THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE
Dear Friends and Colleagues
Glyphosate-Resistant Weeds A Growing Major Problem Worldwide
Glyphosate-resistant soybean was introduced in 1996, the first of several (genetically modified) glyphosate-resistant crops, which allowed the herbicide, glyphosate, to be used as a selective post-emergence herbicide. Glyphosate’s widespread use has resulted in a situation where glyphosate-resistant weed species are evolving at a steady pace and the more virulent existing glyphosate-resistant weeds are spreading rapidly.
A recent review (Item 1) gives us an overview of the incidence of glyphosate-resistant weeds in the world. Thirty-eight weed species have now evolved resistance to glyphosate, distributed across 37 countries and in 34 different crops and six non-crop situations. Glyphosate-resistant weeds in glyphosate-resistant crop systems account for more than 90% of the area infested and the economic damage caused by glyphosate-resistant weeds globally. The USA, Argentina, Brazil, and Canada rapidly adopted GM glyphosate-resistant crops, and these four countries are in the top five countries with the most glyphosate-resistant weed species and have the greatest area infested with glyphosate-resistant weeds.
This problem is exacerbated for farmers because alternative herbicide choices are decreasing, largely because most of these weeds already have resistance traits for older herbicide chemistries and are evolving new resistances. So, it is the combination of multiple herbicide resistance (glyphosate plus others), along with the lack of new herbicide sites of action, which has left modern agriculture with a looming weed management crisis. The researchers expect this problem will increase in importance over the next 30 years, with multiple resistance expanding in broadleaf weeds. Industry has responded by developing herbicide resistance traits in major crops that allow existing herbicides to be used in a new way. However, the researchers warn that over reliance on these traits will result in multiple-resistance in weeds.
Another study (Item 2) focuses on glyphosate-resistant weeds in Australia where high adoption rates of glyphosate-tolerant cotton over the last 14 years have led to an overreliance on glyphosate in weed management and decreased the use of other herbicide options and non-chemical weed-management strategies, possibly leading to the emergence of many resistant weeds. This study was carried out to identify the common weed flora in cotton-growing regions and found a shift in weed dominance in Australian cotton production compared with previous surveys carried out in the region. Most importantly, volunteer cotton has become more prevalent in cotton-production regions, found in 85% of the fields surveyed, compared with 31% of fields in a 2011 survey. Four prevalent weed species are particularly problematic because they have evolved resistance to glyphosate. The changing weed-species patterns identified in this survey demonstrate a need for changes to weed-management practices in cotton to deal with the changing weed flora.
With best wishes,
Third World Network
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Item 1
OVERVIEW OF GLYPHOSATE-RESISTANT WEEDS WORLDWIDE
Heap, I. and Duke, S.O.
Pest Manag Sci. 2017 Oct 10. doi: 10.1002/ps.4760.
https://www.ncbi.nlm.nih.gov/pubmed/29024306
10 Oct 2017
Abstract
Glyphosate is the most widely used and successful herbicide discovered to date, but its utility is now threatened by the occurrence of several glyphosate-resistant weed species. Glyphosate resistance first appeared in Lolium rigidum in an apple orchard in Australia in 1996, ironically the year that the first glyphosate-resistant crop (soybean) was introduced in the USA. Thirty-eight weed species have now evolved resistance to glyphosate, distributed across 37 countries and in 34 different crops and six non-crop situations. Although glyphosate-resistant weeds have been identified in orchards, vineyards, plantations, cereals, fallow and non-crop situations, it is the glyphosate-resistant weeds in glyphosate-resistant crop systems that dominate the area infested and growing economic impact. Glyphosate-resistant weeds present the greatest threat to sustained weed control in major agronomic crops because this herbicide is used to control weeds with resistance to herbicides with other sites of action, and no new herbicide sites of action have been introduced for over 30 years. Industry has responded by developing herbicide resistance traits in major crops that allow existing herbicides to be used in a new way. However, over reliance on these traits will result in multiple-resistance in weeds. Weed control in major crops is at a precarious point, where we must maintain the utility of the herbicides we have until we can transition to new weed management technologies.
Item 2
AN ASSESSMENT OF WEED FLORA 14 YEARS AFTER THE INTRODUCTION OF GLYPHOSATE-TOLERANT COTTON IN AUSTRALIA
Manalil, S., Werth, J., Jackson, R., Chauhan, B. S., & Preston, C. (2017).
Crop and Pasture Science, 68(8), 773-780.
http://www.bioone.org/doi/abs/10.1071/CP17116
11 Oct 2017
Abstract
Glyphosate-tolerant (GT) cotton offers a multitude of benefits such as broad-spectrum and cost-effective weed control, simple weed management, and reduced impact on the environment. However, high adoption rates of GT cotton have led to overreliance on glyphosate in weed management and have decreased the use of other herbicide options and non-chemical weed-management strategies, possibly leading to the emergence of many resistant weeds. Previous surveys in 2006 and 2011 in the cotton-growing regions of New South Wales (NSW) and Queensland, Australia, indicated changes in weed populations over the period and increased prevalence of several weeds. These two surveys indicated increased dominance of Conyza bonariensis, Echinochloa colona, and Chloris virgata in these regions. Periodic weed surveys are necessary to assess weed population dynamics and shifts due to overreliance on glyphosate for weed management. A survey was carried out in the cotton-growing regions of NSW and Queensland in 2014–15, covering 135 fields. Survey results indicated the emergence of volunteer GT cotton as the most common weed present across all of the cotton-growing regions, occurring in 85% of fields, followed by E. colona (67% of fields surveyed), and C. bonariensis and Sonchus oleraceus, which were present in 51% of fields. The most prevalent grass weed after E. colona was C. virgata (37%). Broadleaf weeds Ipomoea lonchophylla and Amaranthus mitchellii were present in 40% and 37% of fields, respectively. Regional-level analysis indicated greater prevalence of Sesbania cannabina and Parthenium hysterophorus in Emerald region of Queensland. Lolium rigidum was present in the Griffith and Warren area of NSW during summer, even though it is a winter weed. The results of this study indicate integration of diversified weed-management options and inclusion of both non-chemical and chemical options because many major weeds observed in this study are tolerant to glyphosate and have already evolved resistance to glyphosate.