What can dry edible bean farmers use to control Palmer amaranth and waterhemp?
Joe Ikley, North Dakota State University Extension weed specialist, conducted trials in edible fields near Fargo, North Dakota, and in Barnes County, North Dakota, on herbicide resistant pigweed.
No one has found Palmer amaranth that is resistant to acetolactate synthase — or ALS — inhibiting herbicides in North Dakota edible bean fields yet, but when it happens, Joe Ikley wants to have an herbicide program in place to control it, along with ALS-resistant waterhemp, which has been reported.
Waterhemp was one of the top three worst weeds in 2022 of the dry bean acres in the Northarvest Bean Growers Association region, according to annual edible bean growers’ surveys. The resistance of waterhemp to ALS-inhibiting herbicides is especially concerning because many dry edible bean herbicide programs rely on an ALS-inhibiting herbicide, imazamox.
Meanwhile, while herbicide-resistant Palmer amaranth hasn't been found in edible beans in North Dakota, it has been found in several counties in the Northarvest region, just not in beans. The challenges of controlling the weed will be similar to controlling ALS-resistant waterhemp if it becomes established in dry edible bean fields, said Ikley, North Dakota State University Extension weed specialist.
Ikley in 2021 and 2022 conducted trials in fields near Fargo, North Dakota, and in Barnes County, North Dakota, to determine the two ALS inhibitor resistant weeds’ reaction to a variety of herbicides. Data on the effectiveness of pre-plant incorporated herbicides on waterhemp and Palmer amaranth is limited.
The two weeds have longer emergence windows than redroot pigweed or Powell amaranth, so herbicides, including Eptam, Sonalan and Treflan that provide longer season residual control, were tested on the Palmer amaranth and water hemp. Spartan, Dual and Outlook were tested as pre-emergence options.
Post-emergence options were limited. Reflex is the most effective product labeled for dry beans. Single applications of full rates, and multiple applications of split rates of bentazon and fomesafen were evaluated for efficacy on ALS-resistant water hemp and Palmer amaranth.
The addition of a residual herbicide to post-emergence applications is important for controlling water hemp and Palmer amaranth in many crops, including dry beans. However, fomesafen and dimethenamid-P are the only herbicides with effective residual activity on ALS-resistant pigweeds that can be applied over the top of dry beans.
If a pigweed population is resistant to protoporphyrinogen oxidase — or PPO — inhibiting herbicides, then dimethenamid-P or S-metolachlor are the only viable options remaining.
Ikley applied the residual products at various dry bean growth stages to determine the optimal time to achieve residual control of late emerging pigweeds until the dry bean crop can canopy.
All field experiments were conducted on a Group 2-resistant water hemp population in Fargo, North Dakota, and on a Group 2-resistant Palmer amaranth population in eastern North Dakota.
All field experiments were conventionally tilled and were conducted in a RCBD with four replications. Experiments on water hemp were established in May 2021 and 2022 at the Fargo location.
Experiments on Palmer amaranth control were established in early June 2021 and 2022 at the research location in Barnes County. Preplant incorporated treatments were applied, then incorporated with a rototiller set to 4 inches. The North Dakota Palomino variety of pinto beans were planted at 70,000 seeds per acre in 30-inch rows, then pre-emergence treatments were applied.
Herbicide efficacy ratings were collected every two weeks after planting. Pigweed biomass was collected and the trials were terminated at eight weeks after planting.
Herbicide efficacy and crop injury ratings were collected one, two and four weeks after initial post-emergence application. Pigweed biomass was collected four weeks after initial biomass application. Yields were collected at the Fargo location for the waterhemp trial.
Overall field conditions at the Palmer amaranth location were dry at trial establishment in both years, and field conditions generally were dry at the Fargo location after planting in both years.. The blanket pre-plant incorporated application of Eptam and Sonalan provided good control several weeks into the season. That led to light infestations of water hemp and Palmer amaranth for the targeted post-emergence applications in these trials. Overall, the treatments with Reflex provided the best control of both water hemp and Palmer amaranth while any treatment without Reflex did not provide adequate control.
Reflex tank-mixed with other herbicides outperformed Reflex by itself. Control of waterhemp with treatments containing Reflex varied between 62% and 96%, while Palmer amaranth control ranged from 36% to 69% with the same treatments.
Ikley knew from earlier research in soybeans that many post-emergence herbicides had failed to control both of those weeds.
His research trials in 2021 and 2022 showed that there were even fewer to control the weeds in edible beans. In fact, Raptor, Pursuit and Permit, the three options, don’t control weeds in edible bean fields with ALS-resistant Palmer Amaranth and water hemp.
Reflex herbicide will control ALS-resistant weeds when the plants are in the recommended growth stage of 2 to 3 inches high.
“We basically have that one chemical,” Ikley said.
That means farmers in counties where or near where waterhemp has been reported should scout their fields early in the growing season for the weed.
Though there only is a single option to control the ALS-resistant waterhemp and Palmer amaranth post-emergence, Ikley’s research trials showed that there are plentiful pre-plant incorporated and pre-emergence herbicides available for control of the weeds.
Pre-plant emergence herbicides are activated by water and pre-planting incorporated herbicides are activated by tillage.
Group 3 herbicides Treflan, Sonalan and Prowl, commonly called the “yellow” herbicides, had a rate of 90% control of ALS inhibitor-resistant waterhemp and ALS inhibitor-resistant Palmer amaranth, Ikley said.
For the most effective control, he suggests edible bean farmers in or near counties where water hemp has been reported use pre-plant emergence or pre-plant incorporated herbicides at planting, followed by a post-emergence treatment if the weeds are found in fields.