17.5 Herbicide Resistance to Growth Regulators

Biotechnology Adding Resistance to Crops

Recently, new crop variety and hybrid products have become available on the market which provide resistance to 2,4-D and Dicamba.

• Enlist corn has tolerance to glyphosate and a new formulation of 2,4-D which has lower drift risks. (Dow AgroSciences)

• Enlist E3™soybeans have tolerance to 2,4-D, glyphosate and glufosinate herbicides. Dow’s new formulation of 2,4-D has lower drift risks. (Developed by Dow AgroSciences and MS Technologies.)
  • o For more information on the science behind these crop products go to: https://www.extension.purdue.edu/extmedia/id/id-453-w.pdf

• Roundup Ready Plus Xtend soybeans have tolerance to glyphosate and dicamba. (Monsanto, BASF, DuPont) However, it is very important to use a certain formulation of dicamba to reduce the risk of volatility and drift injury, as well as be in legal compliance of using the product.
  • o https://extension.psu.edu/which-dicamba-products-can-be-sprayed-burndown-in-xtend-soybeans

Weeds with Resistance

A website has been established and is maintained by a consortium of public and private weed researchers to document and catalog the latest information on herbicide resistant weeds world wide. The International Survey of Herbicide Resistant Weeds is a collaborative effort between weed scientists in over 80 countries and is located at www.weedscience.org .  The consortium’s main aim is to maintain scientific accuracy in the reporting of herbicide resistant weeds globally.  Government, academic, and industry weed scientists worldwide support this collaborative effort.  This resource is funded by the Global Herbicide Resistance Action Committee and CropLife International.

• Ian Heap.  Director of the International Survey of Herbicide-Resistant Weeds.
  IanHeap@weedscience.org
• Mark Peterson. Chairman of the Herbicide Resistance Action Committee.
  mapeterson@dow.com

• Michael Horak. Chairman of the North American Herbicide Resistance Action Committee.  michael.j.horak@monsanto.com

At the time this chapter was written, there were 38 different weed species with documented resistance to the synthetic auxins (Group 4/HRAC Group O).  This would include 2,4-D.  Of the 38 resistant species, eight are in the US.  Two examples are tall waterhemp in Nebraska (2009) and Illinois (2016) and wild carrot in Ohio (1994) and Michigan (1993).  The mechanisms of resistance in weed populations were not known or not yet reported on the International Survey of Herbicide Resistant Weeds resource.

So far, there are no reports of weed populations showing resistance to the auxin transport inhibitors (Group 19/HRAC Group P).