What many might consider an ancient herbicide - 2,4-D - may be the best solution for growers fighting weed resistance today, says Dean Riechers, University of Illinois associate professor of weed physiology.
"Farmers can't imagine going back to 2,4-D or other auxin herbicides," Riechers notes. "But herbicide resistance is bad enough that companies are willing to bring it back. That's how severe this problem is."
In a recently published article in Weed Science, Riechers and his team of research colleagues suggest that tank mixing auxinic herbicides with glyphosate may be the best short-term option available to farmers interested in broad-spectrum, postemergence weed control.
"Resistance has become a big problem," Riechers says. "In 1997, researchers predicted that glyphosate resistance would not be a big issue in Roundup Ready crops. For the most part, they were right. But they underestimated a few weed species and resistance mechanisms."
Since the 1950s, 29 auxin-resistant weed species have been discovered worldwide. In comparison, 21 glyphosate-resistant weed species have been discovered since 1996 when Roundup Ready soybeans were commercialized. And interestingly enough,two of the most problematic weeds in Roundup Ready soybeans and cotton — common waterhemp and Palmer amaranth — are not yet on the list of auxin-resistant
weeds, Riechers adds.
Ideally, chemical companies would come up with a new herbicide to fight these resistant weeds. But new herbicide development is expensive and time-consuming.
Riechers does not know of any new compounds on the horizon. "If we don't fi nd completely novel and new herbicides, our next best bet is to mix glyphosate and another herbicide with relatively minor resistance problems," Riechers adds. "Auxin resistance is not considered a huge problem in the U.S., particularly in corn, soybean and cotton. It has only occurred in isolated incidents."
Riechers says three major reasons explain why resistance to auxin herbicides has not become a big problem yet. First, the auxin family of herbicides has a complicated mode of action. In theory, a weed would have to develop a very complex resistance method to overcome it.
Riechers says the auxin herbicide family is very unusual because it has multiple target sites, which were only recently discovered.
"In addition, resistance to these compounds is rare because a plant that evolves resistance may have a fi tness cost," he adds. "The resistance mechanism that overcomes the herbicide could have a negative consequence to the plant in absence of the herbicide. Basically, for auxin herbicides, there may be a 'penalty' to having resistance."
The third explanation is that auxin herbicides have rarely been relied on by themselves and are normally mixed with other herbicides. A good example is the frequent use of several auxinic herbicides in tankmixes for weed control in home lawn care and golf course applications.
Revisiting the past
Some farmers are concerned about going back to 2,4-D and other auxin herbicides because they are considered old compounds that tend to drift. Riechers says Monsanto and Dow AgroSciences have announced they are working on new formulations to reduce drift, and agricultural engineers are exploring spray application technology to reduce the problems, too.
"This is a risk-reward decision," Riechers adds. "If you have a huge resistance problem in your fi eld and are concerned about losing yield, this may be your best solution for now."
For a list of herbicide-resistant weeds,visit www.weedscience.org.
Source: University of Illinois