Rapid Spread of Resistant Weeds
As known in the industry, weeds can become resistant to ALS-inhibitor herbicides quite quickly when they are used
continuously. Weed resistance to ALS-inhibitors should be widespread inside and outside the U.S. because this mode of action has 54 registered active herbicides in its group.
LOCAL LEVEL DISCOVERY
The rapid increase in discovery of herbicide-resistant weeds makes it likely that more weeds are resistant than those included in the International Survey of Herbicide Resistant Weeds, even though weeds are constantly being added, such as the July listing of smallflower umbrella sedge in California being resistant to ureas and amides.
Taking the discovery of herbicide-resistant weeds to the local level, lower than state level in the U.S., is extremely hard. In most states, the onus lands on the individual farmers.
Resistant weeds being discovered often is the result of growers finding weeds they have not been able to control.
“A farmer will find a weed or a patch of weeds in a field that he is not able to control and will call in a epresentative from the company who supplied the product applied or will check with his local university Extension service,” noted Glasgow.
“The international survey defines what should be done to prove that it is a resistant weed. There is some pretty stringent testing that needs to go on, especially when a new weed species is suspected to be resistant or the first time that a herbicide is showing lack of control of a previously sensitive species,” he added.
Therefore, the listing of weeds is usually behind the curve on what is happening in the real world to some degree. A farmer shouldn’t wait until a weed shows up on the list as resistant to the herbicide mode of action before altering a weed management program.
REASONS FOR RESISTANCE
Julian Smith, Ph.D., director of discovery and innovation with Brandt located in central Illinois, provides a concise thought about management programs that are driving weed resistance. “Farming is more intensive than it was in the past. We tend to have more monoculture environments than we ever had before. Although there still are a multitude of rotations, monocultures are more common. And I think another piece is that there are not as many tillage operations carried out on farms as there used to be. Pre and post tillage used to be a pretty valuable mechanical means of weed control.”
Palmer amaranth and waterhemp are both prolific seed and pollen producers. Smith isn’t saying he is favoring more tillage or against continuous corn and soybean cropping. He simply noted that continuous use of one herbicide with minimum tillage sets the stage for weeds to adapt or for those that thrive in specific management programs to thrive.