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How to beat Palmer amaranth in soybeans

Maybe there will or won’t be football this fall due to COVID-19. Still, get out your clipboard and start drawing X’s and O’s. A football-based strategy may be what it takes to beat back Palmer amaranth in soybeans. 

We modeled this Palmer amaranth playbook after a video created by Tommy Butts, a University of Arkansas (U of A) Extension weed specialist, that matched an Arkansas Razorbacks tight end (Palmer amaranth) running roughshod through multiple herbicide sites of action to which it has resisted. (It’s worth a look here.) We then paired the video concept with a paper Butts wrote with fellow U of A weed scientists Tom Barber and Jason Norsworthy, along with input from other weed scientists and other researchers. See Farmer Fran’s offense vs. Palmer Amaranth’s defensive lineup below, and keep scrolling to see how the game proceeds.

Farmer Fran's Offense

  • Quarterback: Preemergence Pete 

Overlapping burndown and residual preemergence herbicides may not get the glory of postemergence products. These days, though, they’re the workhorses of the herbicide world. Preemergence herbicides like metribuzin (Group 5) laced with Group 15 products like S-metolachlor (Dual Magnum), pyroxasulfone (Zidua), or dimethenamid (Outlook) reduce early-season competition and alleviate weed pressure for post products, say U of A weed scientists.

  • Receiver and backs: Postemergence Paul and His Gang

Overlap residual postemergence herbicides (including Group 15s) 21 to 28 days after planting. This can vary depending on environmental conditions. If needed, repeat with an overlapping postemergence treatment with residual two weeks later. 

  • Left tackle: Diverse Dan

Diverse rotations won’t work everywhere. Still, University of Kentucky researchers found in a 2013 trial that winter wheat helped nix early emerging Palmer amaranth in double-cropped soybeans compared with full-season soybeans.

  • Left guard: Corey Cover Crop

“Cover crops add competition that helps suppress weeds,” says Jennifer Hahn, executive director of the Minnesota Soil Health Coalition.

  • Center: Kayla Crop Competition

Planting early in narrow rows triggers a quick-forming canopy that smothers late emergers like Palmer amaranth. U of A research shows at least 90% of total emerged Palmer amaranth occurred before canopy closure.

  • Right guard: Sanitation Sam

Cleaning tillage and harvest equipment to remove Palmer amaranth seeds can nix transport from field to field, say the U of A weed scientists.

  • Right tackle: Dennis Ditch

In fall 2012, Purdue University weed scientists identified Palmer amaranth scattered along multiple roadside ditches in northwest Indiana that spread the pigweed through pollen and seed.

  • Special teams: Randy Rouger

Pull stray plants before harvest.

  • Special teams: Dave Deep Tillage

Deep tillage can destroy soil structure made possible by no-till. If all else fails, though, deep tillage via moldboard plow can reduce the Palmer amaranth population up to 50% in heavily infested fields, say Purdue University weed specialists. Do it just once, as  buried seed viable up to five years can redeposit in the top layer of soil with repeated deep tillage.

  • Mainstay postemergence soybean herbicides include:
    • ​Glyphosate (Roundup, Group 9)
    • Gllufosinate (Liberty, Group 10)
    • ALS inhibitors (Such as Pursuit, Group 2)
    • PPO inhibitors (Such as Flexstar, Group 14)
    • Synthetic auxins (Dicamba, 2,4-D choline, Group 4)

Palmer amaranth in a green soybean field

Palmer amaranth's defense

  • Right end: Yuri Yield Stealer

Data collected by Purdue University researchers show Palmer amaranth can ravage yields by 79% in soybeans.

  • Right tackle: Tall Thompson

Palmer amaranth can grow 10 feet tall with a circumference as thick as a baseball bat.

  • Left tackle: Prolific Patricia

Female plants can produce 1.8 million seeds per plant.

  • Left end: Dioecious Dick (and Diane)

Having dioecious plants – male and female – helps ensure cross pollination and increases the likelihood of transfer of herbicide-resistant genes.

  • Linebacker: Inchy Iden

Can grow up to 2.5 inches daily.

  • Linebacker: Dispersal Duane

Wind, animals, livestock feed, and equipment can disperse its tiny seed.

  • Linebacker: Seth Season-Long Emerger

Just when you think you have it licked, Palmer amaranth can still pke through a hole in a September soybean canopy.

  • Secondary: Resistance Row

Palmer amaranth resistance has surfaced in seven herbicide sites of action in the United States. They include:

  • ​ALS inhibitors (Group 2, Pursuit, Scepter)
  • Photosystem II inhibitors (Group 5, metribuzin)
  • PPO inhibitors (Group 14, Flexstar, Cobra)
  • HPPD inhibitors (Group 27, Alite 27)
  • EPSP synthase inhibitors (Group 9, glyphosate)
  • Synthetic auxins (Group 4, 2,4-D, and dicamba)
  • Microtubule inhibitors (Group 3, trifluralin, pendimethalin)

The Game: Farmer Fran vs. Palmer amaranth

Play One: 1st and 10 on Farmer Fran's 20

20-yard gain! Farmer Fran’s bulldozer offensive line paves the way for a dash out to the 40-yard line. That’s because of a good cultural program based on: Cover crops and planting soybeans early in narrow rows.

Farmer Fran planted winter cereal rye the previous year into which he plants 15-inch row soybeans early for the area (late April.) Farmer Fran terminates the cover crop shortly after planting. (Note: Planting “green” can carry risk in Southern states for insect infestations, says the U of A’s Butts.)

Play Two: 1st and 10 on Farmer Fran's 40

Another 20-yard gain! Farmer Fran’s preemergence plan pays off, as quarterback Preemergence Pete moves the ball into Palmer amaranth territory. Farmer Fran applies a mix of metribuzin (Group 5) with the Group 15 herbicide pyroxasulfone to further protect the soybeans against emerging Palmer. Farmer Fran also plants  a soybean variety tolerant to metribuzin, as some varieties are metribuzin-sensitive. 

Play Three: 1st and 10 on Palmer's 40

Farmer Fran wants to apply an effective postemergence herbicide on the herbicide-tolerant soybeans. However, rain, rain, and more rain prevents Farmer Fran from applying it 21 days after the preemergence chemical application. 5-yard penalty: Delay of Game.

Play Four: 1st and 15 on Palmer's 45

A post pattern leads to a 15-yard completion! Finally, Farmer Fran is able to apply the effective postemergence herbicide 28 days after applying the preemergence chemical.

Play Five: 1st and 10 on Palmer's 30

A 15-yard gain results from Farmer Fran’s diligence in keeping  farm implements as free of Palmer amaranth seed as possible. This creates a field free (so far) of Palmer amaranth.

Play Six: 1st and 10 on Palmer's 15

Stopped at the line of scrimmage! This was almost a sack because Farmer Fran did not apply a residual herbicide with the first  post application. A scramble in the form of another pass with an effective postemergence herbicide laced with a labeled Group 15 herbicide  adds residual control.

Play Seven: 2nd and 10 on Palmer's 15

White mold — a narrow row drawback— appears after it is too late to apply a fungicide. This removes Farmer Fran’s false sense of security for a bumper crop by cutting yields 5 bushels per acre. Five yard penalty: False start.

Play Eight: 3rd and 15 on Palmer's 20

Farmer Fran picks up 15 yards on a quarterback rollout (out of the pickup, actually). Seeing a few stray Palmer amaranth plants from the road, Farmer Fran removes them from the field.

Play Nine: 1st and goal on Palmer's 5

At harvest, Farmer Fran rolls through a field free of Palmer amaranth into the end zone. Touchdown!

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