Glyphosate For Weed Control Research Articles

The silver bullet that wasn't: Rapid agronomic weed adaptations to glyphosate in North America.

The rapid adoption of glyphosate-resistant crops at the end of the 20th century caused a simplification of weed management that relied heavily on glyphosate for weed control. However, the effectiveness of glyphosate has diminished. A greater understanding of trends related to glyphosate use will shed new light on weed adaptation to a product that transformed global agriculture. Objectives were to (1) quantify the change in weed control efficacy from postemergence (POST) glyphosate use on troublesome weeds in corn and soybean and (2) determine the extent to which glyphosate preceded by a preemergence (PRE) improved the efficacy and consistency of weed control compared to glyphosate alone. Herbicide evaluation trials from 24 institutions across the United States of America and Canada from 1996 to 2021 were compiled into a single database. Two subsets were created; one with glyphosate applied POST, and the other with a PRE herbicide followed by glyphosate applied POST. Within each subset, mean and variance of control ratings for seven problem weed species were regressed over time for nine US states and one Canadian province. Mean control with POST glyphosate alone decreased over time while variability in control increased. Glyphosate preceded by a labeled PRE herbicide showed little change in mean control or variability in control over time. These results illustrate the rapid adaptation of agronomically important weed species to the paradigm-shifting product glyphosate. Including more diversity in weed management systems is essential to slowing weed adaptation and prolonging the usefulness of existing and future technologies.

Efficacy of S-metolachlor + glyphosate for Weed Control in Different Levels of Eucalyptus Straw

The occurrence of weeds in eucalyptus plantations can cause losses in productivity. Chemical control is widely used, but the efficiency of herbicides depends on management and environmental factors. This study aims to evaluate the efficiency of S-metolachlor + glyphosate in the control of grasses in different densities of eucalyptus straw and with simulated rainfall after application of the product. The experiment was conducted in a randomized block design, factorial, with four replications. The first factor represented 0; 1.06 + 0.79 e 2.12 + 1.59 kg i.a. ha−1 of the commercial dose of S-metolachlor + glyphosate, the second 0; 5 and 10 tons ha−1 of straw and, the third 25 and 50 mm of water depth applied in soil with a mix of grasses previously sowed. The evaluations carried out were fresh mass, dry mass, and visual analysis of the control percentage. The fresh and dry mass and the grasses’ dry mass/water ratio decreased with increasing herbicide dosage and straw density. The treatments without straw and with the herbicide application had the highest percentages of control, the highest in the dosage of 2.12 + 1.59 kg i.a. ha−1 of S-metolachlor + glyphosate. Applying different water depths (25 mm or 50 mm) did not influence the control. In conclusion, it was observed that the isolated straw promoted the control of grasses. However, in treatments that included straw and herbicide, there was a decrease in the efficiency of the product, which suggests an antagonism between the vegetation cover and S-metolachlor + glyphosate.

Isolation and Characterization of Glyphosate–Degrading Bacteria from Agricultural Soil in Awka, Anambra State, Nigeria

Glyphosate is a very effective pesticide; hence it has been excessively used in agricultural activities. This led to glyphosate residue accumulation in the soil and resulted in various adverse effects on humans and the ecosystem. This necessitates the need to reduce glyphosate accumulation in the soil and microbial bioremediation can be the solution. Thus, this study aimed to isolate and characterize glyphosate degrading bacteria from agricultural soils exposed to glyphosate weed control method. Bacterial organisms were isolated from glyphosate polluted agricultural soil and were characterized using routine microbiological biochemical methods. The isolates were screened for glyphosate utilization using a mineral salt medium containing glyphosate as a carbon source. Confirmed glyphosate degraders were identified using 16s rDNA sequencing. These isolates were assessed for their ability to degrade 106.16 µg/ml glyphosate in varied volumes (1 ml, 1.5 ml and 2 ml), singly and in consortium. Glyphosate degraders were identified as Exiguobacterium alkaliphiluim, Alcaligenes faecalis, Sinorhizobuim fredii and Acinetobacter nosocomialis. These organisms significantly (p<0.05) degraded different glyphosate volumes as a consortium more than as individual organisms. This study has been able to show the different glyphosate degraders present in some glyphosate-polluted agricultural soils in Awka, Anambra State, Nigeria.

Glyphosate effects on non-target plants: collateral impacts on elemental composition and growth of yerba mate

ABSTRACT This work investigated collateral effects on non-target yerba mate (Ilex paraguariensis A. St.-Hil) plants when using glyphosate for weed control; growth and leaf elemental composition were examined. Special emphasis was placed on examining the heavy metals Cd and Pb due to regulations defining maximum limits in South American infusion products. The experiment was conducted in pots using glyphosate [applied to Congo grass (Brachiaria ruziziensis), soil surface application, and control], two P rates (with and without P), two clonal yerba mate cultivars, and two different soils (basalt- and rhyodacite-derived). When using glyphosate to control Congo grass, total dry matter and basal diameter of yerba mate plants decreased. This effect was enhanced by lack of P. Collateral effects of glyphosate use resulted in increased K, P, and Cu in clone 1 cultivated in basalt-derived soil; with rhyodacite-derived soil, increased levels of Ca, Mg, Mn, Fe, and Zn were observed in clone 1, while clone 2 displayed increases in B, Cd, and Pb. These observations demonstrate that glyphosate use to control undesirable plants can impact initial development and elemental composition of yerba mate, with variations between cultivars and soil type.

Glyphosate Resistance in Sonchus oleraceus and Alternative Herbicide Options for Its Control in Southeast Australia

Sonchus oleraceus is becoming a hard-to-control weed in Australian cropping systems, especially in glyphosate-tolerant cotton and during summer fallows. Several biotypes of this weed have developed resistance to glyphosate as a result of common management practices under conservation agriculture systems in the country. A series of pot experiments were conducted to evaluate the effect of temperature on glyphosate efficacy and performance of several post-emergence and pre-emergence herbicides on a glyphosate-resistant (GR) and a glyphosate-susceptible (GS) biotype of S. oleraceus. At low temperatures (19–24 °C), no plants of the GS biotype survived glyphosate application at 570 g/ha; however, in the high-temperature regime (28–30 °C), 83% of the plants survived this rate of glyphosate. Similarly, for the GR biotype, up to 58% of the plants survived at 2280 g/ha of glyphosate when applied during the high-temperature regime and no plants survived this rate during the low-temperature regime. A number of post-emergence herbicides were found to be effective for S. oleraceus control. However, herbicide application delayed to the six-leaf stage compared with the four-leaf stage reduced control, especially for bromoxynil and saflufenacil herbicides. Glufosinate and paraquat were the most effective herbicides for S. oleraceus control, resulting in no seedling survival for both biotypes. Isoxaflutole, pendimethalin or s-metolachlor efficacy was not reduced by the presence of crop residue, suggesting that these herbicides could be used to control S. oleraceus in conservation agriculture systems. The results of this study suggest that growers will need to reduce over-reliance on glyphosate for weed control in summer fallows and use alternative post-emergence herbicides.

Impact analysis of potential glyphosate regulatory restrictions in the European Union on Turkish hazelnut production and economy

AbstractHazelnut (Corylus avellanaL.) 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.

Increased temperatures and elevated CO2 levels reduce the sensitivity of Conyza canadensis and Chenopodium album to glyphosate

Herbicides are the most commonly used means of controlling weeds. Recently, there has been growing concern over the potential impacts of global climate change, specifically, increasing temperatures and elevated carbon dioxide (CO2) concentrations, on the sensitivity of weeds to herbicides. Here, glyphosate response of both Conyza canadensis and Chenopodium album was evaluated under different environmental conditions. Reduced glyphosate sensitivity was observed in both species in response to increased temperature, elevated CO2 level, and the combination of both factors. Increased temperature had greater effect on plant survival than elevated CO2 level. In combination, high temperature and elevated CO2 level resulted in loss of apical dominance and rapid necrosis in glyphosate-treated plants. To investigate the mechanistic basis of reduced glyphosate sensitivity, translocation was examined using 14C-glyphosate. In plants that were subjected to high temperatures and elevated CO2 level, glyphosate was more rapidly translocated out of the treated leaf to shoot meristems and roots than in plants grown under control conditions. These results suggest that altered glyphosate translocation and tissue-specific sequestration may be the basis of reduced plant sensitivity. Therefore, overreliance on glyphosate for weed control under changing climatic conditions may result in more weed control failures.

Optimum Weed Control Method Increases the Yield of Kinnow by Improving the Physical Properties of Soil

ABSTRACT: Going towards sustainable fruit culture, it is very important that soil fertility should maintain by adopting the proper weeds control and adding the organic matter. This research was carried out to identify the most suitable weed control method for Kinnow orchard. The plants of Kinnow at the age of 6 years grown under drip irrigation were selected. Weeds of orchard were managed by mulching (wood chip and black polyethylene), mechanical (cultivator and rotavator) and chemical (glyphosate) methods. Mechanical weed control was taken as control treatment. The plants where wood chip mulching was used for weed control showed maximum increase in shoot length (24.00 cm), chlorophyll contents (84.00 SPAD value) and yield (11.88 ton ha-1) followed by the plants where weeds were controlled with chemical control while lowest shoot length (12.00 and 12.33 cm), chlorophyll contents (41.00 and 42.67 SPAD value) and yield (3.80 and 4.70 ton) was achieved in cultivator and rotavator treatment respectively. Wood chip mulch also improved soil physical properties by reducing bulk density (1.49 Mg m-3), soil penetration resistance (785.33 kPa) and by enhancing soil organic matter (1.17%), hydraulic conductivity (34.50 mm/hr-1) and soil microbial biomass carbon (159.68 g kg-1) followed by glyphosate control while lowest values of soil physical properties were obtained in cultivator and rotavator weed control. Wood chip mulch also reduced weed dry weight (45.25 g) and efficiently controlled weeds (95.66%) as compared to other weed control methods. In this case, wood chip mulch performed better in all studied parameters than plastic mulch, rotavator, cultivator and glyphosate weed control. So, wood chip mulch should be recommended to citrus grower for improving soil physical properties and yield.

Development of glyphosate-tolerant transgenic cotton plants harboring the G2-aroA gene

Given that glyphosate weed control is an effective strategy to reduce costs and improve economic outcomes of agricultural production in China, the development of glyphosate-resistant cotton holds great promise. Using an Agrobacterium-mediated transformation method, a new G2-aroA gene that encodes 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was transformed into cotton cultivar K312. The transgenic cotton plants were regenerated from a callus tissue culture via kanamycin selection. Ten regenerated cotton plants were obtained and allowed to flower normally to produce fruit. The results from polymerase chain reaction (PCR) and Southern and Western blot analyses indicated that the target gene was integrated into the cotton chromosome and was expressed effectively at the protein level. The glyphosate tolerance analysis showed that the transgenic cotton had a high resistance to glyphosate. Further, even cotton treated with 45.0 mmol L−1 of glyphosate was able to slowly grow, bloom and seed. The transgenic cotton may be used for cotton breeding research of glyphosate-tolerant cotton.

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.

Yield of glyphosate-resistant sugar beets and efficiency of weed management systems with glyphosate and conventional herbicides under German and Polish crop production

In sugar beet production, weed control is one of the most important and most expensive practices to ensure yield. Since glyphosate-resistant sugar beets are not yet approved for cultivation in the EU, little commercial experience exists with these sugar beets in Europe. Experimental field trials were conducted at five environments (Germany, Poland, 2010, 2011) to compare the effects of glyphosate with the effects of conventional weed control programs on the development of weeds, weed control efficiency and yield. The results show that the glyphosate weed control programs compared to the conventional methods decreased not only the number of herbicide applications but equally in magnitude decreased the dosage of active ingredients. The results also showed effective weed control with glyphosate when the weed covering was greater and sugar beets had a later growth stage of four true leaves. Glyphosate-resistant sugar beets applied with the glyphosate herbicide two or three times had an increase in white sugar yield from 4 to 18% in comparison to the high dosage conventional herbicide systems. In summary, under glyphosate management sugar beets can positively contribute to the increasingly demanding requirements regarding efficient sugar beet cultivation and to the demands by society and politics to reduce the use of chemical plant protection products in the environment.

Summer Annual Weed Control with 2,4-D and Glyphosate

The introduction of 2,4-D-resistant crops stacked with glyphosate resistance will enable the use of 2,4-D and glyphosate for weed control in corn, cotton, and soybean. Because there is little reported on the effectiveness of 2,4-D plus glyphosate on summer annual weed control, the objective was to evaluate 2,4-D and glyphosate tank mixtures on summer annual weed control. Six rates of 2,4-D (0, 280, 420, 560, 840, and 1,120 g ae ha−1) and three rates of glyphosate (0, 840, and 1,120 g ae ha−1) were applied to common lambsquarters, common waterhemp, giant ragweed, giant foxtail, and velvetleaf. Glyphosate at 840 g ha−1controlled all weeds 94 to 100%. Giant ragweed was controlled 99 to 100% by 2,4-D alone when rates were 280 g ha−1or higher. Common lambsquarters, common waterhemp, and velvetleaf control increased as 2,4-D rates increased, with 1,120 g ha−1providing 90 to 94% control.

Repeated Sublethal Rates of Glyphosate Lead to Decreased Sensitivity in Palmer Amaranth

Glyphosate-resistant Palmer amaranth is the result of over-reliance on glyphosate for weed control in glyhosate-resistant crops. The use of lower-than-recommended rates of glyphosate is associated with the development of glyphosate resistance; however, the impact of low rates of glyphosate on development of glyphosate-resistant Palmer amaranth is lacking, and hence, an experiment was conducted to determine if glyphosate-resistant Palmer amaranth could be selected through treatment of plants with sublethal glyphosate rates. Three hundred seedlings of a known susceptible Palmer amaranth biotype were treated with glyphosate at 0.094 lb ae/acre at the eight-leaf stage. Plants that survived the glyphosate application (14%) were allowed to produce seeds. The sequence of selection to higher glyphosate rates was continued until the fourth generation (i.e., F0 to F3), after which rate-mortality response was determined. Following four generations of selection, the F4 population showed a 2.15-fold less sensitivity to glyphosate than the initial population based on an LD95. Survival of the F4 generation did not occur when glyphosate was applied at the recommended rate of 0.75 lb/acre. This research provides evidence that repeated use of sublethal rates of glyphosate can contribute to reduced sensitivity of Palmer amaranth to glyphosate, which may play a partial role in the development of glyphosate resistance.

Benchmark study on glyphosate-resistant cropping systems in the United States. Part 6: Timeliness of economic decision-making in implementing weed resistance management strategies

The introduction of glyphosate-resistant (GR) crops in the late 1990s made weed control in maize, cotton and soybean simple. With the rapid adoption of GR crops, many growers began to rely solely on glyphosate for weed control. This eventually led to the evolution of GR weeds. Growers are often reluctant to adopt a weed resistance best management practice (BMP) because of the added cost of additional herbicides to weed control programs which would reduce short-term revenue. This study was designed to evaluate when a grower that is risk neutral (profit maximizing) or risk averse should adopt a weed resistance BMP. Whether a grower is risk neutral or risk averse, the optimal decision would be to adopt a weed resistance BMP when the expected loss in revenue is greater than 30% and the probability of resistance evolution is 0.1 or greater. However, if the probability of developing resistance increases to 0.3, then the best decision would be to adopt a weed resistance BMP when the expected loss is 10% or greater. Given the scenarios analyzed, risk-neutral or risk-averse growers should implement a weed resistance BMP with confidence that they have made the right decision economically and avoided the risk of lost revenue from resistance. If the grower wants to continue to see the same level of return, adoption of BMP is required.

Weed management and irrigation are key treatments in emerging black truffle (Tuber melanosporum) cultivation

Interest in conversion of marginal agricultural lands to small oak woodlands for the production of black truffles (Tuber melanosporum Vittad.) is increasing in the native black truffle areas of France, Spain and Italy as well as suitable or amended sites throughout the world due to high economic returns for gastronomically valuable truffles. Successful long-term management of a perennial plantation replacing an annual crop requires understanding the system in all phases of the life cycle, and in the case of truffles this involves understanding the interactive growth of the aboveground host and the belowground symbiotic ectomycorrhizal fungus whose fruitbody is the desired truffle. Here we focus on the pre-production phase, 4 years after establishing the truffle-oak plantation and prior to truffle production. We tested the influence of weed control, irrigation and fertilizer, each at 3 levels, on plant growth and ectomycorrhizal proliferation in three truffle-oaks plantations in northeast Spain. Results show that adequate weed control improves root and shoot dry weight while fertilizer and irrigation treatments did not influence plant growth. The low dose of irrigation (50% of the estimated water deficit for the site) and glyphosate weed control both increased total root tips/plant and T. melanosporum colonized tips (ectomycorrhizae) by approximately two-fold, compared to control treatments. Distribution of the ectomycorrhizae within the soil profile was significantly influenced by the low dose irrigation treatment, with increases observed in the 10–20 cm and the 20–30 cm deep layers compared to the control and high irrigation treatments. Four years after planting, T. melanosporum remained the dominant fungal symbiont, despite the presence of 14 other ectomycorrhizal morphotypes from these sites.

Assessing the effects of cultivating genetically modified glyphosate-tolerant varieties of soybeans (Glycine max(L.) Merr.) on populations of field arthropods

We assessed the effects of cultivating two genetically modified (GM) glyphosate-tolerant soybean varieties (Glycine max (L.) Merr.) derived from Event 40-3-2 and a Japanese conventional variety on arthropods under field conditions, with weed control using glyphosate and conventional weed control for two years. Plant height and dry weight of the conventional variety were significantly larger than those of the GM varieties, but the GM varieties bore more pods than the conventional variety. We found arthropods of nine taxonomic orders (Araneae, Acari, Thysanoptera, Homoptera, Heteroptera, Coleoptera, Diptera, Lepidoptera, and Hymenoptera) on the plants. The arthropod incidence (number per plant unit weight pooled for each taxonomic order) on the soybean stems and leaves generally did not differ significantly between the GM and conventional varieties. However, the incidence of Thysanoptera and total incidence (all orders combined) were greater on the GM variety in the second year. The weed control regimes had no significant influence on the arthropod incidence on the soybean stems and leaves. The number of flower-inhabiting Thysanoptera (the dominant arthropod in the flowers) was not significantly different between the GM and conventional varieties. Asphondylia yushimai (Diptera, Cecidomyiidae) was more numerous on the pods of the GM variety in both years. Neither the soybean variety nor the weed control regime significantly affected the density of soil macro-organisms. However, the glyphosate weed control affected arthropods between the rows of plants by decreasing the abundances of Homoptera, Heteroptera, Coleoptera and Lepidoptera, and diversity of arthropods.

Épocas de manejo químico de Brachiaria decumbens antecedendo o plantio direto de soja

O manejo químico da cobertura vegetal e a manutenção da palhada sobre o solo são preocupações comuns para os produtores que adotam o sistema plantio direto na região dos cerrados. Com o objetivo de estudar a influência da época de manejo químico de B. decumbens sobre o desenvolvimento e a produtividade da cultura da soja em sistema plantio direto, foram realizados dois ensaios a campo, respectivamente, com os cultivares BRS 154 e MG/BR 46-Conquista, no ano de 2003. Em cada ensaio, testaram-se quatro épocas de manejo de B. decumbens antecedendo a semeadura da soja (28, 14, 7 e 0 dias), utilizando-se o herbicida glifosato. Os ensaios foram conduzidos em delineamento inteiramente casualizado, utilizando-se três repetições no primeiro e quatro no segundo. Determinou-se a matéria seca da cobertura vegetal e avaliou-se a reinfestação de B. decumbens. Na cultura da soja, foram determinados: altura de inserção da primeira vagem, número de vagens por planta, umidade dos grãos, massa de 100 grãos e produtividade. Com base nos resultados obtidos, conclui-se que a época mais adequada para o manejo químico de B. decumbens com glifosato situa-se entre 7 e 14 dias antes da semeadura da cultura da soja e que a escolha da época de manejo químico da cobertura vegetal afeta de forma decisiva o desenvolvimento e a produtividade da cultura.

Glyphosate Weed Control Enhancement with Ammonium Sulfate and Commercial Water Conditioning Agents

Abstract Glyphosate herbicide labels generally recommend addition of ammonium sulfate (AMS) to the spray solution, which often improves weed control, especially when mixed with hard water. AMS in solution disassociates, and the sulfate binds with cations in the spray solution, preventing the development of glyphosate-cation complexes that tend to have lower absorption into plant leaves. In addition, the ammonium ion can associate with the glyphosate molecule, which helps facilitate glyphosate absorption into the leaf. The recommended AMS rate is 1–2 % w/w, and AMS is available in both dry and liquid formulations. Applicators generally find AMS inconvenient to use because of the high use rate and handling issues. Several companies are marketing low-rate water conditioner products to be used as an alternative to AMS with glyphosate. Pesticide applicators like the convenience of low-rate water conditioners, but performance of these products has been inconsistent. Field experiments were conducted near Manhattan, KS, from 2005 to 2008 to compare the efficacy of glyphosate with AMS and various commercial water conditioners on velvetleaf, sorghum, corn, and sunflower. Each experiment consisted of a sublethal (0.31 or 0.43 kg ae/ha) dose of glyphosate applied in combination with the recommended application rate of each adjuvant. Water hardness, environmental conditions, and plant growth stages varied by experiment. Control of all assay species with glyphosate was enhanced by the addition of AMS, unless control was near complete in the absence of AMS. Commercial water conditioner products that included an AMS component at the equivalent rate of 1 % w/w gave equal or slightly better control than glyphosate plus 1 % w/w AMS. Commercial water conditioners that did not provide an equivalent amount of AMS gave less control than glyphosate with 1 % or 2 % w/w AMS and were often no better than glyphosate alone for the low-rate products.

Influence of Glyphosate-Resistant Horseweed (Conyza canadensis) Growth Stage on Response to Glyphosate Applications

Infestations of glyphosate-resistant (GR) horseweed have become widespread in the eastern United States. This biotype is problematic in no-tillage production that relies extensively on glyphosate for weed control. Because horseweed is treated at various stages of growth, a greenhouse study explored rate response of glyphosate-resistant and -susceptible horseweed at three growth stages. GR horseweed was more responsive to glyphosate at the seedling stage than at the large rosette or bolting stages. A field study evaluated GR horseweed response when treated with glyphosate at soybean planting time, POST in-crop (about 45 d after planting), or both at planting and POST in-crop. There was a cumulative effect of the at-planting followed by POST in-crop glyphosate applications. When evaluating single glyphosate applications, the at-planting application was more effective at suppressing GR horseweed than a POST in-crop application. Because glyphosate cannot control GR horseweed, this biotype should be controlled with an herbicide with an alternate mode of action and applied at the most effective timing.

Influence of Glyphosate and Fungicide Coapplications on Weed Control, Spray Penetration, Soybean Response, and Yield in Glyphosate‐Resistant Soybean

Field experiments were conducted to evaluate the effect of various glyphosate plus fungicide coapplications on weed control, spray penetration, phytotoxicity, and yield in glyphosate‐resistant soybean [Glycine max (L.) Merr.] during 2004 and 2005. Glyphosate was applied alone or in combination with the fungicides chlorothalonil [tetrachloroisophthalonitrile], metconazole [5‐[(4‐chlorophenyl)methyl]‐2,2‐dimethyl‐1‐(1H‐1,24‐triazol‐1‐ylmethyl) cyclopentanol], myclobutanil [2‐(4‐chlorophenyl)‐2‐(1H‐1,2,4‐triazol‐1‐yl methyl) hexanenitrile], propiconazole (1‐[[2‐(2,4‐dichlorophenyl)‐4‐propyl‐1,3‐dioxolan‐2‐yl]methyl]‐1H‐1,2,4‐triazole), pyraclostrobin (carbamic acid, [2,[[[1‐(4‐chlorophenyl)‐1H‐pyrazol‐3‐yl]oxy]methyl]phenyl]methoxy‐, methyl ester), tebuconazole [1‐(4‐chlorophenyl)‐4,4‐dimethyl‐3‐(1,2,4,j‐triazol‐1‐ylmethyl)pentan‐3‐ol]‐tetrafluoroetho, tetraconazole [(1‐[2‐(2,4‐dichlorophenyl)‐3‐(1,1,2,2‐tetrafluoroethoxy)propylj]‐1H‐1,2,4‐triazole)], azoxystrobin (methyl (E)‐2‐{2‐[6‐(2‐cyanophenoxy) pyrimidin‐4‐yloxy]phenyl}‐3‐methoxyacrylate) plus propiconazole, propiconazole plus trifloxystrobin [methyl (E)‐methoxyimino‐{(EE)‐alpha‐[1‐alpha,alpha,alpha‐trifluoro‐m‐tolyl) ethy lideneaminooxy]‐o‐tolyl}acetate], and pyraclostrobin plus boscalid at two application timings recommended for optimum weed control. Across all locations and years, similar levels of visual weed control and weed density at harvest were recorded with all glyphosate plus fungicide combinations compared with glyphosate alone. Little soybean plant injury or leaf area index reduction was observed with any glyphosate and fungicide coapplication when compared with glyphosate alone. Only coapplications of either glyphosate and pyraclostrobin or azoxystrobin plus propiconazole increased yields when compared with glyphosate alone. Greater spray penetration was achieved with early compared with late applications. No more than 10% spray card coverage was achieved at any canopy height with late applications. Although the optimum timing of fungicide application(s) will largely be dependent on the time of disease infection, these results suggest that the timing of fungicide application for control of Asian soybean rust (ASR) will rarely coincide with the optimum timing of herbicides like glyphosate for weed control in soybean.