Iowa State University

Two For the Price One  
by Bob Hartzler

December 17, 2004 - The following advertorials¹ appeared in the November issue of Doane's AgProfessional, and I suspect other magazines due to the discussion they have aroused in the weed science community.  It has been more than a year and a half since I have entered anything in the Herbicide Hall of Shame, thus it's only fitting that two ads get nominated at the same time.

No benefit in rotating glyphosate

This advertorial from Monsanto summarizes a long-term weed management study (7 years) evaluating different use patterns of glyphosate, and concludes there is no benefit to rotating glyphosate as long as full rates are used.  While these studies are well designed and managed, I think it is inappropriate to use them to predict the risk of selecting glyphosate resistant weeds.  It is well documented that glyphosate resistance exists at a very low frequency in weed communities.  Because of this low frequency, the likelihood of resistance occurring within the weed community present in a small field trial is very small. 

Using small plots to conclude there is little risk of resistance with systems relying solely on glyphosate can be compared to buying scratch and win tickets in the Iowa Lottery.  If you went to the local Casey’s² and bought every Iowa Lottery ticket in the store, odds are you would not get the $100,000 jackpot ticket.  Based on this experience, is it correct to assume that a $100,000 ticket doesn’t exist?  Of course not!  The reason you didn’t hit the jackpot is that you only sampled a small fraction of the available tickets.  If you had bought all of the tickets at 1000 Casey's stores you would greatly increase the likelihood of striking it rich.  Using the results of a few small field trials to conclude that continuous use of glyphosate poses little threat of selecting resistant weeds is similar to accusing the Iowa Lottery of running a crooked game because you’ve never gotten the winning ticket.

The low frequency of glyphosate resistance in weed communities is the reason glyphosate resistance has not appeared in Iowa after nearly a decade of widespread planting of Roundup Ready soybeans.  However, there is ample proof that glyphosate resistant weeds occur, and the selection of resistant weeds in Iowa is inevitable under current glyphosate use patterns. The increased planting of Roundup Ready corn is bound to speed the process by which resistant weeds are selected.  Adoption of integrated weed management systems that include rotation of herbicides will greatly reduce the likelihood of hitting the unfortunate jackpot of glyphosate resistant weeds appearing in your fields.

Dead weeds don’t become resistant?

A sidebar to the above ad outlined Monsanto's position for managing the risk of selecting glyphosate resistant weeds.  These recommendations focus on the use of full rates of glyphosate rather than the inclusion of alternative management strategies.  The ad concluded with the following marketing gem:  ‘The goal is to kill all the weeds, because we know that dead weeds will not become resistant’.  It seems that Monsanto misses the point that glyphosate resistant weeds will not die when exposed to glyphosate - this is the definition of resistance.  This is why we feel it is important to utilize integrated management systems that rely on a variety of weed control tactics.

¹The advertisements included a header describing the literature as advertorials.  I'm not sure of the difference between an advertisement and editorial since both are used to promote a particular viewpoint.

²For those not familiar with Casey’s, it is a convenience store chain with stores in nearly every Iowa town.

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