Herbicide-resistant weeds often are thought to simply be myths. Like the Loch Ness Monster, UFOs and the Boston Red Sox winning the World Series, herbicide-resistant weeds are something that you have never seen with your own two eyes, so you question whether they really even exist or occur. But wait, the Red Sox did win the World Series. And if that can happen, anything can happen, right?
So what, exactly, are herbicide-resistant weeds? The definition is not as clear as you would think. To better understand resistance, let's first define what makes a susceptible weed species. Herbicide susceptibility is the appearance of phytotoxic injury symptoms by a plant species due to activity of an applied herbicide. The end result of susceptibility can vary from mild phytotoxic symptoms that eventually disappear to lack of reproduction to plant death.
Herbicide resistance is the ability of a plant species to survive and reproduce even after you apply a normal (label-recommended) dose of herbicide — herbicide that one dose would have killed it and provided complete susceptibility. So this specific plant species was once susceptible, but now it can survive a herbicide application. The end result of resistance can vary from no phytotoxic symptoms to visible phytotoxic symptoms; however, the plant now survives and reproduces. This is confusing because with both susceptibility and resistance a plant can be injured, the difference is has the injury decreased over time? If it has, then the species could have developed some resistance or, to increase the confusion, tolerance.
Herbicide tolerance is the naturally occurring ability of a plant to survive and reproduce after a normal recommended label dose of herbicide to which this specific plant species was never susceptible. You may think this is the same as resistance; however, the difference is that a tolerant plant species was never, at any time, susceptible to the herbicide. The tolerance wasn't caused by repeated use of the herbicide. A species simply contains a natural mechanism for tolerating a herbicide that was not selected for over time due to repeated herbicide use.
There is also some gray area between tolerance and susceptible. For example, a plant may be injured but not killed by a herbicide application. So, is it susceptible or tolerant? Well, both. In this case the herbicide does have activity within the plant; however, the plant has a certain degree of tolerance to survive the herbicide.
For this confusing reason, many within the scientific community simply throw out the word tolerance. But, in my mind, tolerance simply defines to a greater degree the over all susceptibility or resistance of a plant species. While a plant may have developed resistance, it may not be able to completely tolerate a herbicide application, displaying no phytotoxic symptoms. Likewise, a plant may be susceptible with some tolerance, displaying injury symptoms but recovering over time.
The basic method of herbicide resistance development within weed populations is selection pressure. When you apply a herbicide, you are applying selection pressure. For example, if you broadcast a herbicide over a soccer field that is covered with a 1,000 plants of a certain weed, each of those plants will have subtle genetic differences. Such genetic variation can translate into subtle differences in the speed of flowering, tolerance to drought, and possibly some mild herbicide tolerance among these plants. If out of those 1,000 plants, one plant survives, it could potentially be due to some herbicide resistance. It may have been injured from the herbicide, but it eventually grew out and produced a few seeds.
1 List compiled from Vencill et al. 2002.
2 List compiled from http://www.weedscience.com.
The next year, the slightly tolerant seed germinates, along with other susceptible seeds. You make another application of herbicide. Again, some plants, while injured, survive the application; but this year some of the plants were not injured quite as badly. This cycle continues until eventually, no injury is observed and the plants continue to grow and reproduce. This is a basic example of selection pressure. Over time, with repeat application of a single family of herbicides, this has happened with many weed species and it will continue to happen under such circumstances.
For the most part, turf and landscape environments have few reported cases of herbicide resistance. The cases that have surfaced most likely developed in the traditional agricultural systems and the weeds have now appeared in turf and landscape. For example, goosegrass resistant to the dinitroaniniline family of herbicides can be found in turf and landscape systems; however, it was first identified in agricultural systems in South Carolina. Other herbicide-resistant weed populations, such as Italian and perennial ryegrass, also can be found in turf and landscape systems; again, however, their origin is in agricultural systems.
The reason for greater herbicide-resistant weeds in agricultural systems compared to turf and landscape is simple: consistent herbicide applications. In agricultural systems, herbicides are consistently applied year after year to insure that the crop yield goal is reached. Weeds must be controlled to eliminate their competition with the crop.
In turf and landscape, rarely are herbicides applied with such consistency. In many instances, managers wait until a problem arises, then make herbicide applications. The one major exception to the rule is the use of pre-emergence herbicides such as Barricade (prodiamine) and Princep (simazine). In both of these cases, herbicide-resistant annual bluegrass (Poa annua) are thought to have arose from turfgrass systems, not traditional agriculture systems, due to seasonal herbicide applications for pre-emergence and, in some cases, post-emergence (with Princep) control.
Genetically-modified Roundup (glyphosate)-resistant crops, such as corn, cotton and soybeans, have been a boon for traditional agricultural farmers. Now, instead of many herbicide programs that were originally used to combat various weed problems, the solution has been reduced to one: Roundup. While the company that invented that herbicide-resistant crop recommended the integration of different herbicide chemistry to thwart the potential develop of glyphosate-resistant weed populations, this advice was — and is today — in large part ignored. And why not? One herbicide vs. three, four or five different herbicides? The simpler, the better.
Eventually, the inevitable happened. Glyphosate-resistant weeds did develop. Populations of horseweed or marestail (Conyza canadensis) have been identified as resistant or tolerant to glyphosate. But the resistance here is somewhat confusing. Applications of glyphosate are herbicidally active on resistant horseweed plants, meaning that the plant does display some injury symptoms. Resistant plants, as we pointed out, simply grow out of the injury and survive and reproduce.
All families of herbicides have had some resistance development against them (see Table 1, page 12). From pre-emergence to post-emergence herbicides, from contact to systemic herbicides, no herbicide family is completely infallible. So if all can fall prey to herbicide resistance, then developing a plan to hold off resistance is absolutely necessary.
While the development of herbicide-resistant populations can happen, there are steps you can take to hold off potential resistance development.
Repeat applications of the same herbicide mode of action will apply pressure to select for resistant plants within a species. Not only do you have to rotate different herbicides, but you also have to use a different mode of action. For example, simply rotating from triazines to substituted ureas will not change the mode of action, only the family.
By utilizing herbicide tank-mixtures with different modes of action that are active on the same species, the species would need to have tolerance to two different modes of action at the same time in order to survive. This greatly decreases the potential for resistance development. Always check the herbicide label to insure that you are tank mixing with a compatible herbicide and that there have not been any reported cases of antagonism with such a mixture.
By integrating both pre- and post-emergence herbicides into your weed management plan, you increase the diversity of mode of action and eliminate weeds before they mature and develop in the system. In such a plan, it would also be necessary to rotate both the pre- and post-emergence herbicide.
The most common approach in many ways is to simply maximize the environment for the plants and turf in the landscape you are managing. By maximizing the growth and competition of the desired plants and turf in the landscape, you decrease the number of weeds that are actually treated by herbicides. By decreasing the number of weeds you are treating, you decrease the potential for resistance development.
Herbicide-resistant weeds are real. Further development of them could damage our ability to utilize many effective herbicides. By optimizing the system for the turf and landscape plants and rotating modes of action to different herbicide chemistry, hopefully we can hold off major resistance development for years to come.