Issue No. 5, Article 7/April 27, 2007
Update on the Occurrence of Glyphosate-Resistant Weeds
Since the commercialization of glyphosate-resistant crops, the question of whether glyphosate-resistant weeds will or will not be selected has been extensively bantered around by individuals involved in virtually every phase of production agriculture. The first contemporary report of glyphosate resistance in a weed species occurred in Australia, where scientists discovered a biotype of rigid ryegrass (Lolium rigidum) that was not controlled by glyphosate. Shortly after this watershed report of glyphosate resistance, another grass species, a biotype of goosegrass (Eleusine indica) in Malaysia, was reported to be glyphosate-resistant. While these initial instances occurred in grass species outside of the United States, it didn't take long for glyphosate resistance to be discovered in broadleaf species within sovereign borders of this country.
A list of confirmed glyphosate-resistant weeds (as well as cases of resistance to myriad other herbicide families) is maintained by The International Survey of Herbicide Resistant Weeds. According to the organization's Web site (www.weedscience.org), "The International Survey of Herbicide Resistant Weeds is a collaborative effort between weed scientists in over 80 countries. Our main aim is to maintain scientific accuracy in the reporting of herbicide resistant weeds globally. This collaborative effort is supported and funded by the Herbicide Resistance Action Committee, the North American Herbicide Resistance Action Committee, and the Weed Science Society of America."
Table 1 provides an updated list of confirmed glyphosate-resistant weeds. The table includes grass and broadleaf species, weeds with an annual or perennial life cycle, and species that occur in the United States and around the globe. Significant progress has been made toward understanding the mechanisms some of these biotypes use to survive glyphosate. A recent article in the journal Weed Technology provided a very good summary of what is currently known about these mechanisms. Table 2 is produced from that article.
As Illinois farmers enter the 2007 growing season, weed scientists continue to stress several significant points related to glyphosate-resistant weeds:
- A selection pressure for herbicide-resistant weeds occurs each time the same herbicide is applied to a particular field.
- Increased adoption of glyphosate-resistant corn hybrids, with a concomitant use of glyphosate to the exclusion of other weed management tools, will speed the selection of glyphosate-resistant weeds.
- Rotating herbicides (sites of action) or tank-mixing herbicides may help slow the selection of glyphosate-resistant weeds, but it is unlikely to completely prevent their selection. Keep in mind that it's nearly impossible to make blanket statements about how effective a particular alternative herbicide or tank-mix partner will be in slowing the selection of glyphosate-resistant weeds.
- Stewardship of glyphosate herbicide is an easy concept to discuss, but it is often more difficult to implement.