Rapid Spread of Resistant Weeds
As ag retailer agronomists and crop consultants try to convince their clients to make changes to their herbicide programs to avoid herbicide-resistant weeds, those agronomists and consultants have had limited ammunition to prove that resistance on a farm is maybe only one growing season away.
“The normal response by many farmers in the past has been to not worry about weed resistance until it is costing them an economic hardship,” said Bill Johnson, Ph.D., professor of weed science at Purdue University. “A resistant weed could be in a neighbor’s field, and they didn’t seem to care. It is my neighbor’s problem; I don’t have it. Usually, the only way that they are going to put some kind of a management program in place is when they have resistance, and it has caused them some problems.”
It is hard to convince many farmers to be proactive in preventing weed resistance to the farming program they have become comfortable using. And if weeds haven’t been identified as a problem in their county, then the farmer figures he still has time to react next season.
The search and discovery of herbicide-resistant weeds in the U.S. has been very unsystematic over the years. Methods and resource investment to identify (ID) resistant weeds has mainly been left to individual state funding and private investment. But even announcing that a specific weed is resistant to a specific herbicide mode of action and has been found in a state doesn’t seem to impress a lot of farmers.
Indiana has been acknowledged as a state that has taken the lead in identifying resistant weeds so that its farmers could turn the tide on the spread of weeds during the past decade.
Surveying and sampling has occurred as new weed problems have emerged. Johnson noted that from about 2003 through 2008 the focus was on marestail/horseweed. Ragweeds became the focus from about 2005 to 2010. And today the focus is on Palmer amaranth and waterhemp.
“When we got started, I was much more concerned about waterhemp than I was Palmer amaranth, but as we went out and did some scouting of waterhemp, we kind of stumbled on the fact that we had a lot of Palmer amaranth out there,” Johnson explained.
“Palmer amaranth is a weed that I think people in the North have misidentified; they have assumed it is waterhemp,” said Les Glasgow, Ph.D., chairman of the North American Herbicide Resistance Action Committee and technical product lead in charge of herbicide resistance management strategy for Syngenta. “It well could be that there is a large number of undetected Palmer amaranth populations. It is an amazing weed considering that it started out in the desert Southwest and can be happy all the way into Michigan.”
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