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Horseweed (Conyza canadensis (L.) Cronq.) is "a summer/winter annual broadleaf weed species that is native to northern North America."[1] In many documented cases, horseweed has evolved resistance to the herbicide glyphosate (Roundup), particularly in fields of genetically engineered Roundup Ready Crops.

According to a 2005 account:[1]

"Widespread evolution of glyphosate resistance in horseweed populations could be attributed to its ability to germinate and establish in undisturbed soils, to colonize no-till fields, to adapt to varying habitats (crop and noncrop lands), together with the popularity of reduced or no-till tillage practices, its vast seed production potential and wind dispersal of seed (Koger et al., 2004; VanGessel, 2001).
"All documented reports of GR horseweed have come from the USA. Monoculture of GR [Glyphosate Resistant] soybean has resulted in less than acceptable control of horseweed populations within 3 years of using only glyphosate for weed control in Delaware (VanGessel, 2001). Greenhouse screenings of one such population from Delaware revealed 8- to 13-fold glyphosate resistance compared to a susceptible population, representing the first documented evidence of evolved glyphosate resistance in a GR crop, as well as the first broadleaf weed to evolve glyphosate resistance. Koger et al. (2004) documented horseweed biotypes from a field planted to GR cotton for three consecutive years (2000–2002) and a field planted to GR soybean (2000) followed by GR cotton (2001 and 2002). These biotypes exhibited an 8- to 12-fold level of resistance to glyphosate compared to a susceptible biotype in Mississippi (Figure 2). Obviously, rotation of GR crops (cotton and soybean) is not a prudent option to prevent evolution resistance in horseweed.
"Feng et al. (2004) found reduced movement of 14C-glyphosate out of a treated single leaf and reduced translocation to other leaves, roots, and crown in the Delaware resistant biotype compared to corresponding plant parts of a susceptible biotype. Similarly, the level of 14C-glyphosate that translocated out of the treated leaf was lower in the resistant biotypes compared with susceptible biotypes. The difference ranged from 28 to 48% between respective resistant and susceptible biotypes from different states in a study comparing absorption and translocation pattern of 14C-glyphosate in resistant and susceptible horseweed biotypes from Mississippi, Arkansas, Delaware, and Tennessee (Koger & Reddy, 2005). All resistant horseweed biotypes were 8- to 12-fold more resistant to glyphosate than a susceptible biotype (Koger et al., 2004). The above results strongly suggest that resistance in horseweed biotypes is due to reduced translocation of glyphosate to growing parts of the plant."

Articles and resources

Related SourceWatch articles


  1. 1.0 1.1 Vijay K. Nandula, Krishna N. Reddy, Stephen O. Duke, and Daniel H. Poston, "Glyphosate-Resistant Weeds: Current Status and Future Outlook," USDA ARS, August 2005.

External resources

  • Koger, C. H., and Reddy, K. N. (2005) Role of absorption and translocation in the mechanism of glyphosate resistance in horseweed (Conyza canadensis). Weed Science 53, 84–9.

External articles

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