The Big Four: Weeds to Watch in 2020
Farmers fight waterhemp, Palmer amaranth, giant ragweed and horseweed in fields across the country. These four major players are some of the most widespread and difficult to control weeds. With multiple management practices, farmers can overcome these weeds in 2020 and in years to come.
Challenging Opponents
Mark Loux, weed scientist at The Ohio State University, says one of the biggest challenges with these four weeds is they tend to develop resistance to not one, but multiple herbicide sites of action.
Over time, weeds have evolved to overcome typical weed control methods. With low- and no-till becoming more prevalent for managing soil health as well, even mechanical methods are less of a solution. These elements together limit the number of effective options for control.
“The biological characteristics of the weeds, such as their long emergence windows, rapid growth and prolific seed production, make them well-adapted to our primary crop rotations and production practices,” Loux says.
Loux cites the following practices to control these weeds:
1. Use an effective herbicide burndown treatment or tillage to kill standing weeds, such as horseweed and giant ragweed, before planting.
2. Use residual herbicide treatments with multiple sites of action to control all four weeds ahead of planting.
3. Make at least one —and sometimes two —post-emergence herbicide applications to control populations of the big four weed species, depending on weed pressure and weather.
4. Add residual herbicides to the first post-emergence application to control waterhemp and Palmer amaranth which emerge later, potentially avoiding the need for a second post-emergence application.
It is also important to note that herbicides are not effective on all weeds, regardless of resistance. When making herbicide applications, farmers should consult their advisor to determine which herbicides have a high efficacy rating for the weeds they’re trying to control.
Resistance Is Looming
When controlling any weed, it’s important to implement the proper practices to mitigate the risk of herbicide resistance. Individual populations of waterhemp, Palmer amaranth, giant ragweed and horseweed all run the risk of developing resistance to multiple herbicide sites of action. That resistance limits farmers’ options for control. Loux says there are two in particular to keep a close watch on.
“Waterhemp and Palmer amaranth are especially prone to developing resistance to any herbicide used repeatedly for their control,” he says. “So we’re at risk of eventually running out of options.”
Staying Ahead of the Problem
Scouting is important to stay on top of weed control. Scouting at the right time is even more crucial. Loux says farmers tend to discover new weeds toward the end of the season around harvest or just before, when they are taller than the crop. These older weeds produce seeds which could easily be spread by the combine, leading to bigger problems next year when different areas or fields have new weed problems.
“Farmers might want to take an extra look at fields during late season to avoid infesting combines with seed from new patches of these weeds when they are harvesting,” Loux says.
He also says field history is important. Scouting for weeds after a post-emergence herbicide application makes farmers aware that there are new weeds in the field and helps them remove them. But farmers make most of their major herbicide decisions on their knowledge of what happened with the weeds in their field the previous years. A pesticide application record is a useful tool to keep track of weed activity and herbicide applications.
If new weeds are discovered when scouting after post-emergence herbicide, Loux says farmers should consult a trusted advisor. Their agronomist, consultant or extension professional can help them decide if they need to adjust their current herbicide plan. These professionals can advise farmers on which herbicide sites of action will work based on the known weed resistance in the area.
Loux says it’s important to get ahead of these weeds by building a management plan around existing resistance.
“My feeling is that a lot of the effort toward control is recognizing what types of resistance the population already has,” Loux says. “Then develop an effective, comprehensive herbicide program that covers the resistance and the biology of the weed.”
When it comes to managing waterhemp, Palmer amaranth, giant ragweed and horseweed, knowledge is power, according to Loux.
“Resistance reduces the number of effective options to control these weeds,” Loux says. “So it’s important to make the right decisions to effectively control these weed populations.”
For more information on herbicide sites of action, check out the Take Action Herbicide Classification Chart or the herbicide lookup tool on the Take Action app.