Broad-spectrum Weed Control Research Articles

Prevalence of herbicide‐resistant weed species in Malaysian rice fields: A review

The management of weeds in Malaysian rice fields is very much herbicide‐based. The heavy reliance on herbicide for weed control by many rice‐growers arguably eventually has led to the development and evolution of herbicide‐resistant biotypes in Malaysian rice fields over the years. The continuous use of synthetic auxin (phenoxy group) herbicides and acetohydroxyacid synthase‐inhibiting herbicides to control rice weeds was consequential in leading to the emergence and prevalence of resistant weed biotypes. This review discusses the history and confirmed cases and incidence of herbicide‐resistant weeds in Malaysian rice fields. It also reviews the Clearfield Production System and its impact on the evolution of herbicide resistance among rice weed species and biotypes. This review also emphasizes the strategies and management options for herbicide‐resistant rice field weeds within the framework of herbicide‐based integrated weed management. These include the use of optimum tillage practices, certified clean seeds, increased crop competition through high seeding rates, crop rotation, the application of multiple modes of action of herbicides in annual rotations, tank mixtures and sequential applications to enable a broad spectrum of weed control, increase the selective control of noxious weed species in a field and help to delay the resistance evolution by reducing the selection pressure that is forced on those weed populations by a specific herbicidal mode of action.

Hoes to Herbicides: Economics of Evolving Weed Management in the United States

Over the past century, U.S. field crop farmers have controlled weeds with progressively less costly technologies, moving from hoeing and draft cultivation to motorized cultivation to selective herbicides to broad-spectrum herbicides associated with herbicide-tolerant (HT) crops. The advent of herbicides had the effect of reducing both capital and labor costs by reducing the number of field passes required for effective motorized weed control. The advent of HT crops again reduced both labor and capital costs. These innovations have attracted widespread adoption by farmers. Today, HT crops and broad-spectrum weed control are used by virtually all field crop farmers except those producing for markets that will not accept genetically modified crops. The accelerating spread of herbicide-resistant weeds (an adaptive evolutionary response to extensive reliance on a few herbicides) is triggering U.S. farmers to increase and diversify their herbicide use, increasing both financial costs and selected health and environmental risks.

Sulfentrazone plus a Low Rate of Halosulfuron for Weed Control in White Bean (<i>Phaseolus vulgaris</i> L.)

Halosulfuron was recently registered as the second soil-applied herbicide for broadleaf weed control in Ontario dry beans, but does not provide an alternative mode of action. Sulfentrazone is used to control broadleaf weeds in soybean and other pulse crops, and its registration for Ontario dry beans would provide a different mode of action for broadleaf weed control. Five field studies were conducted over two years (2014, 2015) to determine if the spectrum of broadleaf weed control is improved by adding a half-rate of halosulfuron to sulfentrazone PRE, and to determine the tolerance of white bean to sulfentrazone (140 or 210 g ai ha-1), s-metolachlor (1050 g ai ha-1), and halosulfuron (17.5 g ai ha-1) applied alone and in combination. Crop injury was assessed at 2 and 4 weeks after crop emergence. Weed control was assessed at 4 and 8 weeks after herbicide application (WAA), and weed density and biomass were determined at 8 WAA. Seed moisture and yield were determined at harvest. Halosulfuron added to sulfentrazone improved the control of Ambrosia artemisiifolia and Sinapis arvensis. Sulfentrazone + s-metolachlor + halosulfuron caused up to 23% crop injury. Therefore, this study concludes that sulfentrazone + s-metolachlor + halosulfuron provides broad spectrum weed control, but is too injurious to white bean for registration in Ontario.

Sequential application of pre-and post-emergence herbicides to control mixed weed flora in maize

A field experiment was laid out in randomized block design with ten weed management practices to determine the bioefficacy of PoE herbicides, halosulfuron-methyl 67.5 g/ha and tembotrione 100 g/ha alone or tank mix and sequential application after PE application of alachlor 1000 g/ha, besides two hand weedings and unweeded control. The broadspectrum weed control and the highest grain yield of rainy season maize was obtained with sequntial application of alachlor 1000 g/ha as PE fb PoE application of halosulfuron-methyl 67.5 g/ha + tembotrione 100 g/ha (tank mix). Post-emergence application of halosulfuron-methyl 67.5 g/ha effectively controlled purple nutsedge.

Effect of weed control on growth and productivity of soybean

A field experiment was carried out at research farm of Rajasthan College of Agriculture, Udaipur during Kharif 2014 in randomized block design (RBD) with 14 treatments pre-emergence application of pendimethalin 0.75 kg/ha + hand weeding 30 DAS resulted broad spectrum of weed control besides gives higher grain yield. This weed management method found to be promising to control weeds of soybean crop and would play an important role in areas where labour is too expensive and time is a constraint.

CONVISO® SMART – an innovative approach of weed control in sugar beet

KWS SAAT SE and Bayer CropScience AG are jointly developing and commercializing an innovative system of weed control in sugar beet for the global market under the name of CONVISO SMART. The technology is based on the breeding of sugar beet cultivars that are tolerant to herbicides of the ALS-inhibitor-class with a broad-spectrum weed control. This will give farmers a new opportunity to make sugar beet cultivation easier, more flexible in its timing and more efficient. The use of CONVISO, as new herbicide in sugar beet, will make it possible to control major weeds with low dose rates of product and reduced number of applications in the future. The tolerance is based on a change in the enzyme acetholactate synthase, which is involved in the biosynthesis of essential amino acids. This variation can occur spontaneously during cell division. During the development, sugar beets with this spontaneously changed enzyme were specifically selected and used for further breeding of CONVISO SMART cultivars. As such, these varieties are not a product of genetic modification. Field studies with CONVISO SMART hybrids showed complete crop selectivity and a broad and reliable efficacy against a large range of major weeds. The bio-dossier for an EU-wide registration of CONVISO was submitted in April in 2015. The variety inscription process is in preparation in different countries. The system CONVISO SMART is scheduled to be available to farmers in 2018 at the earliest.

Herbicide combinations for broad spectrum weed control in wheat

Herbicide combinations for broad spectrum weed control in wheat

A new herbicide mixture: bispyribac sodium+metamifop 14% SE for weed control in wet seeded rice

In direct seeded rice (DSR), weeds are the major biotic constraint in improving the productivity of rice. Acute labour shortage and hike in wage rate force the farmers to rely more on herbicides for weed management in DSR. But the continuous use of same herbicide or herbicides with similar mode of action leads to shift in weed flora and resistance development in weeds. One of the ways to overcome these problems is the use of herbicide mixtures. Hence, field experiments were conducted for two seasons kharif 2014 and rabi 2014–15 to assess the bio-efficacy of bispyribac sodium+metamifop, a post-emergence herbicide mixture for broad spectrum weed control in DSR. The experiment was laid out in randomized block design with seven treatments and three replications. The treatments comprised four doses of bispyribac sodium+metamifop viz., 60, 70, 80 and 90 g/ha, bispyribac sodium @ 25 g/ha, hand weeding twice and weedy check.Schoenoplectus juncoides, Cyperus iria, Cyperus difformis, Fimbristylis miliacea, Isachne miliacea, Ludwigia perennis, Limnocharis flava, Sphenoclea zeylanica, Marsilea quadrifolia, Bergia capensis, Commelina diffussa and Monochoria vaginalis were the major weeds in the experimental field. Among the three morphological groups of weeds, sedges was the most predominant one in the experimental site. Results revealed that compared to bispyribac sodium @ 25 g/ha, bispyribac sodium+metamifop @ 70, 80 and 90 g/ha was found more effective in reducing the density and dry weight of weeds. Application of bispyribac sodium+metamifop @ 90 g/ha registered the highest weed control efficiency of 96.62%. Grain yield was also significantly influenced by the weed control treatments. The highest grain yield (8051 kg/ha) and net returns (Rs. 86238/ha) were recorded in bispyribac sodium+metamifop @ 90 g/ha and it was statistically comparable with the lower doses of 80 and 70 g/ha. Bispyribac sodium+metamifop @ 90, 80 and 70 g/ha was significantly superior to hand weeding twice and bispyribac sodium applied @ 25 g/ha in terms of net returns and B: C ratio.

Effective post-emergence herbicides for weed control in rice nurseries

On farm experiments were conducted during Kharif seasons of 2011 and 2012 to study efficacy of postemergence herbicides bispyribac-sodium 25 g/ha for broad-spectrum weed control in rice nursery. Among the different treatments, post-emergence application of bispyribac-sodium 25 g/ha applied at 15 DAS (days after sowing) significantly reduced total weed density with weed control efficiency of 90.9 to 97.6% for Echinochloa crus-galli, Trianthema portulacastrum and Cyperus iria. Among other treatments, pre-emergence application of pretilachlor + safener 0.15 kg/ha also recorded significantly lower weed density as compared to control (unweeded). Although weed control efficiency of this treatment was relatively higher for E. crus-galli (84.7%) but for T. portulacastrum and C. iria, this treatment recorded weed control efficiency of 75.9 and 71.2%, respectively. Farmers practice (increased seed rate 100 g/m2) also recoded significantly lower weed density as compared to control (unweeded) but the weed control efficiency was quite low against E. crus-galli (36.8%) and C. iria (28.1%); however, it was relatively higher for T portulacastrum (66.0%) with mean of 43.7%.

Effect of herbicides on weed management in dry-seeded rice sown under different tillage systems

In India, dry-seeded rice (DSR) production systems are rapidly replacing conventional rice production systems due to various advantages. DSR systems can be managed under zero-till (ZT) conditions or after a preparatory tillage, often referred to as conventional tillage systems (CONT). Although previous reports indicate the contribution of tillage to weed suppression, the effect of one-time preparatory tillage in a DSR system could vary depending on the dominant weeds in the system, vertical seed distribution and the weed seed dynamics. A study was conducted to test the efficacy of ZT and CONT and their interaction with herbicide treatments on the weed population dynamics and rice grain yield in 2010 and 2011. Tillage systems did not affect weed emergence, weed biomass, tiller production and crop yield. However, herbicide treatments varied in their efficacy on individual weeds. Hand-weeding treatments and pendimethalin combined with hand weeding did not effectively control Cyperus rotundus L. and Panicum maximum Jacq. (a perennial grass weed with underground parts). The herbicide combination of metsulfuron and chlorimuron was effective in controlling C. rotundus but not grass weeds. This indicates the need for sequential applications of herbicides for grass weed control or integration of hand weeding to achieve broad-spectrum weed control. Apart from hand weeding (three times), treatment with penoxsulam–cyhalofop and pendimethalin followed by (fb) hand weeding resulted in low weed density, high tiller production and grain yield. The study clearly indicates that tillage does not always lead to weed suppression compared with ZT, and herbicides must be chosen based on the dominant weeds in a system. The results of this study are pertinent as herbicide-resistant weeds are rapidly evolving under continuous herbicide selection pressure, which warrants studies on enhancing productivity through low-input, environmentally friendly and sustainable production technology.

Co-expression of G2-EPSPS and glyphosate acetyltransferase GAT genes conferring high tolerance to glyphosate in soybean.

Glyphosate is a widely used non-selective herbicide with broad spectrum of weed control around the world. At present, most of the commercial glyphosate tolerant soybeans utilize glyphosate tolerant gene CP4-EPSPS or glyphosate acetyltransferase gene GAT separately. In this study, both glyphosate tolerant gene G2-EPSPS and glyphosate degraded gene GAT were co-transferred into soybean and transgenic plants showed high tolerance to glyphosate. Molecular analysis including PCR, Sothern blot, qRT-PCR, and Western blot revealed that target genes have been integrated into genome and expressed effectively at both mRNA and protein levels. Furthermore, the glyphosate tolerance analysis showed that no typical symptom was observed when compared with a glyphosate tolerant line HJ06-698 derived from GR1 transgenic soybean even at fourfold labeled rate of Roundup. Chlorophyll and shikimic acid content analysis of transgenic plant also revealed that these two indexes were not significantly altered after glyphosate application. These results indicated that co-expression of G2-EPSPS and GAT conferred high tolerance to the herbicide glyphosate in soybean. Therefore, combination of tolerant and degraded genes provides a new strategy for developing glyphosate tolerant transgenic crops.

Herbicide and Mulch Interactions: A Review of the Literature and Implications for the Landscape Maintenance Industry

Use of organic mulch is one of the most common methods of weed control in landscape planting beds and provides other benefits, including improved soil characteristics, increased growth of ornamental plants, and enhanced property aesthetics. In the landscape maintenance industry, it is common to apply mulch and herbicides concurrently to landscape beds to provide long-term, broad-spectrum weed control. It is known that herbicides behave differently when applied to different soil types and organic materials; however, research is lacking concerning which herbicides are most effective with different mulch materials in the landscape. Determining the most effective herbicide–mulch combinations could potentially improve weed control, reduce labor costs from hand weeding, and mitigate negative environmental impacts resulting from off-site herbicide movement. The objective of this paper is to review the research that has been conducted pertaining to various mulch–herbicide combinations in the landscape and in other areas of agricultural production while also identifying key knowledge gaps that should be addressed in future research. Review of the literature suggests satisfactory weed control can be achieved with high mulch depths (≥ 7 cm) regardless of herbicide use, and herbicide–mulch interactions become more pronounced as mulch depth decreases. Additionally, future research is needed to determine which herbicides are best suited for different mulch types to improve weed control and potentially reduce environmental impacts, including herbicide leaching and runoff into urban and suburban waterbodies.

Photochemical changes and oxidative damage in the aquatic macrophyte Cymodocea nodosa exposed to paraquat-induced oxidative stress

The non-selective herbicide paraquat (Pq) is being extensively used for broad-spectrum weed control. Through water runoff and due to its high water solubility it contaminates aquatic environments. Thus, the present study was carried out to investigate the photochemical changes and oxidative damage in the aquatic macrophyte Cymodocea nodosa to short- (2h) and long-term (24h) exposure to 2, 20, 200 and 1000μM paraquat (Pq) toxicity by using chlorophyll fluorescence imaging and H2O2 real-time imaging. The effective quantum yield of PSII (ΦPSII) show a tendency to increase at 2μM Pq after 2h exposure, and increased significantly at 20 and 200μM Pq. Τhe maximum oxidative effect on C. nodosa leaves was observed 2h after exposure to 200μM Pq concentration when the highest increases of ΦPSII due to high electron transport rate (ETR) resulted in a significant increase of H2O2 production due to the lowest non-photochemical quenching (NPQ) that was not efficient to serve as a protective mechanism, resulting in photooxidation. Prolonged exposure (24h) to 200μM Pq resulted in a decreased ΦPSII not due to an increase of the photoprotective mechanism NPQ, but due to high quantum yield of non-regulated energy loss in PSII (ΦNO), resulting to the lowest fraction of open PSII reaction centers (qp). This decreased ΦPSII has resulted to less Pq radicals to be formed, with a consequence of a small increase of H2O2 production compared to control C. nodosa leaves, but substantial lower than that of 2h exposure to 200μM Pq. Exposure of C. nodosa leaves to 1000μM Pq toxicity had lower effects on the efficiency of photochemical reactions of photosynthesis under both short- (2h) and long-term (24h) exposure than 200μM Pq. This was evident by an almost unchanged ΦPSII and qp, that remained unchanged even at a longer exposure time (48h), compared to control C. nodosa leaves. Thus, the response of C. nodosa leaves to Pq toxicity fits the “Threshold for Tolerance Model”, with a threshold concentration of 200μM Pq required for initiation of a tolerance mechanism, by increasing H2O2 production for the induction of genes encoding protective processes in response to Pq-induced oxidative stress. Overall, it is concluded that chlorophyll fluorescence imaging constitutes a promising basis for investigating herbicide mode of action in aquatic plants and for detecting their protective mechanisms.

Synthesis and Herbicidal Activity of Triketone-Quinoline Hybrids as Novel 4-Hydroxyphenylpyruvate Dioxygenase Inhibitors.

4-Hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27, HPPD) is one of the most important targets for herbicide discovery. In the search for new HPPD inhibitors with novel scaffolds, triketone-quinoline hybrids were designed and subsequently optimized on the basis of the structure-activity relationship (SAR) studies. Most of the synthesized compounds displayed potent inhibition of Arabidopsis thaliana HPPD (AtHPPD), and some of them exhibited broad-spectrum and promising herbicidal activity at the rate of 150 g ai/ha by postemergence application. Most promisingly, compound III-l, 3-hydroxy-2-(2-methoxy-7-(methylthio)quinoline-3-carbonyl)cyclohex-2-enone (Ki = 0.009 μM, AtHPPD), had broader spectrum of weed control than mesotrione. Furthermore, compound III-l was much safer to maize at the rate of 150 g ai/ha than mesotrione, demonstrating its great potential as herbicide for weed control in maize fields. Therefore, triketone-quinoline hybrids may serve as new lead structures for novel herbicide discovery.

Weed control in dry direct-seeded rice using tank mixtures of herbicides in South Asia

Dry direct-seeded rice (DSR) faces with complex weed problems particularly when farmers missed pre-emergence herbicide applications. Thus, an effective and strategic weed control in DSR is often required with available options of post-emergence herbicides. In such situations, tank mixtures of herbicides may provide broad spectrum weed control in DSR. Field experiments were conducted in the wet seasons of 2013 and 2014 to study weed control in response to tank mixtures of herbicides currently applied in DSR in South Asia. Results revealed that the tank mixtures of the currently available herbicides (azimsulfuron plus bispyribac or fenoxaprop, bispyribac plus fenoxaprop, and azimsulfuron plus bispyribac plus fenoxaprop; all applied as post-emergence) rarely resulted in antagonistic effects. Highest weed control efficiency (∼98%) was recorded with the tank mixture of azimsulfuron plus bispyribac plus fenoxaprop during both the years. This treatment also produced highest grain yield (7.2 t ha−1 in 2013 and 7.9 t ha−1in 2014), which was similar to the grain yield in the plots treated with the tank mix of azimsulfuron plus fenoxaprop, pendimethalin (applied as pre-emergence) followed by (fb) bispyribac, pendimethalin fb fenoxaprop, as well as pendimethalin fb azimsulfuron. Plots treated with the post-emergence application of single herbicide (i.e., azimsulfuron, bispyribac, or fenoxaprop) had lower grain yield (3.0–5.2 t ha−1 in 2013 to 3.5–6.1 t ha−1in 2014) than all the sequential herbicide treatments and tank mixtures (azimsulfuron plus fenoxaprop and azimsulfuron plus bispyribac), owing to a broad spectrum weed control. The study suggested that if farmers missed the pre-emergence application of herbicides (e.g., pendimethalin) due to erratic rains or due to other reasons, good weed control and high yield can still be obtained with tank mix applications of azimsulfuron plus fenoxaprop or azimsulfuron plus bispyribac plus fenoxaprop in DSR.

Biologically effective rate of sulfentrazone applied pre-emergence in soybean

Walsh, K. D., Soltani, N., Hooker, D. C., Nurse, R. E. and Sikkema, P. H. 2015. Biologically effective rate of sulfentrazone applied pre-emergence in soybean. Can. J. Plant Sci. 95: 339–344. Sulfentrazone is a protoporphyrinogen (PPO)-inhibiting herbicide under evaluation for use in soybean in Ontario, Canada. The primary objective of this study was to determine the dose of sulfentrazone applied pre-emergence (PRE) needed to provide 50 and 90% control of redroot pigweed, common ragweed, common lambsquarters and green foxtail. Seven field trials were conducted over a 3-yr period (2007, 2008 and 2009) in southwestern Ontario to evaluate the efficacy of sulfentrazone applied PRE at doses ranging from 26 to 1120 g a.i. ha−1. The doses of sulfentrazone applied PRE to reduce redroot pigweed, common ragweed, common lambsquarters and green foxtail dry weight by 50% were 104, 139, 15 and 65 g a.i. ha−1; doses of 241, 514, 133 and 721 g a.i. ha−1of sulfentrazone were required for 90% reduction in above-ground biomass of those weed species, respectively. Sulfentrazone applied PRE caused soybean injury only at 560 and 1120 g a.i. ha−1, with 6 and 13% soybean injury at 4 wk after herbicide application (WAT), respectively. Weed control provided by sulfentrazone applied PRE at a dose of 600 g a.i. ha−1was sufficient to maintain 90% of the soybean yield compared with the weed-free control. Therefore, PRE application of sulfentrazone has the potential to provide excellent (>90%) control of selected weeds with minimal to no crop injury; however, weed control varied by species, and thus broad spectrum weed control is not feasible using sulfentrazone alone.

Weed control spectrum and turfgrass tolerance to bioherbicide Phoma macrostoma

Phoma macrostoma is a bioherbicide being developed for selective weed control in turfgrass. The previous research with this product is limited to cool-season turfgrass systems with little information available on appropriate application rates or weed control spectrum for weeds common to warm-season turf. Studies were conducted to evaluate application rates and weed control spectrum of P. macrostoma on common dandelion (Taraxacum officinale G.H. Weber ex Wigg.), slender aster (Aster subulatus var. ligulatus Shinners), common mallow (Malva neglecta Wallr.), common purslane (Portulaca oleracea L.), California burclover (Medicago polymorpha L.), and annual sow thistle (Sonchus oleraceus L.). Although most weeds exhibited susceptibility to P. macrostoma, control differed by weed species and study. P. macrostoma had the greatest efficacy on dandelion, but provided little efficacy on common purslane and common mallow. Warm-season turfgrass tolerance was also evaluated across five commonly used warm-season turfgrass species. No turfgrass injury was observed at any rate on any species; however, weed control data suggest that P. macrostoma may not offer effective broad-spectrum weed control similar to common synthetic herbicides, when applied at the rates used in this study.

Synthesis and herbicidal evaluation of triketone-containing quinazoline-2,4-diones.

Exploring novel 4-hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27, HPPD) inhibitors is one of the most promising research directions in herbicide discovery. To discover new triketone herbicides with broad-spectrum weed control as well as excellent crop selectivity, a series of (total 52) novel triketone-containing quinazoline-2,4-dione derivatives were synthesized and further bioevaluated. The greenhouse testing indicated that many of the newly synthesized compounds showed better or excellent herbicidal activity against broadleaf and monocotyledonous weeds at the dosages of 37.5-150 g of active ingredient (ai)/ha. The structure and activity relationship in this study indicated that the triketone-containing quinazoline-2,4-dione motif has possessed great impact on herbicide activity and may be used for further optimization. Among the new compounds, III-b and VI-a-VI-d displayed a broader spectrum of weed control than mesotrione. In addition, the compound III-b also demonstrated comparatively superior crop selectivity to mesotrione, thus possessing great potential for weed control in the field.

Determination of genotoxic effects of Imazethapyr herbicide in Allium cepa root cells by mitotic activity, chromosome aberration, and comet assay

Imazethapyr (IM) is an imidazolinone herbicide that is currently used for broad-spectrum weed control in soybean and other legume crops. In this study, cytotoxic and genotoxic effects of IM were investigated by using mitotic index (MI), mitotic phases, chromosomal abnormalities (CAs) and DNA damage on the root meristem cells of Allium cepa. In Allium root growth inhibition test, EC50 value was determined as 20 ppm, and 0.5xEC50, EC50 and 2xEC50 concentrations of IM herbicide were introduced to onion tuber roots. Distilled water and methyl methane sulfonate (MMS, 10 mg/L) were used as a negative and positive control, respectively. As A. cepa cell cycle is 24 hours, so, application process was carried out for 24, 48, 72 and 96 hours. All the applied doses decreased MIs compared to control group and these declines were found to be statistically meaningful. Analysis of the chromosomes showed that 10 ppm IM except for 48 h induced CAs but 40 ppm IM except for 72 h decreased CAs. DNA damage was found significantly higher in 20 and 40 ppm of IM compared to the control in comet assay. These results indicated that IM herbicide exhibits cytotoxic activity but not genotoxic activity (except 10 ppm) and induced DNA damage in a dose dependent manner in A. cepa root meristematic cells.

Environmental Impact of Glyphosate-Resistant Weeds in Canada

Glyphosate-resistant (GR) giant ragweed, horseweed, and common ragweed were confirmed in southwestern Ontario, Canada in 2008, 2010, and 2011, respectively. In the western prairie provinces of Alberta and Saskatchewan, GR (plus acetolactate synthase inhibitor-resistant) kochia was discovered in 2011. This symposium paper estimates the environmental impact (EI) of the top herbicide treatments or programs used to manage these GR weed species in the major field crops grown in each region. For each herbicide treatment, EI (per ha basis) was calculated as the environmental impact quotient (EIQ), which quantifies the relative potential risk of pesticide active ingredients on human and ecological health based on risk components to farm workers, consumers, and the environment, multiplied by the application rate (kg ai ha−1). Total EI is defined as EI (per ha basis) multiplied by the application area (i.e., land area affected by a GR weed). It was assumed that all herbicide treatments would supplement the continued usage of glyphosate because of its broad spectrum weed control. For the control of these GR weeds, most treatments contain auxinic or protoporphyrinogen oxidase (PPO)-inhibiting herbicides. The majority of auxinic herbicide treatments result in low (EI ≤ 10) to moderate (11 to 20) EI, whereas all treatments of PPO inhibitors have low EI. Total EI of GR horseweed and kochia will generally be greater than that of giant or common ragweed because of rapid seed dispersal. For recommended herbicide treatments to control GR weeds (and herbicide-resistant weeds in general), EI data should be routinely included with cost and site of action in weed control extension publications and software, so that growers have the information needed to assess the EI of their actions.