Iowa State University

Herbicide Combinations to Sustain Glyphosate
by Bob Hartzler

March 10, 2004 -  Several factors (convenience, performance, economics) are leading many farmers to adopt rotations utilizing continuous Roundup Ready systems.  While the benefits of these systems are readily apparent, most University weed scientists feel the risk of selecting glyphosate resistant weeds in this rotation outweighs the weed management and economic advantages.  Numerous rationalizations have been proposed as to why continuous Roundup Ready systems will not rapidly select resistance weeds, including the low frequency of glyphosate resistance within weed populations and the fact that additional herbicides are used in these systems.  This article will address the impact of alternative modes of action in a continuous Roundup Ready rotation on the selection pressure placed on weeds.

The evolution of herbicide resistance within a weed population is based on selection pressure.  The more frequently a herbicide is used, the more pressure placed on a weed population, and the sooner resistance will appear at a troublesome level in the population.  Using alternative modes of action can reduce the potential for selecting resistant weeds by placing different selection pressures on weed populations.  In a system relying only on glyphosate, a weed possessing a trait allowing it to survive glyphosate will rapidly increase in frequency.  But if a second herbicide is used with glyphosate, this alternative herbicide may kill the weed with the glyphosate resistant trait and prevent it from increasing within the weed population.  Theoretically this approach is sound and can reduce the potential for herbicide resistance. 

Australia researchers developed a model that simulated the development of resistance within a population under different selection pressures from two herbicides (Diggle et al.  2003).  When herbicides were rotated on an annual basis, resistance increased exponentially and resistant plants exceeded susceptible plants within ten years.  Using herbicides in combination greatly delayed the increase in resistance, and was determined to be a better approach to resistance management than herbicide rotation.  Based on these results, continuous Roundup Ready crops with the use of alternate modes of action seems an appropriate approach to glyphosate resistance management.

So why is there such concern regarding continuous Roundup Ready rotations when studies support the practice of herbicide combinations to reduce resistance risks?  The problem lies in the fact that most Midwestern fields contain at least fifteen to twenty weed species, each having a unique range of responses to herbicides.  The Australian model described above utilized a single hypothetical weed species that was controlled 95% by both herbicides evaluated in the model.  For herbicide combinations to reduce resistance risks, they both must achieve efficacy that is high enough against all weeds to ensure redundant kill.  This redundant kill rarely, if ever, occurs in the real world.

To illustrate the flaw in this approach to resistance management, a typical herbicide program that might be used in a continuous Roundup Ready rotation is described in Table 1.  In the corn year a reduced rate of Harness Xtra (acetochlor + atrazine) is applied preemergence.  Acetochlor and atrazine reduce early-season competition and provide greater flexibility in timing for the postemergence glyphosate application.  A two-pass glyphosate program is used in soybean, with Select included in the second glyphosate treatment to control volunteer Roundup Ready corn.

Table 1.  Typical herbicide program used in a corn-soybean rotation relying on Roundup Ready crops.

Relying on alternative modes of action to reduce resistance risks will rarely result in the redundant kill essential for this strategy to be effective against resistance selection.  The relative effectiveness of the herbicides against several weeds is shown in Table 2.  In this situation the only weed that redundant kill is achieved is giant foxtail.  In soybean the alternate mode of action (Select) has no activity on broadleaf species.  In corn, acetochlor would provide good control of giant foxtail, but the atrazine is used at too low of a rate ( 0.65 lb / acre) to provide effective control of most important broadleaf weeds.  Thus, for the majority of weed species the described herbicide program has a similar risk of glyphosate resistance as if glyphosate had been used alone.

Table 2.  Activity of individual herbicides used in management program on selected weed species.

The benefits of Roundup Ready crops provide significant incentive to growers to maximize the use of this technology.  However, growers should be aware of the risks associated with 'using too much of a good thing'.  Decisions should be based on sound, research-based information.  Inclusion of alternative modes of action can reduce selection pressure when herbicides provide redundant control of weeds; however, this is rarely achieved with the herbicide programs used in a Midwestern corn-soybean rotation.  To achieve redundant control, many of the benefits of the Roundup Ready system are lost, specifically low cost, application flexibility and reduced potential for crop injury.  The Australian model demonstrates that simply rotating glyphosate resistant crops will not eliminate the risk of glyphosate resistance, but this approach is likely to delay the onset of resistance much more effectively than the strategy of planting continuous Roundup Ready crops with the inclusion of alternate modes of action.

Diggle, A.J., P.B. Neve and F.P. Smith.  2003.  Herbicides used in combination can reduce the probability of herbicide resistance in finite weed populations.  Weed Res.  43:371-382.

Related articles:
        A World Apart - comparison of Monsanto resistance management recommendations in U.S. and Australia
        Are Roundup Ready Weeds in Your Future II?  - Status and impact of glyphosate resistant weeds
        What, Me Worry? - Commentary on Monsanto promotional literature
        Do Reduced Herbicide Rates Increase the Risk of Resistance?  -  Discussion of basis for Monsanto's full rate approach to manage resistance

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