sistance today, said Dean Riechers, University of Illinois associate professor of weed physiol- ogy. “Farmers can’t imagine going back to 2,4-D


Which Direction Are Herbicides Heading? 2,


URBANA, ILL.


4-D is coming back. What many might consider a “dinosaur” may be the best solution for growers fighting weed re-


or other auxin herbicides,” Riechers said. “But herbicide resistance is bad enough that com- panies are willing to bring it back. That illus- trates how severe this problem is.” In a recently published article in Weed Sci-


ence, Riechers and his team of research col- leagues


suggest that


tank-mixing auxinic herbi- cides with glyphosate may be the best short-term op- tion available to farmers in- terested in broad-spectrum, postemergence weed con- trol. “Resistance has become a


uncontrolled growth called epinasty in sensitive


big problem,” Riechers said. “In 1997, researchers pre- dicted that glyphosate re- sistance would not be a big issue in Round-Up 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


such as giant ragweed (Ambrosia trifida).


broadleaf weeds


University and provided by Dean Riechers, University of Illinois


Photo by William Johnson, Purdue


species have been discovered worldwide. In comparison, 21 glyphosate-resistant weed species have been discovered since 1996 when Round-Up Ready soybeans were commercial- ized. And interestingly enough, two of the most problematic weeds in Round-Up Ready soybean and cotton – common waterhemp and Palmer amaranth – are not yet on the list of auxin-resis- tant weeds, Riechers said. Ideally, chemical companies would come up


with a new herbicide to fight these resistant weeds. But new herbicide development is expen- sive and time-consuming. Riechers said he does not know of any new compounds on the horizon. “If we don’t find completely novel and new her-


bicides, our next best bet is to mix glyphosate and another herbicide with relatively minor resistance problems,” Riechers said. “Auxin resistance is not considered a huge problem in the United States, particularly in corn, soybean and cotton. It has only occurred in isolated incidences.” Why have the


Until the next novel herbicide comes out, Riechers said you only have to look back at what happened with glyphosate to see how important it is to be a good steward by using herbicides in a sustainable, beneficial way.


auxinic herbicides escaped the re- sistance problems of the more mod- ern


used today? Riechers


herbicides said


there are three major


reasons


that help explain why resistance to auxin herbicides has not become a


big problem yet. First, the auxin family of herbi- cides has a very complicated mode of action. In theory, a weed would have to develop a very com- plicated resistance method to overcome it. Riech- ers said 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 fitness cost,” he said. “The resistance mechanism that overcomes the herbicide could have a negative consequence to the plant in ab- sence of the herbicide. Basically, for auxin herbi- cides there may be a ‘penalty’ to having resistance.” The third explanation is that auxin herbicides have rarely been relied on by themselves and are


12• MidAmerica Farmer Grower / October 21, 2011


Auxinic herbicides like 2,4-D and dicamba trigger


normally mixed with other herbicides. A good ex- ample is the frequent use of several auxinic her- bicides in tank mixes for weed control in home lawncare and golf course applications. Some farmers are con-


cerned about going back to 2,4-D and other auxin her- bicides because they are considered old compounds that tend to drift and move off-target


to sensitive


plants. Riechers said Mon- santo and Dow Agro- Sciences have announced that they are working on new formulations to reduce drift, and agricultural engi- neers are exploring spray application technology to re- duce the problems, too. “This is a risk/reward de-


cision,” Riechers said. “If you have a huge resistance problem in your field and are concerned about losing yield, this may be your best solution for now. The alter- native is to give up and do nothing. For some growers, this technology may be worth the risk because they have no other choices.” So the question remains.


How long will it take for plants to form resistance to the combination of auxin herbicides and glyphosate? “We are trying to predict


the future, but all we can do at this point is speculate,” Riechers said. “However, we can use the past to help us make wise choices for the future. We have resistance to almost all herbicide fami- lies now. Tank-mixing auxin herbicides with glyphosate


www.jamarmfg.com


may work for the short term, but I expect that auxin resistance will likely increase over time. Na- ture always finds a way.”


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