Iowa State University

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Glyphosate Resistance in the Cornbelt
by Bob Hartzler
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December 22, 2005 - The following is a portion of a paper prepared for the Pennsylvania Agronomic Education Conference, State College, PA, January 17-18, 2006.

Historically, herbicide resistant weeds have not been as big a problem for Iowa and Midwest farmers as for Pennsylvania farmers. Several factors have reduced selection of resistant weeds in the western Cornbelt: 1) lower annual rainfall in the western cornbelt results in less reliance on preemergence herbicides than in the East; 2) interrow cultivation remained a routine practice on most farms until the 1990’s; and 3) in Iowa, high pH soils reduce the amount of atrazine that can be applied in order to avoid carryover injury to soybean. These factors lead to the use of integrated weed management programs that have slowed the evolution of resistance within weed populations.

The big question is ‘when will glyphosate resistance become a problem in the western Cornbelt?’ Glyphosate resistance is not as easy to characterize as resistance to other herbicide classes. There is considerable debate/disagreement among weed scientists as to when a weed should be classified as glyphosate resistant and how to manage the risk of selecting glyphosate resistant weeds. Thus, the following discussion should be viewed as my viewpoint rather than the collected vision of my weed science colleagues in the region.

One factor that muddies the water when discussing glyphosate resistance is that in most instances the level of resistance is relatively small (Table 1). With triazine and ALS herbicides, resistant biotypes typically have at least a 100 fold level of resistance (resistant individuals able to survive a dose more than 100 times that used to control susceptible biotypes). Biotypes with a high level of resistance show little response to label rates and can these weeds can increase rapidly in a field due to prolific seed production. Weeds with a low level of resistance, as often seed with glyphosate, may be able to survive use rates of the herbicide, but they will be suppressed by the herbicide. The non-lethal effects of the herbicide on the resistant weed reduces its competitiveness, seed production and the rate that it increases within the field.

Table 1. Response of susceptible and resistant horseweed populations to
chlorimuron and glyphosate. GR 90 = dose required to reduce biomass by 90%.

chlorimuron (Classic)
0.2 g/ha
200 g/ha
0.6 kg/ha
16 kg/ha

Trainer et al. 2005. Weed Technol.

The one species that there is general agreement regarding glyphosate resistance is horseweed (marestail). While widespread in the East and Southeast, Missouri is the only state west of the Mississippi River reporting glyphosate resistant horseweed. In the past year Missouri has also reported glyphosate resistance in common ragweed and waterhemp. These resistant biotypes are believed to be restricted to a few fields.

Resistance management In addition to debating whether a difficult-to-control weed should be considered resistant to glyphosate, weed scientists also differ on how best to manage the risk of glyphosate resistance. Different approaches to resistance management create a continuum of selection pressure with varying risks of selecting glyphosate resistant weeds (Figure 1). At one end of the spectrum is continuous planting of RR crops and sole reliance on glyphosate to control weeds. At the other end is the decision to completely avoid the use of glyphosate. Most farmers implement programs that place them somewhere in between these two extremes.

Monsanto promotes the ‘Right Rate at the Right Time’ approach. This strategy assumes that using too low of a rate will allow weeds to escape control. The survival of weeds exposed to non-lethal rates will allow a buildup of resistance within the weed population. This type of resistance could be caused by multiple genes or by an incompletely dominant trait. With multiple gene resistance, several genes are involved - each gene providing a small level of tolerance to glyphosate. If non-lethal glyphosate rates are used, weed biotypes having these genes will survive. After repeated use of non-lethal rates, biotypes may accumulate a sufficient number of these genes to render glyphosate ineffective.

An interesting phenomenon observed with glyphosate in the 10 years since the introduction of RR soybean is a steady increase in rates needed to obtain effective weed control. This has been referred to as rate creep. Rate creep suggests that weeds are slowly increasing their tolerance to glyphosate – consistent with the Right Rate approach to resistance management. Two studies have looked for a correlation between history of glyphosate exposure within a weed population (rigid ryegrass and waterhemp) and the population’s tolerance to glyphosate. Neither found the weeds’ tolerance to glyphosate was related to its prior exposure to the product, and thus fail to support this hypothesis. However, controlled studies with rigid ryegrass confirm that repeated sub-lethal rates can rapidly select for resistant biotypes.

While we know that the use of sub-lethal rates can lead to resistance under certain situations, a flaw to this approach is that it ignores the definition of resistance. The accepted definition for herbicide resistance is ‘the heritable ability of a weed to survive a dose of a herbicide normally lethal to that species’. Thus, according to this definition the ‘right rate’ won’t kill a resistant individual. Simply applying the label rate may reduce the risk of multiple gene resistance or resistance provided by an incompletely dominant gene, but it does little to reduce the selection pressure. Selecting the proper rate is an important component of a weed management plan, but it should not be the keystone of a resistant management plan.

Another approach to reducing selection pressure is to use additional herbicide modes of action in combination with glyphosate. This can be accomplished with sequential applications of a preemergence herbicide followed by a postemergence application of glyphosate or by adding a second herbicide to the postemergence glyphosate treatment. The sequential application approach has the advantages of reducing early-season competition, therefore providing a wider application window, and avoids antagonism that sometimes occurs with glyphosate tank-mixes.

The weakness in this strategy is that the alternative herbicide must control weeds as effectively as glyphosate to eliminate selection of glyphosate resistant weeds. It is unlikely that growers will apply a full rate of a broad spectrum herbicide when a planned application of glyphosate is part of the weed management program. Thus, there will be species on which glyphosate places significant selection pressure. Reduced rates of Harness Xtra and INTRRO were effective at reducing the number of foxtail and waterhemp exposed to post applications of glyphosate (Table 1). However, they had little effect on velvetleaf due to this weed’s tolerance to the alternative herbicides. INTRRO actually increased selection pressure on velvetleaf by eliminating susceptible weeds that suppressed velvetleaf populations early in the season.

Table 1. Reduction in glyphosate selection pressure by preemergence applications
of 1.2 qt Harness Xtra and 2 qt INTRRO in RR corn and soybean.

Weed species

Harness Xtra


% reduction in selection pressure

Giant foxtail



Common waterhemp






Bob Hartzler, Iowa State University, 2005.

The final option for managing resistance is limiting glyphosate use. The simplest approach to managing resistance would be to avoid continuous planting of RR crops. Each year that glyphosate is not used should delay the appearance of glyphosate resistant weeds by at least a year. This is the only approach that equally reduces selection pressure on all weeds found within the agronomic system. In the western Cornbelt, we have more herbicide options in conventional corn than conventional soybean, so for most growers it would make sense to plant RR soybean followed by conventional corn.

What about burndown applications of glyphosate in no-till? A burndown application is unlikely to significantly alter selection pressure on the summer annual weeds that are the major concern for glyphosate resistance. The majority of summer annual weeds will not be established at planting, thus they are not exposed to burndown applications of glyphosate. Selection of glyphosate resistant marestail undoubtedly was influenced by burndown applications of glyphosate, but this weed acts primarily as a winter annual. Although winter annuals are an important problem, I do not think they threaten the value of glyphosate and RR crops as much as summer annual weeds. Thus, I would not hesitate to use glyphosate for burndown uses in conventional crops.

In summary, a number of resistance management strategies are available to protect the value of glyphosate. Each strategy has its own strengths and weaknesses. If we knew in advance which weed species possessed glyphosate resistance it would be simple to design an effective resistance management strategy. However, since we can’t predict the next glyphosate resistant weed, I think we should take a conservative approach to resistance management. The foundation of glyphosate resistance management is avoiding continuous planting of RR crops. The other approaches (Right Rate, alternative modes of action) compliment this strategy and should be incorporated within the weed management program.

The only 100% effective strategy to prevent selecting glyphosate resistant weeds is to completely avoid glyphosate use. Most would agree glyphosate is too valuable of a tool to adopt this approach. In addition, no one can accurately predict how long the different approaches to resistance management will delay the onset of glyphosate resistant weeds. Glyphosate resistant weeds will not eliminate the value of glyphosate. The economic impact of glyphosate resistant weeds will vary widely depending on the characteristics of the species and the effectiveness of alternative control tactics. Each individual must determine their own comfort level when developing a long-term resistance management program. Hopefully these decisions will be based on a sound understanding of herbicide resistance and its implications to crop production.


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