Evolution Of Herbicide Resistance In Weeds Research Articles

Integrating cover crops and herbicides for weed control in soybean

Abstract In mid-southern, southeastern, and northeastern U.S. soybean production regions, the evolution of herbicide-resistant weeds has become a significant management challenge for growers. The rising herbicide costs for managing herbicide-resistant weeds are also a growing concern, leading to the utilization of cover crops as an integrated weed management strategy for addressing these challenges. Field experiments were conducted at two locations in Alabama in 2022 to evaluate winter cereal cover crops, including a mixture and herbicide system integration in soybean. Treatments included five cover crops: oats, cereal rye, crimson clover, radish, and a cover crop mixture. Cover crops were evaluated for their weed-suppressive characteristics compared to a winter fallow treatment. Additionally, four herbicide treatments were applied: a preemergence (PRE) herbicide, a postemergence (POST) herbicide, PRE plus POST herbicides, and a nontreated (NT) check. The PRE herbicide was S-metolachlor; the POST treatment contained a mixture of dicamba and glyphosate. The PRE plus POST system contained the PRE application followed by POST application. Results show that cereal rye and the cover crop mixture provided weed biomass reduction compared to all cover crop treatments across both locations. Furthermore, we observed greater soybean yield following the cereal rye cover crop than following the winter fallow treatment at one location. POST and PRE plus POST herbicide treatment resulted in greater weed biomass reduction and improved soybean yield than the PRE herbicide treatment alone and the NT check at both locations.

Detoxification mechanism of herbicide in Polypogon fugax and its influence on rhizosphere enzyme activities

The excessive use of chemical herbicides has resulted in evolution of herbicide-resistant weeds. Cytochrome P450 monooxygenases (P450s) are vital detoxification enzymes for herbicide-resistant weeds. Herein, we confirmed a resistant (R) Polypogon fugax population showing resistance to quizalofop-p-ethyl, acetolactate synthase (ALS)-inhibiting herbicide pyroxsulam, and several other ACCase (acetyl-CoA carboxylase)-inhibiting herbicides. Molecular analysis revealed no target-site gene mutations in the R population. Foliar spraying with malathion clearly reversed the quizalofop-p-ethyl phytotoxicity. Higher level of quizalofop-p-ethyl degradation was confirmed in the R population using HPLC analysis. Subsequently, RNA-Seq transcriptome analysis indicated that the overexpression of CYP89A2 gene appeared to be responsible for reducing quizalofop-p-ethyl phytotoxicity. The molecular docking results supported a metabolic effect of CYP89A2 protein on most herbicides tested. Furthermore, we found that low doses of herbicides stimulated the rhizosphere enzyme activities in P. fugax and the increase of rhizosphere dehydrogenase of R population may be related to its resistance mechanism. In summary, our research has shown that metabolic herbicide resistance mediated by CYP89A2, contributes to quizalofop-p-ethyl resistance in P. fugax.

Absorption, Translocation, and Metabolism of Glyphosate and Imazethapyr in Smooth Pigweed with Multiple Resistance

The evolution of herbicide-resistant weeds is the major challenge for chemical management worldwide, increasing production costs, and reducing yield. This work aimed to evaluate the putative resistance of the Amaranthus hybridus population from Candido Mota (CMT) to glyphosate and imazethapyr and to investigate the non-target site mechanisms involved. Dose–response studies were conducted under greenhouse conditions and the control and biomass reduction were evaluated 28 days after application (DAA). Absorption, translocation, and metabolization studies were evaluated at 72 h after treatment (HAT) using radiometric techniques. The dose–response results show different responses among populations to glyphosate and imazethapyr. The CMT population was not controlled with labeled herbicide doses. Based on biomass reduction, the resistance factor was 16.4 and 9.4 to glyphosate and imazethapyr, respectively. The CMT absorbed 66% of 14C-glyphosate and 23% of 14C-imazethapyr at 72 HAT. Although the CMT population absorbed more glyphosate than the susceptible population (12.6%), translocation was impaired for both herbicides in the CMT when compared to the SUS population. There was no evidence that herbicide metabolization was involved in CMT resistance to the herbicides studied. Understanding the mechanisms endowing resistance allows better decision-making. This is the first study that describes non-target-site resistance mechanisms in an Amaranthus hybridus population from Brazil.

Evolution of Herbicide Resistant Weeds in Agro-ecological Systems

Weed management through herbicides continues to be the most efficient and cost-effective component of integrated weed management (IWM) in crop production systems. However, the current production systems are characterised by intensive, inappropriate use of herbicides along with misapplication which creates selection pressure and leads to the rapid evolution of herbicide resistance in weeds. Currently, there are 511 unique cases of herbicide-resistant weed species globally, involving 266 species and these comprise 153 and 113 dicots and monocots respectively. To date, herbicide resistance has been reported from 96 crops in more than 71 countries. Furthermore, weeds have evolved resistance to 21 of the 31 known sites of herbicide action in different crops. Understanding the mechanisms of herbicide resistance is therefore essential which when coupled with the ecological and management factors that affect herbicide resistance would lead to the development of appropriate, profitable, and sustainable weed management strategies. The review is aimed at the development of herbicide resistance, mechanisms of herbicide resistance, factors that influence the rate of resistance development and management of herbicide resistance by growers.

Widespread Occurrence of Glyphosate-Resistant Hairy Fleabane (Erigeron bonariensis L.) in Colombia and Weed Control Alternatives

Glyphosate, the most applied herbicide globally, offers effective non-selective and post-emergent weed control. Evolution of herbicide-resistant weeds is commonly associated with the recurrent application of herbicides with the same mode of action. Native to South America, hairy fleabane (Erigeron bonariensis L.) is the most problematic weed in this sub-continent and has previously been confirmed glyphosate resistant. This research aimed at characterizing glyphosate-resistant populations, thus estimating the frequency of resistance, resistance levels and identifying effective herbicide alternatives to control glyphosate-resistant populations. Glyphosate resistance characterization was initially conducted on ten suspected populations collected in plantain, banana, cassava, passionfruit, papaya, and drybean crops. Two resistant populations were selected and further characterized through dose-response tests; in addition, response to alternative herbicides (paraquat, glufosinate, 2,4-D, pyraflufen-ethyl, and mesotrione) was determined. All surveyed hairy fleabane populations survived (≥80% of individuals) the recommended glyphosate rate of 1080 g ae ha−1; conversely, five populations collected from non-cropping areas were effectively controlled at this same rate. Dose-response tests estimated resistance factors of 3.15- to 22.3-fold versus the most susceptible population. Interestingly, resistance profile and dose-response tests detected hormesis responses at field-recommended rates. The most effective herbicide alternatives to control glyphosate-resistant hairy fleabane were pyraflufen-ethyl and mesotrione.

Evaluation of the Allelopathic Activity of Albizia procera (Roxb.) Benth. as a Potential Source of Bioherbicide to Control Weeds

Agricultural production depends heavily on the application of synthetic herbicides. Using these herbicides results in the development of herbicide-resistant weeds, poses hazards to human and animal health, and pollutes the environment. To solve these problems, developing and using bioherbicides must be increased. Although different uses of Albizia procera have been well reported, its allelopathic activity against weeds and crop species has not. Hence, we evaluated the allelopathic activity of the A. procera plant and isolated its allelopathic compounds. Extracts of A. procera significantly suppressed the seedling growth of the tested species (cabbage, alfalfa, lettuce, barnyard grass, timothy, and Italian ryegrass). The seedling growth decreased with increasing extract concentrations. The concentrations required for 50% growth inhibition (I50 value) of the tested plants were 0.0225–0.4935 mg/mL. The A. procera extracts were separated using different column chromatography, and two active fractions (AP-5 and AP-7) were isolated. Cress seedling growth was completely restricted by fraction AP-5, and fraction AP-7 restricted the cress shoots to 83.10% and roots to 85.65% of the control treatment. The findings of this study indicate that A. procera extracts have allelopathic activity and these fractions might contribute to the activity.

Allelopathy: an eco-friendly approach to control palmer amaranth using allelopathic sweetpotato

Palmer amaranth (PA) is one of the major weeds in sweetpotato reducing its quantity and quality. The widespread and repeated use of chemical herbicides has led PA to develop resistance for such chemicals. In addition, chemical herbicides are incompatible with the organic production system. It is imperative to find sustainable weed management strategies to provide weed control suitable for organic cultivation and detain the development of herbicide-resistant weeds under conventional crop production. In the present study, seventeen sweetpotato varieties were screened for their allelopathic (weed-suppressing) effect on the growth of PA. The experiment was conducted in a greenhouse using a stair-step system. Each plant in the stair-step system had its height (cm), chlorophyll concentration (cci) and shoot biomass (g) measured. The variation in the height, chlorophyll and shoot biomass reduction of PA was significant after the third week of transplanting. Three weeks after transplanting (WAT), only three varieties, i.e., Morado (75%), Bayou belle (62%), and Vardaman (61%), reduced PA’s height by >60% compared to the control. While 5 WAT, four varieties, i.e., 529 (93%), Morado (93%), Heartogold (85%), and Centennial (81%), reduced PA height by >80%. Hatteras, Centennial, and 529 reduced the chlorophyll content of PA by >50%. In the presence of Beauregard, the commercial cultivar, there was no reduction in shoot biomass of PA. Cluster analysis also demonstrated that the four allelopathic sweetpotato varieties, i.e., Heart-O-Gold, Centennial, 529, and Morado, were clustered together, indicating that these varieties have similar potential to suppress the growth and development of PA. Combining allelopathic sweetpotato cultivars (Heart-O-Gold, Centennial, 529, and Morado) with other sustainable weed control measures, such as cover crops and hand-weeding, can improve the weed management, espicially in organic farming. However, field experiments should be conducted to confirm the allelopathic as well as yield potential of these varieties in an agronomic setting. The availability of the allelopathic sweetpotato cultivars will benefit organic producers by enhancing crop productivity and decrease reliance on chemical herbicides in conventional farming systems.

Compositional and Animal Feeding Assessments of a Novel Herbicide-Tolerant Maize Variety

ZDAX5 is a variety of herbicide-tolerant maize that contains the modified P450-N-Z1 gene isolated from Cynodon dactylon (L.) Pers. and the cp4 epsps gene isolated from the Agrobacterium tumefaciens strain CP4 and exhibits high tolerances to flazasulfuron and glyphosate under field conditions. Once ZDAX5 corn is available on the market, the evolution of herbicide-resistant weeds will be delayed by applying glyphosate and flazasulfuron to corn fields. Prior to commercialization, it is critical to assess the safety of ZDAX5 maize. Compositional analysis and feed consumption studies in rodents are an important consideration in the safety assessment of genetically modified crops. The nutritional components of ZDAX5 were analyzed and compared with those of its non-transgenic counterpart. The data showed that all the analyzed components in the herbicide-tolerant maize plants were substantially equivalent to those of its non-transgenic counterpart. Furthermore, most of the measured values from ZDAX5 were within the range of values reported for other commercial maize varieties. The sub-chronic feeding trial was carried out with grains from GM, and non-GM maize were independently added into rodent diets at concentrations of 12.5%, 25% and 50%. As a control, another set of rats was fed with a marketed diet. At the end of the 90-day feeding study, no negative effects associated with the consumption of GM maize were found. These results indicate that the herbicide-tolerant maize ZDAX5 is as nutritious and safe as non-transgenic maize.

Weed Management Practices in Wheat (Triticum aestivum L.): A Review

Wheat is India’s second-largest staple food crop grown in different agro climatic zones within different crop sequences, as a result of which a diversity of weeds infests to this crop. Diversity of weeds including grassy, broadleaf weeds and sedges are mainly infested to the crop. Phalaris minor (grassy) and Rumex dentatus (broadleaf) are the most dominant weeds in northern region of country. For receiving fast results, use of herbicides is most popular method of weed control and used all over the world. But over reliance on herbicides has created several complications such as environmental quality deterioration, shifting in weed flora and development of herbicide resistant weeds. These problems can be abridged to a great extent by decreasing reliance on chemical method which can be done by integration of various cultural and mechanical methods to the weed management program. Various cultural practices such as nutrient management, seeding rate, spacing, cropping sequence, water management, sowing methods, sowing time and competitive varieties should be followed in such a way that crop plants utilize much more resources in comparison to weed plants. As a mechanical measure, Furrow Irrigated Raised Bed (FIRB) method and stale seed bed technique are recommended for effective weed management. Timely application of herbicides with recommended doses with the help of efficient spray technology improves the efficacy of herbicides. So with the help of Integrated Weed Management (IWM) i.e integration of various cultural, mechanical and chemical methods, we can efficiently managed the diverse weed flora in wheat crop.

Prospects for Bioherbicides

New solutions to weed management are needed now more than ever. Ag retailers, university extension specialists and farmers consider the evolution of herbicide-resistant weeds "one of the most significant developments in agriculture today." Indeed, weed resistance is not just a US problem, but a global one. There are currently 505 unique cases of herbicide resistant weeds globally, with 264 species. Weeds have evolved resistance to 21 of the 31 known herbicide sites of action and to 164 different herbicides. Herbicide resistant weeds have been reported in 95 crops in 71 countries. According to Duke, no new herbicide modes of action have been introduced since the 1980s. Since this paper, only one herbicide with a new mode of action (cyclopyrimorate) has been commercialized. Some major agrichemical companies have announced new discoveries and new modes of action coming in the future, but these are some years off. Given this state of weed management, one would think that biological or natural product herbicides with new modes of action would already be making headway in the market. Unfortunately, few new biological products have been commercialized. The technical difficulty in finding bioherbicides that can compete with the spectrum and price of chemical herbicides has left agriculture with a paucity of new herbicides based on natural products. Most products are targeted at organic agriculture because of their higher manufacturing cost. These products are typically short residual, burndown products that require high volumes and multiple applications. Examples are clove and orange oils. Other natural active ingredients include acetic acid (vinegar), FeHEDTA, NaCl, pelargonic acid, and caprylic and capric acids. This paper reviews the reasons for the lack of new bioherbicides, current and pipeline products and how new technologies may accelerate their progress.

Herbicidal properties of antihypertensive drugs: calcium channel blockers

Herbicide resistance is a worldwide problem in weed control. This prompts researchers to look for new modes of action to slow down the evolution of herbicide-resistant weeds. This research aims to determine the herbicidal action of thiazolo[3,2-a]pyrimidines derivatives, which are well known as antihypertensive drugs. The phytotoxic effects of ten compounds were investigated using leaf disc discoloration test and seed germination bioassay. At concentrations of 125 to 250 mg/L, the 5-(3-Fluoro-phenyl)-7-methyl-5H-thiazolo[3,2-a]pyrimidine-6-carboxylic acid ethyl ester (c) was highly active against Oldenlandia verticillata and Eleusine indica. At application rates of 1.25 to 2.5 kg ai/ha, formulated c demonstrated selective post-emergence and pre-emergence herbicidal activity against O. verticillata, E. indica and Cyperus iria. In the crop tolerance test, formulated c outperformed the commercial herbicide diuron, with aerobic Oryza sativa being the most tolerant, followed by Zea mays, and Brassica rapa. The addition of calcium chloride partially nullified compound c's inhibitory effects on weed shoot growth, indicating that it has potential as a calcium channel blocker. Compound c acted by triggering electrolyte leakage without affecting photosystem II. These findings imply that c could be explored further as a template for developing new herbicides with novel modes of action.

Effects of cereal rye seeding rate on waterhemp (Amaranthus tuberculatus) emergence and soybean growth and yield

AbstractThe evolution of herbicide-resistant weeds has resulted in the necessity to integrate nonchemical control methods with chemicals for effective management in crop production systems. In soybean, control of the pigweed species, particularly herbicide-resistant waterhemp and Palmer amaranth, have become predominant concerns. Cereal rye planted as a winter cover crop can effectively suppress early-season weed emergence in soybean, including waterhemp, when planted at a rate of 123 kg ha−1. The objectives of this study were to determine the effects of different cereal rye seeding rates (0, 34, 56, 79, 110, and 123 kg ha−1) on early-season waterhemp suppression and soybean growth and yield. Soybean was planted into fall-seeded cereal rye, which was terminated within 4 d of soybean planting. The experiment was conducted over the 2018, 2019, and 2020 growing seasons in Columbia, Missouri. Effects of cereal rye on early-season waterhemp suppression varied by year and were most consistent at 56 kg ha−1 or higher seeding rates. Linear regression analysis of cereal rye biomass, height, or stand at soybean planting showed inverse relationships with waterhemp emergence. No adverse effects on soybean growth or yield were observed at any of the cereal rye seeding rates relative to plots that lacked cereal rye cover. Result differences among the years suggest that the successfulness of cereal rye on suppression of early-season waterhemp emergence is likely influenced by the amount of waterhemp seed present in the soil seed bank.

Corrigendum: An EU Perspective on Biosafety Considerations for Plants Developed by Genome Editing and Other New Genetic Modification Techniques (nGMs).

[This corrects the article DOI: 10.3389/fbioe.2019.00031.].

An EU Perspective on Biosafety Considerations for Plants Developed by Genome Editing and Other New Genetic Modification Techniques (nGMs).

The question whether new genetic modification techniques (nGM) in plant development might result in non-negligible negative effects for the environment and/or health is significant for the discussion concerning their regulation. However, current knowledge to address this issue is limited for most nGMs, particularly for recently developed nGMs, like genome editing, and their newly emerging variations, e.g., base editing. This leads to uncertainties regarding the risk/safety-status of plants which are developed with a broad range of different nGMs, especially genome editing, and other nGMs such as cisgenesis, transgrafting, haploid induction or reverse breeding. A literature survey was conducted to identify plants developed by nGMs which are relevant for future agricultural use. Such nGM plants were analyzed for hazards associated either (i) with their developed traits and their use or (ii) with unintended changes resulting from the nGMs or other methods applied during breeding. Several traits are likely to become particularly relevant in the future for nGM plants, namely herbicide resistance (HR), resistance to different plant pathogens as well as modified composition, morphology, fitness (e.g., increased resistance to cold/frost, drought, or salinity) or modified reproductive characteristics. Some traits such as resistance to certain herbicides are already known from existing GM crops and their previous assessments identified issues of concern and/or risks, such as the development of herbicide resistant weeds. Other traits in nGM plants are novel; meaning they are not present in agricultural plants currently cultivated with a history of safe use, and their underlying physiological mechanisms are not yet sufficiently elucidated. Characteristics of some genome editing applications, e.g., the small extent of genomic sequence change and their higher targeting efficiency, i.e., precision, cannot be considered an indication of safety per se, especially in relation to novel traits created by such modifications. All nGMs considered here can result in unintended changes of different types and frequencies. However, the rapid development of nGM plants can compromise the detection and elimination of unintended effects. Thus, a case-specific premarket risk assessment should be conducted for nGM plants, including an appropriate molecular characterization to identify unintended changes and/or confirm the absence of unwanted transgenic sequences.

A novel triple amino acid substitution in the EPSPS found in a high-level glyphosate-resistant Amaranthus hybridus population from Argentina.

The evolution of herbicide-resistant weeds is one of the most important concerns of global agriculture. Amaranthus hybridus L. is a competitive weed for summer crops in South America. In this article, we intend to unravel the molecular mechanisms by which an A. hybridus population from Argentina has become resistant to extraordinarily high levels of glyphosate. The glyphosate-resistant population (A) exhibited particularly high parameters of resistance (GR50 = 20 900 g ai ha-1 , Rf = 314), with all plants completing a normal life cycle even after 32X dose application. No shikimic acid accumulation was detected in the resistant plants at any of the glyphosate concentrations tested. Molecular and genetic analyses revealed a novel triple substitution (TAP-IVS: T102I, A103V, and P106S) in the 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS) enzyme of population A and an incipient increase on the epsps relative copy number but without effects on the epsps transcription levels. The novel mechanism was prevalent, with 48% and 52% of the individuals being homozygous and heterozygous for the triple substitution, respectively. In silico conformational studies revealed that TAP-IVS triple substitution would generate an EPSPS with a functional active site but with an increased restriction to glyphosate binding. The prevalence of the TAP-IVS triple substitution as the sole mechanism detected in the highly glyphosate resistant population suggests the evolution of a new glyphosate resistance mechanism arising in A. hybridus. This is the first report of a naturally occurring EPSPS triple substitution and the first glyphosate target-site resistance mechanism described in A. hybridus. © 2018 Society of Chemical Industry.

Multiple Herbicide–Resistant Italian Ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] in California Perennial Crops: Characterization, Mechanism of Resistance, and Chemical Management

AbstractItalian ryegrass [Lolium perenne L. spp. multiflorum (Lam.) Husnot] is a troublesome weedy species in many regions of California. Its control has been chiefly dependent on herbicides due to their effectiveness and practicality and, as result, herbicide-resistant populations have been selected. Poor control of a population of L. multiflorum with paraquat was recently reported in a prune orchard in Hamilton City, CA. A series of experiments were carried out to characterize the response of this population to several POST herbicides, study the mechanisms of resistance, and investigate alternative chemical management options in tree crops. A known susceptible (S) and the suspected resistant population (PRHC) were subjected to greenhouse dose–response experiments with clethodim, fluazifop-P-butyl, glufosinate, glyphosate, paraquat, pyroxsulam, rimsulfuron, and sethoxydim. A 310-bp fragment of the EPSPS gene containing position 106 was sequenced from PRHC and S. Field experiments were carried out in a prune orchard with PRE herbicides commonly used by perennial crop growers in California. Greenhouse dose–response experiments confirmed that PRHC is resistant to paraquat, as well as multiply resistant to clethodim and glyphosate. The EPSPS gene of PRHC is heterozygous for glyphosate resistance at position 106, where one allele exhibited proline substituted by serine and the other by alanine. Field experiments with PRE herbicides indicated that tank mixes containing indaziflam and flumioxazin can provide adequate L. multiflorum control up to 150 d after treatment. Poor weed management practices, such as overreliance on a single site of action, have frequently been associated with the selection of herbicide-resistant L. multiflorum populations around the world, and adequate herbicide-resistance management programs are necessary for growers to maintain economic sustainability even after evolution of herbicide-resistant weeds in their fields.

Effect of Seeding Rate on Dose Response of Wild Mustard (Sinapis arvensis) to Fluthiacet-Methyl

Concern over the development of herbicide-resistant weeds has led to interest in integrated weed management systems that reduce selection pressure by utilizing mechanical and cultural weed control practices in addition to herbicides. Increasing crop seeding rate increases crop competitive ability and thus can enhance herbicide efficacy. However, it is unknown how increasing the seeding rate affects an herbicide’s efficacy. The objective of this study was to examine the interaction between increasing seeding rate and herbicide dose to control weeds. To meet this objective, the herbicide fluthiacet-methyl was applied to field-grown lentil, with Indian mustard, a proxy for wild mustard, used as a model weed. The experiment was a factorial design with four lentil seeding rates and seven herbicide rates. Overall the herbicide dose response was altered by changing lentil seeding rate. Increasing lentil seeding rate decreased the weed biomass production when herbicides were not applied. In two of the four site-years, increasing lentil seeding rate lowered the herbicide ED50, the dose required to result in a 50% reduction in weed biomass. Increasing the crop seeding rate altered the dose response to provide greater weed control at lower herbicide rates compared with normal crop seeding rates. Increased seeding rates also resulted in higher and more stable crop seed yields across a wider range of herbicide dosages. These results suggest that dose–response models can be used to evaluate the efficacy of other weed management practices that can interact with herbicide performance.

Evaluation of 2,4-D-Choline Based Herbicide Systems in 2,4-D Tolerant Soybean (<i>Glycine max</i> L.)

Weeds are the most limiting factor in soybean production in South Carolina. With early emergence and rapid growth, weeds effectlively compete for water, nutrients, and light resources. The recent evolution of herbicide resistant weeds has made it increasingly difficult for growers to effectively control weeds in soybean. Glyphosate and ALS-resistant Palmer amaranth biotypes have spread rapidly throughout South Carolina, especially in areas where resistance management is lacking. Soybean varieties have been recently developed with tolerance to 2,4-D. Field experiments were conducted at the Clemson University Edisto Research and Education Center located near Blackville, SC in 2012 and 2013 to evaluate selected 2,4-D choline based herbicide programs for weed management in 2,4-D tolerant soybean. Overall, all herbicide treatments were effective in controlling weeds at the POST2 timing. Palmer amaranth control was excellent; however, pitted morningglory was the most difficult. The 2,4-D plus glyphosate pre-mixture provided excellent control for all three weed species with >95% control at POST2 timing. In these treatments, the rate of 2,4-D choline plus glyphosate (1.09 kg ae ha-1 or 1.64 kg ae ha-1) did not have a significant effect on weed control (P = 0.3772). There was a decrease in pitted morningglory control 3 WAP in 2012 vs 2013 in plots treated with S-metolachlor + fomesafen because of a lack of activating soil moisture in 2012. Results from this study showed that all treatments evaluated provided good to excellent control of all 3 weed species. Based on the herbicide programs evaluated in the study, herbicide resistant weeds, such as Palmer amaranth, can be effectively controlled when treated at the correct growth stage.

Monitoring Landsat vegetation indices for different crop treatments and soil chemistry

ABSTRACTThe timing and quantity of fertilizer and herbicide applications in agricultural systems are critical where maximizing vigour and yield is the ultimate goal. While fertilizers are applied to the soil to promote plant growth, herbicides are commonly used to control weeds in order to reduce the weeds’ competition for nutrients. Satellite imagery is frequently used to monitor agricultural activities and vegetation indices (VIs) are widely applied in temporal analysis of crop status. This study considers monitoring Landsat VIs for the period between 5 June and 27 October 2014 in agricultural systems under four different management treatments at the Kellogg Biological Station (KBS), in Michigan, USA. The results show that (1) fine-tuning conventional treatments by intense early herbicide applications in combination with no-tilled soil results in significantly higher VIs during the early growth stage, a more rapid maturity rate, and the highest crop yield; (2) nitrogen uptake from nitrate-based rather than from ammonium-based fertilizers might be more beneficial in terms of crop vigour and yield return; (3) organic treatments, with organic corn and no agricultural chemicals, keep higher VIs longer in the season at the cost of lower yield; and (4) genetically modified (GM) breeds under conventional or reduced-chemical treatments have synchronized early senescence. A positive correlation between VIs during the early growth stage and yield is observed for conventional no-till treatment (coefficient of determination, R2 = 0.70). The correlation becomes gradually weaker with each month from late June to October (29 June: R2 = 0.70; 16 August: R2 = 0.61; 17 September: R2 = 0.44; 27 October: R2 = 0.01). The analysis of variance (ANOVA)–Tukey–Kramer approach suggests significant differences in VIs between organic and GM corn (treated conventionally or with reduced chemicals) for the preharvest season (27 October 2014). The leave-out-one cross-validation analysis confirms the predictive accuracy of the model (mean square error (MSE) = 0.0014). The rapid evolution of herbicide-resistant weeds requires constant refinement of chemical inputs to agricultural systems, thus making the monitoring of (Landsat) VIs important in the years to come.

A New Approach to Weed Management to Mitigate Herbicide Resistance in Argentina

The evolution of herbicide-resistant weeds is a major concern in the corn- and soybean-producing Pampas region of Argentina, where growers predominantly plant glyphosate-resistant crop varieties and depend heavily on glyphosate for weed control. Currently, 16 weed species in Argentina are resistant to one or more of three different herbicide mechanisms of action, and resistant weed populations continue to increase, posing a serious threat to agricultural production. Implementation of integrated weed management to address herbicide resistance faces significant barriers in Argentina, especially current land ownership and rental patterns in the Pampas. More than 60% of Pampas cropland is rented to tenants for periods that rarely exceed 1 yr, resulting in crop rotation being largely abandoned, and crop export taxes and quotas have further discouraged wheat and corn production in favor of continuous soybean production. In this paper we discuss ways to facilitate new approaches to weed management in Argentina, including legal and economic reforms and the formation of a national committee of stakeholders from public and private agricultural sectors.