More than 15 years after their first successful commercial introduction in the United States, genetically engineered (GE) seeds have been widely adopted by U.S. corn, soybean, and cotton farmers. Still, some questions persist regarding the potential benefits and risks of GE crops. The report finds that, although the pace of research and development (measured by the number of USDA-approved field tests) peaked in 2002, other measures show that biotech firms continue to develop new GE seed varieties at a rapid pace. Also, U.S. farmers continue to adopt GE seeds at a robust rate, and seed varieties with multiple (stacked) traits have increased at a very rapid rate. Insecticide use has decreased with the adoption of insect-resistant crops, and herbicide-tolerant crops have enabled the substitution of glyphosate for more toxic and persistent herbicides. However, overreliance on glyphosate and a reduction in the diversity of weed management practices have contributed to the evolution of glyphosate resistance in some weed species.

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Genetically Engineered Crops in the United States

Herbicide-resistant weeds have been observed since the early years of synthetic herbicide development in the 1950s and 1960s. Since that time there has been a consistent increase in the number of cases of herbicide resistance and the impact of herbicide-resistant weeds. Although the nature of crop production varies widely around the world, herbicides have become a primary tool for weed control in most areas. Dependence on herbicides continues to increase as global populations migrate away from rural areas to cities and the agricultural labor force declines. This increased use of herbicides and the concurrent selection pressure have resulted in a rise in cases of multiple resistance, leaving some farmers with few or no herbicide options for certain weed infestations. Global population and economic forces drive many farmer choices regarding crop production and weed control. The challenge is how to insert best management practices into the decision-making process while addressing various economic and regulatory needs. This review endeavors to provide a current overview of herbicide resistance challenges in the major crop production areas of the world and discusses some research initiatives designed to address portions of the problem. © 2017 Society of Chemical Industry.

The challenge of herbicide resistance around the world: a current summary

Weeds are a nuisance in a variety of land uses. The increasing prevalence of both herbicide resistant weeds and bans on cosmetic pesticide use has created a strong impetus to develop novel strategies for controlling weeds. The application of bacteria, fungi and viruses to achieving this goal has received increasingly great attention over the last three decades. Proposed benefits to this strategy include reduced environmental impact, increased target specificity, reduced development costs compared to conventional herbicides and the identification of novel herbicidal mechanisms. This review focuses on examples from North America. Among fungi, the prominent genera to receive attention as bioherbicide candidates include Colletotrichum, Phoma, and Sclerotinia. Among bacteria, Xanthomonas and Pseudomonas share this distinction. The available reports on the application of viruses to controlling weeds are also reviewed. Focus is given to the phytotoxic mechanisms associated with bioherbicide candidates. Achieving consistent suppression of weeds in field conditions is a common challenge to this control strategy, as the efficacy of a bioherbicide candidate is generally more sensitive to environmental variation than a conventional herbicide. Common themes and lessons emerging from the available literature in regard to this challenge are presented. Additionally, future directions for this crop protection strategy are suggested.

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Controlling weeds with fungi, bacteria and viruses: a review.

Arundo donax L.: a non-food crop for bioenergy and bio-compound production.

https://bsppjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3059.1999.0378b.x

Fundamentals of Weed Science

Glyphosate was compared with other commonly used corn herbicides at three locations in Kentucky in 1998 and 1999. Sequential glyphosate treatments provided greater than 87% control of common cocklebur, ivyleaf morningglory, common lambsquarters, and giant ragweed. Control of these species with glyphosate treatments was similar to the control with atrazine plus other postemergence (POST) herbicides. Generally, treatments containing s-triazines and chloracetamides applied to the soil surface were not as effective as sequential glyphosate or atrazine plus POST herbicides. Corn yield was not significantly different at any location or for any year, whereas differences in net return occurred at one location in 1998. Nomenclature: Atrazine; chloracetamides; glyphosate; s-triazines; common cocklebur, Xanthium strumarium L. #3 XANST; common lambsquarters, Chenopodium album L. # CHEAL; giant ragweed, Ambrosia trifida L. # AMBTR; ivyleaf morningglory, Ipomoea hederacea L. Jacq. # IPOHE; corn, Zea mays L. ‘...

Comparison of Glyphosate with Other Herbicides for Weed Control in Corn (Zea mays): Efficacy and Economics1

Flumioxazin adsorption kinetics were described using a Greenville sandy clay loam soil. Adsorption kinetics experiments showed that 72% of total herbicide was absorbed after 1 h of continuous shaking and continued to increase to 78% after 72 h. Flumioxazin adsorption was then tested on seven agriculturally important soils throughout the southern USA. Adsorption isotherms for all soils had K(f) (Freundlich distribution coefficient) values that ranged from 8.8 to 0.4, with many near 1.5. Soil organic matter content was the parameter most highly correlated with flumioxazin adsorption (r(2) = 0.95, P < 0.001). Sorption to clay minerals had K(f) values ranging from 50 for bentonite to 4.7 for kaolinite. However, normalizing K(f) for sorbent surface area revealed that aluminum hydroxide (gibbsite) possessed the greatest flumioxazin sorption per unit area. Sorption to anionic exchange resin (K(f) 676) was greater than cationic exchange resin (K(f) 42). Molecular model calculations were performed to elucidate why sorption was greater to anionic exchangers. These calculations indicated that a region of dense electronegativity exists on the 3-dione moiety of the molecule. This would lead to greater flumioxazin sorption by positively charged surface sites. Desorption isotherms from soil exhibited no effect of hysteresis. Desorption from clay minerals was very rapid and flumioxazin in solution was undetectable after three desorption steps. From these data it was concluded that flumioxazin can become readily available in soil solution with increase in soil water content.

Sorption and desorption of flumioxazin to soil, clay minerals and ion‐exchange resin

Palmer amaranth's ability to evolve resistance to different herbicides has been studied extensively, but there is little information about how this weed species might be evolving other life-history traits that could potentially make it more aggressive and difficult to control We characterized growth and morphological variation among 10 Palmer amaranth populations collected in Florida and Georgia from fields with different cropping histories, ranging from continuous short-statured crops (vegetables and peanut) to tall crops (corn and cotton) and from intensive herbicide use history to organic production Palmer amaranth populations differed in multiple traits such as fresh and dry weight, days to flowering, plant height, and leaf and canopy shape Differences between populations for these traits ranged from 36% up to 87% Although glyphosate-resistant (GR) populations collected from cropping systems including GR crops exhibited higher values of the aforementioned variables than glyphosate-susceptible (GS)

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Differentiation of Life-History Traits among Palmer Amaranth Populations (Amaranthus palmeri) and Its Relation to Cropping Systems and Glyphosate Sensitivity

Palmer amaranth is a troublesome weed in cotton production. Yield losses of 65% have been reported from season-long Palmer amaranth competition with cotton. To determine whether water is a factor in this system, experiments were conduced in 2011, 2012, and 2013 in Citra, FL, and in Tifton, GA. In 2011, infrequent rainfall lead to drought stress. The presence of Palmer amaranth resulted in decreased soil relative water content up to 1 m in depth. Cotton stomatal conductance (gs) was reduced up to 1.8 m from a Palmer amaranth plant. In 2012 and 2013 higher than average rainfall resulted in excess water throughout the growing season. In this situation, no differences were found in soil relative water content or cotton gs as a function of proximity to Palmer amaranth. A positive linear trend was found in cotton photosynthesis and yield; each parameter increased as distance from Palmer amaranth increased. Even in these well-watered conditions, daily water use of Palmer amaranth was considerably higher...

https://bioone.org/journals/Weed-Science/volume-63/issue-4/WS-D-15-00062.1/Palmer-Amaranth-Amaranthus-palmeri-Competition-for-Water-in-Cotton/10.1614/WS-D-15-00062.1.pdf

Palmer Amaranth (Amaranthus palmeri) Competition for Water in Cotton

Experiments were conducted between 2002 and 2004 at multiple locations in Florida to determine the efficacy of aminopyralid and other herbicides on tropical soda apple (TSA) control. Aminopyralid applied at rates ≥0.08 kg ai/ha consistently provided >96% TSA control up to 335 d after treatment (DAT), while applications <0.06 kg/ha were less effective as well as inconsistent. Control of TSA with aminopyralid was often not statistically different from control with triclopyr, picloram, or dicamba. However, these responses were likely due to the variability in TSA control by triclopyr, picloram, or dicamba across several locations. For example, the standard error of the mean for TSA control with picloram at 335 DAT was 8, compared to 1 for aminopyralid. Herbicides were applied in April, January, and June, but time of year did not affect the efficacy of aminopyralid. Aminopyralid possesses soil residual activity and controlled 98% of germinating seedlings at 75 DAT, compared to 0% control for triclopyr or 2,4-...

Jason A. Ferrell

Papers

Comparison of Glyphosate with Other Herbicides for Weed Control in Corn (Zea mays): Efficacy and Economics1

Sorption and desorption of flumioxazin to soil, clay minerals and ion‐exchange resin

Differentiation of Life-History Traits among Palmer Amaranth Populations (Amaranthus palmeri) and Its Relation to Cropping Systems and Glyphosate Sensitivity

Palmer Amaranth (Amaranthus palmeri) Competition for Water in Cotton

Control of tropical soda apple (Solanum viarum) with aminopyralid.