From Kansas Agland:
With “bulletproof” weeds like palmer amaranth and kochia becoming ever more resistant across the Great Plains, farmers must focus on rotating modes of action, using pre-emergent herbicides and following the label when mixing products, experts say.
For 25 years, kochia and other weeds were successfully controlled by glyphosate, a broad-spectrum herbicide initially sold under the brand name Roundup. Now, these weeds are showing resistance to the herbicide in fields from Texas to Canada, according to Kansas State University.
“We were in the honeymoon period of weed control in the late 1990s and early 2000s when glyphosate was working,” said K-State weed scientist Curtis Thompson.
Some farmers are pulling out the tillage equipment, said Thompson. But if they can, there are more advantages to sticking with a no-till system.
Research conducted by Dr. Alan Schlegel at K-State Southwest Research and Extension Center at Tribune shows 13-year average yields of wheat/sorghum/fallow benefit from a straight no-till system, Thompson said. The research compares three systems: conventional, reduced tillage (ground is only tilled as needed between sorghum harvest and wheat planting), and complete no-till. Sorghum hybrid, soil fertility and in-crop weed control remain the same in all three systems.
The results: the 13-year average yield of wheat that was conventional – 13 bushels an acre; reduced tillage – 16 bushels an acre; and no-till – 21 bushels an acre. The 13-year average yield for sorghum was: conventional – 18 bushels an acre; reduced till – 30 bushels an acre; and no-till – 58 bushels an acre.
“I do think it is going to require a higher level of management in all phases of crop production,” he said of sticking with no-till. “I think it can be done.”
“Timeliness of effective herbicide applications is key so successful control,” he added. “It may mean that we apply herbicides in late fall or in January or February to control a severe kochia population, or perhaps fall applications to manage marestail.”
Crop rotation is also a key component, which allows the use of multiple modes of action of herbicides and different timings of application based on the crop planted, Thompson said.
“We aren’t ready to throw out the no-till technology and go back to the moldboard plow,” he said.
Terry Faurot, a Scott County farmer and chemical applicator, said his business has been busier due to the growing resistant-weed problem – and he’s busier earlier in the season.
“Farmers are jumping in early,” he said. “In the January to March range, I’m putting on pre-emergents.”
He advises farmers to follow a course of action that catches the weeds before they come out of the ground.
“What farmers are doing, they are coming in February and March and putting something like Dicamba and Atrazine, and creating a barrier. So, when the ground warms, it blocks (the weeds).”
Then, he said, as the herbicide wears out and it gets closer to planting, farmers can come back with another pre-emergent herbicide.
“The whole thing is to keep it from seeding out,” he said. “The biggest thing is to not let those weeds go to seed.”
Killing a growing weed crop
If kochia does emerge, don’t wait until the weeds are tall to try to kill it, he said. “Then they are really hard to kill.”
The best time to kill growing kochia is when it’s between 3 and 8 inches in height. Once it gets too tall, the stem gets woody and the plant won’t take in the chemical.
For palmer amaranth – or any weed – the earlier farmers catch them, the better.
Other modes of action
Faurot also recommends that farmers change up their mode of action.
“I usually spray with three modes of action to attack weeds,” he said, adding many farmers use a combination of 2,4-D, Dicamba and glyphosate.
But there are others. For instance, mixing atrazine and paraquat is a good combination for controlling weeds in the fall. Paraquat is a potent chemical and it defoliates the plant, Faurot said.
“I wouldn’t go with the same stuff all the time,” he said.
He also recommends mixing in ammonium sulfate to the tank mix, as well as surfactant, an additive that will help farmers get better coverage.
Also, don’t cut back on chemical. Follow the label. Once you damage the weed, it becomes even tougher to kill.
“You just can’t skimp on chemicals,” Faurot said. “You have to kill those weeds the first time around. If you damage it, you won’t kill it the second time around.”
Sometimes it is better to invest in more expensive chemical, he said.
“Sometimes it it is cheaper to put on the expensive stuff the first time than come in and do a rescue treatment later,” he said.
What other producers are doing
In Reno County, farmer Jud Hornbaker is using different mixes of chemicals to combat weeds. That includes the herbicide Sharpen for pre-plant burndown. For post emergence, he uses Anthem and Roundup.
Next year he will have Liberty in the tank mix, another post-emergent herbicide. He also uses Warrant, 2,4-D and Dicamba. He uses about 17 to 20 gallons of water an acre.
“The main thing is to get the weeds small, not wait until they are 3 feet high,” he said.
McPherson County farmer Monte Dossett also has several different modes of action to combat Palmer amaranth pigweed, including pre-emergent herbicide Authority and Zidua.
He has also used Fierce and Warrant. The chemicals work best when he can maintain soil moisture. The water in the soil helps make them work.
He applies a pre-emergent a month before planting and right after planting – along with Roundup once the plants emerge.
Some chemicals have gone up considerably, Dossett said.
“Before the weed resistance, I’d do one pre-emergent, and a lot of people wouldn’t do any,” he said.
Why were weeds so bad in Kansas this year?
Why is pigweed pressure heavier this year than in recent years? Increasing pigweed pressure in soybeans has been the trend in Kansas over the past 10 years or so, increasing a bit overall every year. But this year pigweed pressure has taken a big step up.
There are two main reasons for what is happening this year:
1. Glyphosate resistance is spreading. More populations of Palmer amaranth and waterhemp are now resistant to glyphosate. Waterhemp populations have been resistant to glyphosate for several years. Glyphosate resistance in Palmer amaranth has been a more recent occurrence, and resistant populations are now increasing rapidly within the state.
2. Wet weather in May and early June. The rainy pattern in May and early June delayed planting and caused producers in some areas of Kansas to plant later than expected. As a result of both the wet soils and the delayed planting, the effectiveness of early preplant herbicides had worn off by the time the beans were planted. Pigweeds began emerging in some cases before the beans could be planted. Then when the soils dried out enough to plant beans, producers had to hurry their operations and may not have had time to apply burndown or preemergence residual herbicides.
Producers who are still trying to rely primarily on post-emergence herbicides to control pigweeds are having an increasingly hard time getting good control. It used to be that glyphosate would provide 95 percent or more control of both waterhemp and Palmer amaranth even if those weeds were a foot tall or more. But now, glyphosate provides poor control of pigweeds on many fields in Kansas.
Weed Scientist, Kansas State University