Weed mitigation in soybeans
Herbicide resistance is an increasing problem for soybean growers. Resistance leaves fewer tools for operators to combat weeds.
Vipan Kumar, Ph.D., presented “Integrated Weed Management Strategies in New York Soybean Production” at the recent Soybean & Small Grains Congress hosted by Northwest New York Dairy, Livestock & Field Crops Program. Kumar is an associate professor and Extension weed management specialist.
He said that glyphosate- and atrazine-resistant Palmer amaranth has been confirmed in seven New York counties, including “multiple populations from Ontario and Yates counties in 2024.”
Kumar shared the results of research on Palmer amaranth plants grown in a greenhouse for experimentation purposes. Roundup® PowerMAX at 32 fl. oz./acre and atrazine at the same rate had little effect on the amaranth plants. Callisto® at the rate of 6 oz./acre proved the most effective of all the treatments.
Herbicide-resistant waterhemp has also lowered yields for New York soybean growers. Kumar said nearly two dozen sites across several counties statewide have waterhemp populations.
“Out of 23 tested populations, nine exhibited four-way resistance and different levels of resistance,” Kumar said. “Whole plant dose response confirms the first report of multiple herbicide resistant waterhemp populations to atrazine (six- to 28-fold), Synchrony® (more than 130), glyphosate (19- to 22-fold) and Callisto (nine- to 15-fold).
“Based on survival percent, that is a concern for managing resistant waterhemp,” he said.
In Kumar’s on-farm study, “Roundup was not very effective, which is not surprising considering this species is resistant.”
He added that a wet spring helped to activate pre-emergent herbicides and using pre-emergent herbicides caused soybean injury.
“A two-pass approach of Trico® and Classic™ followed by Enlist One® and Liberty® provided consistent control all season,” Kumar said.
Italian ryegrass has also become a problematic weed. “We have established a glyphosate resistance with Italian ryegrass for farmers using it as a cover crop,” Kumar said.
After 21 days, Durango® applied at 432 oz./acre provided “pretty consistent control,” he said.
In a greenhouse study, Kumar tested Select Max, Accent Q, Axial and Gramoxone on suspected resistant plants. Accent Q provided the best weed control.
Kumar also shared research on glyphosate-resistant horseweed, which is becoming more widespread in New York State. Sharpen®, Liberty Ultra and gramoxone all “did a pretty good job of burning down glyphosate-resistant horseweed,” Kumar said, compared with Enlist Duo® and Enlist One.
At one of his test sites researching the efficacy of Roundup PowerMAX 3 and Roundup PowerMAX 3 + Dual II Magnum®, Kumar saw some benefits from two-pass programs. “This was not very well established. We will repeat these trials this year,” he said.
He also compared chemical and non-chemical weed interventions in 2024 and 2025. The treatments included inter-row mower, electric weeder, Liberty + Enlist One, seed impact mill and inter-row mower + electric weeder + impact mill.
Compared with the non-treated control area, the lowest weed biomass was found when using an herbicide, then inter-row mower followed by electric weeder; then inter-row mower; and then electric weeder. Soybean yields in this trial reflected the weed biomass results, as the highest yields were in the areas with the most effective weed suppression.
Kumar also noted new EPA restrictions for 2026 registration of over-the-top dicamba use, including:
- A maximum of two applications of 0.5 lbs./acre may be made annually, for a maximum of 1 lb. of all dicamba products annually
- Doubled volatility reduction agents – 40 oz./acre of approved volatility reduction agents must be added to every application
- Mandatory conservation practices – growers must achieve three runoff/erosion mitigation points from EPA’s certified conservation practices menu on each treated field to protect endangered and threatened species
- Temperature-based application limits – for applications occurring on a day with a forecasted temperature between 85º and 95º F on the day of or the day after application, a user may only treat up to 50% of their untreated dicamba-tolerant soybean acres in a county
