of herbicide-resistant biotypes and unwanted side effects of herbicides, there is a need for new integrated weed management strategies to control weeds. Therefore, reducing weed infestations by targeting seed production during crop harvest should be considered. In 2017 and 2018, we estimated the fraction of the total seed production of A. myosuroides and A. spica-venti in a field that potentially could be collected by a grain harvester during winter wheat (Triticum aestivum L.) harvest. Twenty plants of each species were surrounded by a porous net before flowering to trap shed seeds during reproductive development. Seeds were collected and counted weekly up until and immediately before wheat harvest, and the ratio of harvestable seeds to shed seeds during the growing season was determined. Alopecurus myosuroides produced on average 953 seeds plant−1 in 2017 and 3,337 seeds plant−1 in 2018. In 2017 and 2018, 29% and 37% of the total A. myosuroides seeds produced, respectively, were retained on plants at maturity. Apera spica-venti produced on average 1,192 seeds plant−1 in 2017 and 5,678 seeds plant−1 in 2018, and retained 53% and 16% of the seeds at harvest, respectively. If a grain harvester potentially collected approximately 30% of the total seed production of the two grass weeds and removed or killed them, it would reduce seed input to the soil seedbank. However, such methods cannot stand alone to reduce weed pressure.

Knowledge of crop–weed interference effects on weed biology along with yield penalties can be used for the development of integrated weed management (IWM) tactics. Nevertheless, little is known about the beneficial effects of soybean [Glycine max (L.) Merr.] density, an important aspect of IWM, on late Palmer amaranth (Amaranthus palmeri S. Watson) establishment time. Two field experiments were conducted in 2014 and 2015 to investigate how various soybean densities and A. palmeri establishment timings in weeks after crop emergence (WAE) affect height, biomass, and seed production of the weed but also crop yield in drill-seeded soybean. Soybean density had a significant impact on dry weight and seed production of A. palmeri that established within the first 2 wk of crop emergence, but not for establishment timings of the weed 4 wk and later in relation to crop emergence. Differential performance of A. palmeri gender was observed, regarding greater biomass production of female than male plants under crop presence, and merits further investigation. Grain yield reductions were recorded at earlier A. palmeri establishment timings (i.e., 0 and 1 WAE) compared with 8 WAE establishment timing in 2014 and 2015. High soybean densities resulted in greater soybean yields compared with low soybean density, but no grain yield benefits were observed between medium and high soybean densities. Crop budget analysis revealed the benefits of moderate seeding rate (i.e., 250, 000 seeds ha−1) increases in comparison to lower (i.e., 125,000 seeds ha−1) or high (i.e., 400,000 seeds ha−1) on crop revenue, net income returns, and breakeven price. Earlier A. palmeri establishment timings (i.e., 0, 1, and 2 WAE) resulted in lower crop revenue and net income returns compared with later establishment timings of the weed.

Metabolic resistances to atrazine (atz-R) and mesotrione (meso-R) occur in several waterhemp [Amaranthus tuberculatus (Moq.) Sauer] populations in the United States. Interestingly, although metabolic atz-R but mesotrione-sensitive A. tuberculatus populations have been reported, an Amaranthus population has not been confirmed as meso-R but atrazine-sensitive, implying an association between these traits. Experiments were designed to investigate whether the single gene conferring metabolic atz-R plays a role in meso-R. An F2 population was generated from a multiple herbicide–resistant A. tuberculatus population from McLean County, IL (MCR). A cross was made between a known meso-R male clone (MCR-6) and a herbicide-sensitive female clone from Wayne County, IL (WCS-2) to develop an F1 population. Survival of MCR-6 plants following atrazine POST treatment (14.4 kg ha−1) indicated the male parent was homozygous atz-R. F1 plants were intermated to obtain a segregating pseudo-F2 population. Dose–response and metabolic studies conducted with mesotrione using F1 plants indicated intermediate biomass reductions and metabolic rates compared with MCR-6 and WCS. F2 plants were initially treated with either mesotrione (260 g ha−1) or atrazine (2 kg ha−1) POST, and after 21 d of recovery, vegetative clones from surviving resistant plants were subsequently treated with the other herbicide. When mesotrione was applied first, the meso-R frequency was 8.2%, and when atrazine was applied first, the atz-R frequency was 75%. However, the meso-R frequency increased to 16.5% following preselection for atz-R, and 100% of surviving meso-R plants were atz-R. Our findings indicate that the gene conferring metabolic atz-R is also involved with the meso-R trait within the population tested.

Southern crabgrass [Digitaria ciliaris (Retz.) Koeler] is an annual grass weed that commonly infests turfgrass, roadsides, wastelands, and cropping systems throughout the southeastern United States. Two biotypes of D. ciliaris (R1 and R2) with known resistance to cyclohexanediones (DIMs) and aryloxyphenoxypropionates (FOPs) previously collected from sod production fields in Georgia were compared with a separate susceptible biotype (S) collected from Alabama for the responses to pinoxaden and to explore the possible mechanisms of resistance. Increasing rates of pinoxaden (0.1 to 23.5 kg ha−1) were evaluated for control of R1, R2, and S. The resistant biotypes, R1 and R2, were resistant to pinoxaden relative to S. The S biotype was completely controlled at rates of 11.8 and 23.5 kg ha−1, resulting in no aboveground biomass at 14 d after treatment. Pinoxaden rates at which tiller length and aboveground biomass would be reduced 50% (I50) and 90% (I90) for R1, R2, and S ranged from 7.2 to 13.2 kg ha−1, 6.9 to 8.6 kg ha−1, and 0.7 to 2.1 kg ha−1, respectively, for tiller length, and 7.7 to 10.2 kg ha−1, 7.2 to 7.9 kg ha−1, and 1.6 to 2.3 kg ha−1, respectively, for aboveground biomass. Prior selection pressure from DIM and FOP herbicides could result in the evolution of D. ciliaris cross-resistance to pinoxaden herbicides. Amplification of the carboxyl-transferase domain of the plastidic ACCase by standard PCR identified a point mutation resulting in an Ile-1781-Leu amino acid substitution only for the resistant biotype, R1. Further cloning of PCR product surrounding the 1781 region yielded two distinct ACCase gene sequences, Ile-1781 and Leu-1781. The amino acid substitution, Ile-1781-Leu in both resistant biotypes (R1 and R2), however, was revealed by next-generation sequencing of RNA using Illumina platform. A point mutation in the Ile-1781 codon leading to herbicide insensitivity in the ACCase enzyme has been previously reported in other grass species. Our research confirms that the Ile-1781-Leu substitution is present in pinoxaden-resistant D. ciliaris.

Quantifying the level of ecophysiological, biochemical, and agronomical fitness of herbicide-resistant (R) and herbicide-susceptible (S) weeds is useful for understanding the evolutionary development of herbicide resistance, but also for implementing herbicide-resistance management strategies. Although germination is a key fitness component in the life cycle of weeds, germinability of S and R weeds has rarely been evaluated under stressful conditions. Germinability traits of S and non–target site resistant subpopulations of blackgrass (Alopecurus myosuroides Huds.) sharing closely related genetic background were tested under salinity, drought stress, and accelerated seed-aging (i.e., exposed to 100% relative humidity at 45 C from 0 to 134 h) conditions. In addition, the activity of three antioxidant enzymes and protein concentration of accelerated aged seeds of the subpopulations were studied. There were no differences in maximum seed germination (Gmax) and time to 50% germination between the two subpopulations under optimum conditions. However, under salinity, drought stress, and accelerated aging conditions, there were differences between the subpopulations. The salinity, drought, and accelerated aging treatments reducing Gmax of the S subpopulation by 50% were 18 dS m−1, 0.75 MPa, and 90 h, respectively, while for the R subpopulation the corresponding values were 15 dS m−1, 0.66 MPa, and 67 h. No differences were found in the activity of the antioxidant enzymes and the content of protein between non-aged seeds of the subpopulations. The aging treatments reducing the activity of catalase and superoxide dismutase enzymes by 50% were 118 and 82 h for the S subpopulation, respectively, while they were 54 and 58 h for the R subpopulation. In contrast, there were no differences in the effect of the aging treatments on the peroxidase activity and protein content between subpopulations. The results provided clear evidence that the non–target site resistant loci of blackgrass is associated with fitness costs under environmental stress.

South American countries are important agricultural players worldwide. Pesticides are key components of their production systems and, in some cases, complement environmentally sound systems, such as no-till, which contributes to preserving soil productivity. In this review, presented in the symposium Global Perspective on Herbicides Being Banned during the 2019 Weed Science Society of America meeting, we describe the regulatory framework and current situation of restricted and banned herbicides in South America. We also discuss where the pressure for herbicide bans is coming from and the opportunities for improving herbicide use and public perception. Argentina, Brazil, Colombia, and Uruguay were chosen as representative countries of the region. They all have regulatory systems in place for pesticide registration and reevaluation based on science. Glyphosate, paraquat, and some 2,4-D formulations are in the spotlight. Glyphosate is being reevaluated in Brazil and, although banned within the city limits in some cities in Argentina and Uruguay, it can still be used in agriculture. Paraquat is prohibited for aerial applications in Colombia and is the only herbicide that needs a professional prescription in Uruguay. It was reevaluated in Brazil, resulting in a use-restriction phase in effect until 2020, when it will be permanently banned. Ester formulations of 2,4-D have been banned in Brazil since the early 2000s and have restrictions in some provinces in Argentina, where 2,4-D butyl and isobutyl esters will be prohibited starting April 2021. In Uruguay, atrazine is the only herbicide banned for agricultural use. The regulatory frameworks ensure that herbicides on the market are effective and safe. Reevaluation is an important part of the system and is conducted when there are reasonable concerns. There are opportunities to continue training pesticide handlers and applicators and to communicate the importance of adopting the best management practices where herbicides are part of the production system.

Organophosphate insecticides, which have the capacity to inhibit specific herbicide-degrading (cytochrome P450) enzymes, have been used to explore metabolic herbicide-resistance mechanisms in weeds. This study investigates the response of seven field-selected rigid ryegrass (Lolium rigidum Gaudin) populations to herbicides from three different sites of action in the presence or absence of the P450 inhibitor phorate. Phorate antagonized the thiocarbamate herbicides triallate and prosulfocarb (8-fold increase in LD50) in multiple resistant L. rigidum populations with resistance to three different site-of-action herbicides. In contrast, phorate synergized trifluralin and propyzamide in some populations, reducing the LD50 by 50%. Conversely, treatment with phorate had no significant effect on the LD50 for S-metolachlor or pyroxasulfone (inhibitors of very-long-chain fatty-acid synthesis). Phorate has diverse effects that are herbicide and population dependant in field-selected L. rigidum, suggesting P450 involvement in the metabolism of trifluralin and failure to activate thiocarbamate herbicides in these populations. This research highlights the need for implementation of diverse approaches other than herbicide alone as part of a long-term integrated strategy to reduce the likelihood of metabolism-based resistance to PPI herbicides in L. rigidum.

The repetitive use of ALS inhibitors for smallflower umbrella sedge (Cyperus difformis L.) control has selected for herbicide-resistant (R) populations that threaten the sustainability of rice (Oryza sativa L.) production and demand alternative control measures be developed. A better understanding of seedling recruitment patterns at the field level is required to optimize the timing and efficacy of control measures. Therefore, a population-based threshold model was developed for optimizing germination prediction in multiple acetolactate synthase (ALS)-R and ALS-susceptible (ALS-S) C. difformis biotypes and applied to field-level emergence predictions. Estimated base temperatures (Tb) ranged from 16.5 to 17.6 C with no clear pattern between biotypes; such values are higher than Tb values of other important rice weeds, as well as for rice. Germination rates increased linearly from 16 to 33.7 C. ALS-R seeds germinate faster due to smaller median thermal times to germination (θT(50)) while also displaying lower germination synchronicity across water potentials. Interestingly, ALS-R biotypes were capable of germinating under lower moisture availability, as indicated by their lower (more negative) base water potential values (Ψb(50)) for seed germination; Ψb(50) values ranged from −0.24 to −1.13 MPa. In-field soil germination measurements found thermal times to emergence varied across three water regimes (daily water, flooded, or saturated). Seedling emergence under the daily water treatment was fastest; however, total seedling density was lower than for the other water regimes. In order to optimize springtime C. difformis seedling emergence, soil moisture should be kept around field capacity, as germination is hindered at lower moisture contents. By predicting when most of the seed population germinates, the thermal-time model can address issues regarding the optimal timing for herbicide applications, thereby allowing for improved C. difformis management in rice fields.