January 29, 2003
By Bob Hartzler
Source: ISU Extension Agronomy
Weed management generally doesn’t generate much interest among the general public, but the topic of glyphosate resistant weeds was recently featured on the front page of the Des Moines Register and New York Times. By now most Midwest farmers have experience with at least one herbicide resistant weed species, so resistance is not a new problem. The question is whether glyphosate resistance should be treated different than resistance to other herbicides. This talk will review where we stand in terms of glyphosate resistance and potential impacts of this problem.
Current status The first documented case of glyphosate resistance was reported in 1996 involving rigid ryegrasss in Australia. The resistant biotype was obtained from an orchard near Orange, New South Wales, Australia. The site had intensive selection pressure, with two or three applications per year of glyphosate for 15 years. Roundup was used to control weeds within rows of trees. Greenhouse research confirmed that this population was 7 to 11 times more resistant to Roundup than susceptible populations. Since the original report several additional glyphosate resistant weed populations have been identified: rigid ryegrass in a wheat production system in Australia and in California, Italian ryegrass in Chile, goosegrass in Malaysia and horseweed (marestail) in the east, midwest and southeast U.S. Resistance in goosegrass is due to an altered target site, whereas the mechanism of resistance in the other weeds is currently under investigation.
The identification of glyphosate resistant horseweed is the first case of a weed developing resistance in Roundup Ready soybeans. The resistant biotype first appeared in Delaware in 2000 and since has spread as far west as Indiana and has been identified in the Southeastern U.S. where Roundup Ready cotton is grown. The first population originated in a field in which glyphosate was the only herbicide used in Roundup Ready soybeans in 1999 and 2000. Prior to 1999 glyphosate had been used infrequently as a preplant burndown herbicide in no-tillage systems. The horseweed biotype has exhibited 8 to 13 fold resistance to glyphosate.
There has been considerable discussion whether waterhemp should be included on the list of glyphosate resistant weeds. Waterhemp populations with individuals capable of surviving ‘normal’ use rates were identified in Iowa and Missouri the first year that Roundup Ready soybeans were marketed. Research in the greenhouse and laboratory has shown that the enhanced tolerance/resistance exhibited by these plants is controlled genetically, rather than environment, coverage, and other management practices. A student working for Mike Owen at Iowa State University has been able to increase the tolerance in one of these problem populations by 3.5 fold after two generations with recurrent selection (see article).
In academic terms, the research at both Missouri and Iowa could be interpreted to support that glyphosate resistant waterhemp is indeed present in the Midwest. Both groups have identified waterhemp populations that survive higher levels of glyphosate than most populations, and they have documented that the trait can be passed on to new generations. However, I believe there are a few important pieces missing, and in my mind prevent the problem waterhemp populations from being classified as glyphosate resistant.
First, the populations were identified the first time glyphosate was used as a postemergence herbicide in the field (i.e. the first year RR soybeans were marketed). Resistance is defined as the ability of a plant to survive a dose of herbicide that was toxic to the original population. This implies that the resistance is identified after selection pressure has weeded out the susceptible individuals. Since the problem was identified the first time glyphosate was used for general weed control, I feel these populations fail to meet the criteria of being selected from the original population. Second, I am not aware of these problem populations increasing following continued use of glyphosate. I am not aware of any farmers or dealers in Iowa who have given up on controlling waterhemp with glyphosate. Higher rates of glyphosate are currently being used than when RR soybeans were first introduced, and the percentage of RR soybean fields treated with a preemergence herbicide has increased dramatically. Waterhemp may be largely responsible for both of these occurrences in many fields. However, growers are still relying on glyphosate to control waterhemp postemergence, and I’m not aware of anyone who adds a diphenylether to glyphosate to control waterhemp in RR soybeans. For these reasons, I do not think waterhemp should be included in the list of weeds resistant to glyphosate. However, the research has documented that within the waterhemp gene pool there is the potential for resistance to develop, and it is something that should be watched closely.
Impact of glyphosate resistant weeds There has been a lot of talk lately about the potential impacts of glyphosate resistant weeds. Some persons have described them as super weeds, and there have even been inferences that the presence of glyphosate resistant weeds could reduce the value of farmland. There is no question that the development of glyphosate resistant weeds will increase the cost of weed management for farmers, but the question is by how much. I think there are several possible scenarios: some situations would have relatively little impact whereas others would pose a major problem for farmers. Which scenario develops depends upon the characteristics of the resistant weed, primarily the effectiveness of alternative tactics on this species and how quickly the weed spreads.
For most weed species we have alternatives to glyphosate that are highly effective and provide good flexibility in application timing. For these weeds a farmer could simply add another herbicide to glyphosate to control the resistant species. In this situation, the primary impact of the glyphosate resistance is the added cost of the additional herbicide, otherwise the farmer could use the identical weed management program used prior to the development of the resistant velvetleaf population. Farmers who rotate Roundup Ready corn and Roundup Ready soybeans already do this by using Select or a similar herbicide to control RR volunteer corn in their beans.
The more costly scenario would involve a weed for which the alternative herbicides have limited flexibility in application timing. A weed species that requires postemergence applications to be made before weeds reach a 4-inch height would have a major impact on weed management systems. In this situation, the loss of application flexibility would present a greater cost to many growers than the additional herbicide expense. The continued growth in farm size increases the importance of the application flexibility provided by glyphosate.
Since the first report of glyphosate resistant rigid ryegrass in 1996, four additional resistant species with this trait have been identified. While not quite one new species per year, this rate of development suggests that we will continue to see new resistant biotypes. Eventually one of these weeds will appear in Iowa and surrounding states, and my guess is it will happen sooner than later. However, the ability to survive glyphosate does not create a ‘super weed’, and there is no reason to use scare tactics to try and change farmers' perceptions and practices.
I believe it is makes good sense for farmers to implement a long-term plan to reduce the selection pressure placed on weeds by glyphosate. The simplest way to do this is to avoid planting continuous Roundup Ready crops. Using additional modes of actions with glyphosate provides alternative selection pressures on certain weeds, and in some situations this will reduce the likelihood of resistance. However, since we do not know which weed is likely to develop resistance, it is impossible to know whether the alternative mode of action is reducing glyphosate selection pressure on the appropriate speces. Thus, we believe an annual rotation of herbicides should be the foundation of resistance management.
With the manner that glyphosate is being used in the Midwest, resistance is inevitable. When resistance develops, we will need to control these biotypes with existing herbicides - no new modes of action are coming down the pipeline in the foreseeable future. The large number of alternative products for use in corn and soybean will reduce the impact of glyphosate resistance, but there can be significant costs associated with the problem. The need for application flexibility in today's agriculture increases the cost of glyphosate resistance compared to previous cases of resistance experienced by Iowa farmers. Because of this, evaluating weed management programs in terms of selection pressure placed on weeds should be an important component of crop management planning.