This review focuses on proactive and reactive management of glyphosate-resistant (GR) weeds. Glyphosate resistance in weeds has evolved under recurrent glyphosate usage, with little or no diversity in weed management practices. The main herbicide strategy for proactively or reactively managing GR weeds is to supplement glyphosate with herbicides of alternative modes of action and with soil-residual activity. These herbicides can be applied in sequences or mixtures. Proactive or reactive GR weed management can be aided by crop cultivars with alternative single or stacked herbicide-resistance traits, which will become increasingly available to growers in the future. Many growers with GR weeds continue to use glyphosate because of its economical broad-spectrum weed control. Government farm policies, pesticide regulatory policies and industry actions should encourage growers to adopt a more proactive approach to GR weed management by providing the best information and training on management practices, information on the benefits of proactive management and voluntary incentives, as appropriate. Results from recent surveys in the United States indicate that such a change in grower attitudes may be occurring because of enhanced awareness of the benefits of proactive management and the relative cost of the reactive management of GR weeds. Copyright © 2011 Society of Chemical Industry

Herbicide-resistant weed management: focus on glyphosate.

Glyphosate resistance was first discovered in populations of rigid ryegrass in Australia in 1996. Since then, glyphosate resistance has been detected in additional populations of rigid ryegrass and Italian ryegrass in several other countries. Glyphosate-resistant rigid ryegrass and Italian ryegrass have been selected in situations where there is an overreliance on glyphosate to the exclusion of other weed control tactics. Two major mechanisms of glyphosate resistance have been discovered in these two species: a change in the pattern of glyphosate translocation such that glyphosate accumulates in the leaf tips of resistant plants instead of in the shoot meristem; and amino acid substitutions at Pro 106 within the target site, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). There are also populations with both mechanisms. In the case of glyphosate resistance, the target site mutations tend to provide a lower level of resistance than does the altered translocation mechanism. Each of these resis...

https://bioone.org/journals/weed-science/volume-57/issue-4/WS-08-181.1/A-Decade-of-Glyphosate-Resistant-Lolium-around-the-World/10.1614/WS-08-181.1.pdf

A Decade of Glyphosate-Resistant Lolium around the World: Mechanisms, Genes, Fitness, and Agronomic Management

Field studies were conducted in 2005 and 2006 to determine the most effective chemical options within three individual herbicide-based burndown programs, glyphosate, paraquat and glufosinate, for controlling glyphosate-resistant horseweed in Mississippi. Burndown treatments were applied April 5, 2005 and March 15, 2006 to horseweed plants 15 to 30 cm in height. Glyphosate at 0.86 kg ae/ha alone provided 60 to 65% horseweed control 4 wk after treatment (WAT). Control 4 WAT ranged from 73 to 74% when the glyphosate rate was increased to 1.25 kg/ha. Glyphosate at 0.86 kg/ha applied in combination with 2,4-D at 0.84 kg ae/ha or dicamba at 0.28 ae/ha maximized control of horseweed (≥ 90%) 4 WAT and soybean yield. Horseweed control 4 WAT with paraquat alone at 0.84 kg ai/ha ranged from 55 to 63% and control did not improve by increasing the rate to 0.98 kg/ha. Addition of 2,4-D or dicamba to paraquat maximized horseweed control both years (78 to 89%), whereas soybean yield was maximized with addition of dicamba...

https://www.jstor.org/stable/pdfplus/25194985.pdf

Glyphosate-resistant Horseweed (Conyza canadensis) Control Using Glyphosate-, Paraquat-, and Glufosinate-Based Herbicide Programs

Weed management is one of the prime concerns for sustainable crop production Conyza bonariensis and Conyza canadensis are two of the most problematic, noxious, invasive and widespread weeds in modern-day agriculture The biology, ecology and interference of C bonariensis and C canadensis have been reviewed here to highlight pragmatic management options Both these species share a unique set of biological features, which enables them to invade and adapt a wide range of environmental conditions Distinct reproductive biology and an efficient seed dispersal mechanism help these species to spread rapidly Ability to interfere strongly and to host crop pests makes these two species worst weeds of cropping systems These weed species cause 28–68 % yield loss in important field crops such as soybean and cotton every year These weeds are more prevalent in no-till systems and, thus, becoming a major issue in conservation agriculture Cultural practices such as crop rotations, seed rate manipulation, mulching, inter-row tillage and narrow row spacing may provide an effective control of these species However, such methods are not feasible and applicable under all types of conditions Different herbicides also provide a varying degree of control depending on crop, agronomic practices, herbicide dose, application time and season However, both these species have evolved resistance against multiple herbicides, including glyphosate and paraquat The use of alternative herbicides and integrated management strategies may provide better control of herbicide-resistant C bonariensis and C canadensis Management plans based on the eco-biological interactions of these species may prove sustainable in the future

Biology and management of two important Conyza weeds: a global review.

Plants are exposed to increasingly severe drought events and roots play vital roles in maintaining plant survival, growth, and reproduction. A large body of literature has investigated the adaptive responses of root traits in various plants to water stress and these studies have been reviewed in certain groups of plant species at a certain scale. Nevertheless, these responses have not been synthesized at multiple levels. This paper screened over 2000 literatures for studies of typical root traits including root growth angle, root depth, root length, root diameter, root dry weight, root-to-shoot ratio, root hair length and density and integrates their drought responses at genetic and morphological scales. The genes, quantitative trait loci (QTLs) and hormones that are involved in the regulation of drought response of the root traits were summarized. We then statistically analyzed the drought responses of root traits and discussed the underlying mechanisms. Moreover, we highlighted the drought response of 1-D and 2-D root length density (RLD) distribution in the soil profile. This paper will provide a framework for an integrated understanding of root adaptive responses to water deficit at multiple scales and such insights may provide a basis for selection and breeding of drought tolerant crop lines.

/pdf/responses-of-root-system-architecture-to-water-stress-at-bub9663e.pdf

Responses of root system architecture to water stress at multiple levels: A meta-analysis of trials under controlled conditions

Prickly lettuce is an annual weed that germinates in both the fall and the spring. It is often found in no-till soybeans and winter wheat in Ontario, Canada, as well as along the edges of fields. Field studies were conducted from 2001 to 2004 to estimate crop yield losses, and to characterize the phenology and seed production of prickly lettuce in relation to time of emergence. Prickly lettuce had a large impact on soybean yield, with losses of 60 to 80% at densities of 50 plants m−2 or more. Prickly lettuce density estimated to cause a 10% soybean yield loss varied from 0.2 plants m−2 in 2002 to 1.2 plants m−2 in 2003 and 2004. In winter wheat, prickly lettuce at densities up to 200 plants m−2 caused no detectable yield loss in this study. Plants that emerged in the fall generally were larger, flowered earlier. and produced more seeds than those emerging in spring, but size and fecundity were strongly density-dependent. The number of flowers produced per plant could be estimated from the height ...

Prickly lettuce (Lactuca serriola) interference and seed production in soybeans and winter wheat

Root traits and growth of spreading orach and common lambsquarters were compared in response to soil compaction, drought, and waterlogging under controlled environment conditions. On the basis of the typical habitats occupied, the hypothesis was that spreading orach would be more tolerant of compaction and waterlogging and common lambsquarters more tolerant of drought. When grown in buckets with two soil bulk densities (1.2 and 1.6 g cm−3) for 8 wk, the two species responded similarly to compaction, with the fraction of fine roots reduced by 10%, total root length by 70%, root and shoot dry weight and leaf area by 50 to 60%, and plant height by 30% at the high compared with the low bulk density. When grown for 6 wk in soil columns 1 m long, which were watered daily or allowed to dry, common lambsquarters was deeper rooted than spreading orach at both moisture levels and better able to sustain growth in the drying columns. The watering regime did not alter the rooting depth of either species. Tota...

https://bioone.org/journals/Weed-Science/volume-53/issue-1/WS-04-143R/Responses-of-spreading-orach-Atriplex-patula-and-common-lambsquarters-Chenopodium/10.1614/WS-04-143R.pdf

Responses of spreading orach (Atriplex patula) and common lambsquarters (Chenopodium album) to soil compaction, drought, and waterlogging

Horseweed is a winter annual weed that has evolved resistance to multiple herbicide modes of action in 11 countries worldwide. A paraquat-resistant horseweed population in an Ontario orchard that was being managed by a rotation of herbicides began to show increased tolerance to the herbicide linuron. Experiments were conducted in the greenhouse to compare the response of this population and several paraquat-susceptible populations to linuron, diquat, and oxyfluorfen. Plants were sprayed with a range of doses of each herbicide when they were from 5 to 10 wk old, and the ED50, or dose at which shoot dry weight was reduced by 50%, was estimated. There was a sevenfold difference in the ED50 values of the paraquat-resistant and -susceptible populations in response to diquat and a threefold difference in response to linuron. The response to oxyfluorfen was age dependent. The ratio of resistance to susceptible ED50 values was estimated as 57 for 5-wk-old plants and 11 for 8-wk-old plants in response to ...

Response of paraquat-resistant and -susceptible horseweed (Conyza canadensis) to diquat, linuron, and oxyfluorfen

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Prickly lettuce (Lactuca serriola) interference and seed production in soybeans and winter wheat

Responses of spreading orach (Atriplex patula) and common lambsquarters (Chenopodium album) to soil compaction, drought, and waterlogging

Response of paraquat-resistant and -susceptible horseweed (Conyza canadensis) to diquat, linuron, and oxyfluorfen