Iowa State University

A World Apart
by Bob Hartzler

January 14, 2004 (Updated January 30, 2004) -  Glyphosate is the most widely used herbicide in Iowa and the world.  More than 80% of the soybean planted in Iowa are Roundup Ready varieties, and the acres of Roundup Ready corn are increasing.  This heavy reliance on glyphosate results in significant selection pressure on weed communities, therefore creating the potential for selection of glyphosate resistant weeds.   In an earlier article (What - Me Worry?) I questioned the message Monsanto was presenting concerning stewardship of glyphosate.  Monsanto distributed a flyer¹ in early 2003 recommending the following steps to minimize the resistance risks associated with using Roundup in RR corn and soybean¹:

1)  Base weed control programs on local agronomic needs;
2)  Always start with a weed-free field;
3)  Apply Roundup WeatherMax when weeds are within labeled heights;
4)  Always use full, labeled rates;
5)  Use higher rates if applications are delayed and made to larger weeds;
6)  Scout fields and take appropriate actions if escapes are present; and finally
7)  Report any incidence of non-performance to local retailer or Monsanto representative.

While these are excellent recommendations for managing weeds in the short term, they fail to address long-term shifts in weed communities in response to repeated herbicide use.  Without going into detail, my concerns with these recommendation are that neither the frequency of Roundup use nor the inclusion of alternative control strategies are mentioned.  It appears that Monsanto believes that as long as these steps are followed, no weeds will survive.  A key message repeated throughout the promotional flyer is that "Dead weeds don't produce seeds".  Anyone involved in weed management realizes that no economical weed management program used in agronomic crops is capable of providing complete weed control.  Thus, weed seed production will occur, and the more glyphosate is used the greater the risk of the evolution of resistance.

Recently I ran across a piece of informational literature² developed by Monsanto in Australia.  The bulletin describes strategies to manage glyphosate resistance in Roundup Ready Canola.  Included in their detailed recommendations were the following:
1)  Use alternative burndown herbicides;
2)  Mix herbicides with other modes of action with Roundup during preplant and fallow applications; and
3)  Rotate the use of herbicide groups during cropping rotations.

These are similar recommendations made by most U.S. Land Grant Universities for managing resistance in cropping systems using Roundup Ready corn and soybean, but steps that Monsanto suggest are unnecessary in the Cornbelt.  The only reason I can think why Monsanto would take such a different approach in Australia than in the U.S. is the experience Australian farmers have with herbicide resistance.  Perhaps Monsanto believes that farmers Down Under are more aware of what is needed to manage resistance due to their past experiences, and thus they feel obligated to promote rational strategies to manage resistance.   The webpage that featured these recommendations was changed in late January, and now describes how Monsanto will work with farmers to develop effective Resistance Management Plans.

Evolution of resistance in weeds is the same in both Australia and the U.S., and thus resistance management practices should be based on the same principles.  I believe the practices recommended by Monsanto for avoiding glyphosate resistance in Roundup Ready Canola are much more appropriate than the strategies suggested for Midwest farmers producing Roundup Ready corn and soybean.  This is one time I would recommend farmers avoid using local recommendations in favor of those made for another region.

Appendix:  The complete list of glyphosate resistance tactics provided on Monsanto's Australian website:

1. The use of alternative knockdown herbicides (e.g. SpraySeed ®);
2. Mixing herbicides with other modes of action with Roundup herbicide during pre-plant and fallow applications;
3. Rotating the use of herbicide groups during cropping rotations;
4. Introducing a pasture phase into crop systems and Pasture Topping with paraquat;
5. Crop Topping with paraquat;
6. Burning header trash;
7. Using full soil disturbance where environmental conditions are favourable (soils are not exposed to erosion).

Related articles:   Do reduced rates increase the risk of resistance?

¹ The Facts:  Managing Weed Resistance.  2003.  Monsanto Company.
² Weed Management.  2003.  Monsanto Company.  http://www.monsanto.com.au/canola/weedManagement.htm#2   

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University