The Threat of Multiple-Resistant Weeds
Oftentimes when you hear discussions about weed resistance, it is referring to a particular weed species that is resistant to one mode-of action, such as glyphosate. However, there are also many cases in which weeds develop resistance to more than one mode of action, becoming multiple-resistant weeds. In these cases, management becomes even more difficult and yield potential becomes even harder to preserve.
This article in Delta Farm Press reports there is a possible case of ragweed in Arkansas that was already resistant to glyphosate, and is now under suspicion of ALS-resistance as well. Unfortunately, if this proves to be the case, it won’t be the first state to report multiple resistance that includes glyphosate.
According to www.weedscience.com*, Missouri has confirmed waterhemp that is resistant to ALS, PPO and glyphosate modes of action. Additionally, Ohio has a population of horseweed that is resistant to ALS chemistries and glyphosate. In fact, across the U.S., there are 45 states that have weeds that are herbicide resistant, and of those, 12 have at least one weed population that is confirmed resistant to more than one mode-of-action.
In fields that already have herbicide-resistant weeds and are now being treated with glyphosate as an alternative, it is even more important to implement management practices to discourage glyphosate resistance. With no new chemistries coming down the pipeline, there is no easy cure for this situation. Before glyphosate-tolerant technology was widespread, growers were facing a frustrating battle with ALS- resistant weeds. Unfortunately, ALS-resistant weeds are still prominent and growers may be faced with multiple resistance to glyphosate and ALS chemistries if preemptive practices aren’t put in place.
The best thing to do is fight resistance before you see it by rotating crops and chemistries; employing a pre-emergence, residual herbicide in corn and soybeans; limiting glyphosate use; using full herbicide rates; and tank mixing where applicable.
How much of a problem was herbicide resistance for you in the heyday of chemistries such as Pursuit? Has that experience changed your perspective of the current resistance situation with glyphosate? How so?
*Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. September 13, 2007