Warning - Some Weeds Resisting Glyphosate Herbicides

A narrow-leaved plantain (Plantago lanceolata) that is resistant to the herbicide glyphosate was discovered in South African orchards and vineyards in 2003. This follows the discovery in 2002 of glyphosate resistant Lolium multiflorum in Chile. This brings to six the number of weed species that have developed resistance to this popular herbicide.

Research in 2000 at the University of Delaware determined that horseweed, a.k.a. Canada fleabane (Conyza canadensis) had developed resistance to glyphosate herbicide in some east coast farm fields. By 2003, resistant Canada fleabane was known from 9 U.S. states. In the late 1990’s, glyphosate resistant populations of ryegrass (Lolium rigidum) were found in Australia orchards following 10 to 15 years of annual exposure. Subsequent populations were found in California and resistant goosegrass (Eleusine indica) populations were found in Malaysia. Horseweed is common throughout many agricultural areas of B.C.

Most herbicide-resistant weed populations develop following repeated use of the same or related herbicide group for a number of years on the same field. Weed populations contain a very small number of individual plants that are naturally able to withstand a particular herbicide (or herbicide group) that is registered to control the weed. This naturally occurring resistance is not due to weather conditions or application technique but is an inherent characteristic of the genetic makeup of the resistant plants.

Because of their low frequency of occurrence, these resistant plants go undetected. The continual use of the same herbicide (or herbicide group) year after year allows them to set seed and multiply. Removal of the susceptible members of the population allows more opportunity for the resistant ones to flourish.

Managing Resistance

• Rotate herbicides. Classification tables are available that list herbicides according to the mechanism by which they kill weeds. Rotate between herbicides with a different mechanism of action (ie. if you find a herbicide that works well for you, don’t stick with it!)
• Rotate crops, if practical. This usually results in using a diversity of herbicides.
• Use tank mixes that control target weeds by different mechanisms of action.
• Limit use of herbicides that remain active in the soil for long periods.
• Integrate herbicide use with other weed control strategies.

There are no reported cases of weed resistance to glyphosate herbicides in British Columbia. Triazine group resistance is known to occur however in populations of redroot pigweed, lamb’s-quarters, groundsel and barnyardgrass. Included in the triazine group are popular herbicides such as atrazine, simazine and metribuzin.

For more information, please contact, BCMAFF, Victoria or Kamloops.

Dec 2003

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