Think outside the jug

As weeds evolve to gain the upper hand against herbicides, control strategies must advance to tackle multiple forms of resistance. Single-year solutions are rarely enough. It’s time to create a four-year plan for a corn-soybean rotation that thinks outside the jug.

Palmer amaranth, the aggressive pigweed cousin of nefarious waterhemp, has created havoc for farmers across many states. It has joined the ranks of multiple-herbicide-resistant weeds such as waterhemp, kochia, marestail and common ragweed.

Amit Jhala, a University of Nebraska weed scientist, said nine weeds have confirmed resistance to one or more herbicides in Nebraska. Added to the list above are giant ragweed, Johnsongrass, redroot pigweed and shattercane.

Weed resistance has occurred to these widely used herbicides: ALS-inhibitors (Group 2), atrazine (Group 5), glyphosate (Group 9) and PPO-inhibitors (Group 14). Jhala said some populations of Palmer amaranth and waterhemp are resistant to two or three herbicide groups with varying modes of action (MOA), including groups 2, 5, 6, 9, 14 and 27.

“Palmer amaranth has been the greatest challenge for the past three to four years. Our take-home message at field days and Extension presentations is for growers to reduce the overall weed seed bank in the soil,” Jhala said.

The weed scientist explains three critical challenges with Palmer amaranth that make this task daunting: 

1. It emerges from May through August (in Nebraska).
2. One female plant can produce from 200,000 to 500,000 seeds, depending on growing conditions and competition.
3. Seeds can survive five to six years in the soil.

“We’ve even seen 6-inch-tall Palmer amaranth female plants that emerged in July or August produce 10,000 seeds at the end of the season,” Jhala said.

Developing a four-year plan

While success can be a long and winding road, herbicide-resistant Palmer amaranth can be beaten. To start, preemergence residual herbicides are essential as the foundation for preventing the emergence of herbicide-resistant weeds. 

“We are fortunate that we have several really good preemergence herbicides labeled for corn and soybeans, which are a premix of two to four different herbicides and MOA,” Jhala said.

Given a four-year alternating corn-soybean rotation, Jhala encourages growers to use the university’s Guide for Weed, Disease, and Insect Management in Nebraska mode of action (MOA) charts to avoid repeating the exact herbicide MOA in corn and soybeans.

“By planning effective herbicide MOA used for Year 1 in soybeans, Year 2 corn, Year 3 soybeans and Year 4 corn, growers can ensure they don’t repeat MOA,” Jhala said. “Using this four-year cycle, you can then repeat the same herbicide selection in years 5 through 8 without increasing herbicide resistance.”

Since no herbicide combination eliminates all weed emergence, more growers are turning to cover crops and planting green to achieve the biomass needed for soil armor to suppress weeds.

“Our studies with fall-seeded cereal rye cover crops after harvesting corn or soybeans over the past four years have shown there’s no need to terminate the cover crop until about 10 days after planting corn or soybeans,” Jhala said. “Longer cover crop growth can achieve 5,000 pounds per acre of biomass. Planting green into the actively growing cereal rye before terminating creates a solid residue mass on the soil surface to reduce Palmer amaranth emergence.”

Stop seed production

The second message Jhala emphasizes to growers is not to allow emerged weeds to go to seed. To accomplish this, stacked-trait corn and soybean varieties, as well as postemergence herbicides, play a critical role.

“For example, Enlist E3 soybeans were planted on 70% to 75% of the soybean acres in Nebraska this year. It gave growers the flexibility to use glyphosate to kill grass weeds or glufosinate-based herbicides to kill glyphosate-resistant Palmer amaranth,” Jhala said.

Enlist One^® (2,4-D choline) can also be used to kill numerous broadleaf weeds, including glyphosate-resistant broadleaf weeds. The main caution is to stay within the four-year plan without duplicating the application of the same MOA. With the Enlist® corn trait available in the market for the last few years, growers must be conscious to incorporate additional MOAs if planting Enlist soybeans as part of the four-year rotation.

Monitor your plans

Harvest is a good time to take notes on which fields had weed escapes and then employ cover crops after harvest. It’s also a time to tweak strategies if a component of your four-year plan didn’t deliver the desired control.

“For example, some growers find greater success by overlapping residual herbicides,” Jhala said. “Within your four-year plan, follow a preemergence residual herbicide combination with a post program that includes residual herbicides that can be tank-mixed. This can further reduce late-season Palmer amaranth emergence.”

As multiple herbicide-resistant weeds take over fields, some farmers are taking drastic measures, such as adding weed seed destruction devices to the back of combines. 

“My colleagues have tested some of these attachments, which can crush 85% of the weed seeds into dust,” Jhala said.

Weed control is no longer a simple thought process of using this herbicide to control those weeds.

“Growers must make longer-term plans to reduce the overall weed seedbank in the soil and use different management practices with herbicides to solve these difficult problems,” Jhala said.

For a deeper dive:

• Guide for Weed, Disease, and Insect Management in Nebraska
• Herbicide Classification Chart
• Difference between Mode of Action (MOA) and Site of Action (SOA)

 Content provided by DTN/The Progressive Farmer

Atrazine is a Restricted Use Pesticide.
Enlist E3^® soybeans are jointly developed and owned by Corteva Agriscience LLC and M.S. Technologies LLC. Enlist One^® is not registered for sale or use in all states or counties. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Enlist One is the only 2,4-D product herbicide authorized for use in Enlist crops. Always read and follow label directions.