Historically, there has been talk about the economic threshold in weed control and how it is OK to leave a few weeds in a field, but the dynamics of herbicide resistance by weeds has changed that philosophy.
In today’s situation, if farmers are going to preserve the weed-control technologies available, then they have to make plans to eliminate every weed that will produce seed because it might be resistant to one or more herbicides and spread like lightning from one year to the next. That is why farmers have to be cognizant of any weeds they see in their fields during the growing season and at harvest.
“The best way to preserve the effectiveness of herbicides we currently have available is to assure we don’t have seed production by weeds,” said Jason Norsworthy, Ph.D., University of Arkansas professor and Elms Farming Chair of Weed Science.
“When a farmer is out there and sees one plant in the field, at the end of the day it is going to be worth his time and effort to remove that plant, if possible before it sets seed, to try and minimize the amount of seed going back into the soil. If that plant is resistant to a specific herbicide and thousands of seeds go into the soil, then it will be a very short time until the herbicide is totally ineffective,” Norsworthy said.
He explains that once a weed population in a field is herbicide resistant, the weeds will never again be susceptible to that herbicide mode of action. He notes the example of weeds still being ALS herbicide-resistant 15 years after the last such herbicide was sprayed on a field. And he said the same will be true of weed populations resistant to glyphosate.
The solution of relying completely on glyphosate has been replaced in general with the use of a preemergence herbicide prior to the postemergence application of glyphosate, but that solution hasn’t solved a lot of problems when the weeds that escape are glyphosate resistant. That is why LibertyLink soybeans are becoming the alternative to Roundup Ready beans.
“LibertyLink soybeans give the grower more tools to fight glyphosate-resistant weeds that escape the preemergence herbicides,” said Alan Hopkins, seed technology account manager for Bayer CropScience.
“No preemergence herbicide is 100 percent effective. Even if you get 90 percent control in a heavy weed pressure field, the grower is still in a very difficult situation with those 10 percent escapes if they are glyphosate resistant in a Roundup Ready field.”
Mike Thompson, Beck’s Hybrids, seed advisor with the Atlanta, Ind., based company, explained how glyphosateresistant marestail and giant ragweed is a “big issue in Indiana.”
He said farmers with high-seed-producing weeds are being directed toward the use of LibertyLink soybeans because Liberty herbicide, if applied correctly, will take care of those escaped marestail, waterhemp, giant ragweed and even Palmer amaranth weeds.
In the “old days before Roundup Ready,” growers were told to look at fields to spray 21 days after planting when weeds were quite small and an ALS or minimally damaging herbicide might be used. With the adoption of Roundup Ready, Thompson said, farmers began expecting completely clean fi elds and they saw a little increase in yield because of less weed competition.
He said, today where glyphosate-resistant weed-escapes occur, “we see the yield advantage with the Liberty program fi elds over the Roundup program fields.” Thompson said, “We are trying to educate growers about glyphosate-resistant weeds and that they need to do everything possible to prolong the life of glyphosate, and that means using multiple modes of action including a LibertyLink system.”