(The following information is from a paper presented by the author at a recent meeting in Missouri.)
AMES, Iowa – 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 differently than resistance to other herbicides.
Current status. The first documented case of glyphosate resistance was reported in 1996 involving rigid ryegrasss in 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.
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 United States.
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 United States where Roundup Ready cotton is grown.
The horseweed biotype has exhibited eight- to 13-fold resistance to glyphosate.
Impact of resistance. 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.
It depends. 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.
Application timing. 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
Not a ‘super weed.’ 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 species.
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.
(The author is an extension weed management specialist at Iowa State University.)
