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Weed Madness

Driven mad by herbicide-resistant weeds? Here’s how they start, what they are, and how to manage them.

How Resistance Spreads
Surprised at how a field full of herbicide-resistant weeds results? It had its origins years earlier. Birds do it, bees do it, even educated fleas do it, as the old Cole Porter song goes.

So why can’t waterhemp (or weeds) do it?

That last line probably isn’t funny if you’re battling herbicide-resistant weeds. Still, it says much about the way herbicide-resistant waterhemp infests your fields in the first place.

That field full of weeds – waterhemp, Palmer amaranth, marestail, or something else – started some time before.

Field Flirters
It’s akin to when a point guard catches a cute courtside TV reporter’s eye. Waterhemp is dioecious, with male and female plants. Sparks quickly fly.

“Since waterhemp is dioecious, pollen is already moving around,” says Pat Tranel, University of Illinois (U of I) weed scientist. “Pollen can be viable up to 120 hours, and it can move ½ mile from the pollen source.”

Initially, resistance is rare
Every herbicide selects for its own failure, even a brand-spanking-new one. In every weed species, rare genetic biotypes exist that resist a herbicide. This varies among herbicide sites of action, says Ian Heap, director of the International Survey of Herbicide-Resistant Weeds.

Rare herbicide-resistant biotypes exploit management uniformity, much as a star center who shreds double-teaming defenders.  Every herbicide application heightens the odds that resisters survive.

How quickly?
Really quick in the case of some weeds like Palmer amaranth. It’s double trouble if the weeds are herbicide-resistant biotypes. By producing up to 1.8 million seeds per plant, glyphosate Palmer amaranth infested 20% of the field areas in less than two years in a 2008 University of Arkansas analysis.

You may be doing a laundry list of best-management practices to forestall resistance. Rotating herbicides’ sites of action. Lacing your row-crop rotation with small grains. Regularly scouting fields and rounding oddball weeds.

Unfortunately, all can be for naught due to an often overlooked resistant-weed spreader: waterfowl.

Kevin Bradley, University of Missouri Extension weed specialist, reports an MU trial found that weed seeds can remain intact after passing through a mallard duck’s digestive system.

Think you’re cruising in a corn planter that goes 10 mph? It has nothing on mallard ducks, which can fly up to 48 mph for up to 38 hours. Potentially, they can move weeds like Palmer amaranth, waterhemp, common lambsquarters, giant foxtail, and smartweed nearly 1,740 miles in 1½ days.

Glyphosate seems synonymous with herbicide-resistant weeds. In most cases, though, weeds resist multiple sites of action. Here are herbicide action sites that waterhemp resists and initial U.S. confirmation year.

  • ALS Inhibitors - 1993
  • Triazines - 1994
  • PPO inhibitors - 2001
  • Glycines (Glyphosate) - 2005
  • Synthetic auxins (2,4-D) - 2009
  • HPPD inhibitors - 2009

“Waterhemp won’t stop at six herbicide sites of action,” says U of I’s Tranel. “It will develop resistance to anything being developed, whether it’s the seventh, eighth, or ninth.”

It’s important to note that no waterhemp biotype exists in all six herbicide sites of action. Still, multiple site resistance abounds. In 2009, for example, weed scientists confirmed an Illinois  waterhemp biotype that resisted four sites of action.

Nightmare crosses between pigweeds occur, but so far at a low rate.

Glyphosate-tolerant waterhemp and Palmer amaranth are bad now, but imagine how fearsome they could be if this weed equivalent of pro basketball’s Stephen Curry and LeBron James teamed up together! Could they cross-pollinate to create some sort of freak frankenweed?

Now that would be bad – and it is currently happening with pigweed species like waterhemp, Palmer amaranth, and spiny amaranth.  

“There is some hybridization occurring,” says Kevin Bradley, University of Missouri Extension weed specialist. “There are some weeds I walk by and say, ‘Wait a minute. That doesn’t look quite right.’ The leaves will look like waterhemp, but the head looks like Palmer.”

2012. That’s the year Bill Molin, USDA-ARS plant physiologist in Stoneville, Mississippi, began tracking spiny amaranth and Palmer amaranth hybrid. Resistant to both glyphosate and ALS herbicides, the hybrid is the result of a cross between Palmer amaranth growing in a cotton field and spiny amaranth in a neighboring pasture.

Palmer and waterhemp are doing it, too
Researchers at the University of Illinois and Colorado State University have also discovered amaranthus hybrids of Palmer amaranth and waterhemp.

“The hybrids often look similar to the normal variation of the species,” says Pat Tranel, University of Illinois weed scientist. In many cases, hybridization between species can only be confirmed by genetic analysis, says University of Missouri’s Bradley.

The Good News
Imagine if that imaginary basketball player brimmed with hybrid vigor by crossing Curry with James. Fortunately for you, this hybrid vigor so far doesn’t extend to amaranthus species. Palmer amaranth and waterhemp shoot bricks when it comes to hybridizing. In 2011, field studies by Colorado State University weed scientists showed that glyphosate-resistant Palmer amaranth transferred glyphosate resistance to:

  • Spiny amaranth 0.4% of the time.
  • Waterhemp 0.2% of the time.
  • Smooth pigweed 0.01% of the time.

Remain watchful, though.

“I think the danger in Missouri is with pastures between crop fields,” says University of Missouri’s Bradley. “There, Palmer amaranth that transfers resistance to spiny pigweed in the middle of a pasture is something of which we are aware.”

It’s akin to the anxiety a coach feels when his star forward, at midshot, flashes an on-court smile at a cheerleader. “Wherever these two plants are close together, there is a chance to hybridize,” Molin says.

How to control?
Double- and triple-team it just as you would a 40-point-a-night forward. “Broaden the spectrum of herbicides used,” says Molin. It’s important that this spectrum include herbicides with multiple effective modes of action, he says.

Look beyond the field, too. Control weeds in fencerows, ditches, and other places these weeds can grow, he says.

Even more good news
Low hybridization of pigweed species also means low rates of hybridized seed being shed.

Still, bear in mind that the average waterhemp plant sheds 250,000 seeds. If waterhemp hybridizes just 0.2% of the time, the plant still sheds 500 seeds that can germinate and emerge the next year.

Precision weed control

No new tools are on the horizon, but there’s still technology to help you manage weed issues.

Herbicide-resistant weeds aren’t man-made, but that’s not to say man isn’t to blame. “Farmers have been selecting for herbicide-resistant weeds,” says Lisa Behnken, Extension educator for the University of Minnesota. “They do that by being very predictable.”

Herbicide-resistant weeds occur naturally in the weed population. Farmers became more predictable, and the repeated use of the same chemical tools caused resistant weeds to become a larger part of the population. “Waterhemp became resistant to ALS herbicides before glyphosate technology was even around,” says Behnken.

Missing free throws frustrate the crowd on game day. The same goes for weed specialists when applicators continue to use flat-fan spray nozzles. “They haven’t adopted the air-induction nozzles,” says Fritz Breitenbach, Extension specialist for the University of Minnesota.  “We’ve done enough work to show that they’re superior.” It’s a simple correction that could change the outcome of the game.

In a University of Minnesota study, Behnken found that only 9% of farmers were mapping weeds. “We’d like to see that change,” says Behnken. As you are going across the fields during harvest, map the weeds. That allows you to spend money wisely on your herbicide program, and you can focus on the spots where you have more difficult problems.

You only have one first chance to prevent a weed before it gets established, says Russ Higgins, University of Illinois Extension educator. Once it’s established, it becomes widespread across the field, and then it becomes costly and difficult to control.  

Make Multiyear Plans
Last-minute-play calls are no longer acceptable management plans. Weed control needs to be a system approach, which includes multiple practices, says Behnken. “The majority of farmers are lucky if they make a one-year plan.”

Instead, she recommends farmers create multiyear plans with backup plans. In those plans, Behnken recommends tactics that have both chemical and nonchemical tools. Also, don’t include the same herbicide or mode of action every year.

Few new weed-control tools exist, says Breitenbach. “They have to rediscover old tools,” he says. Breitenbach recommends that you go back to incorporating cultural practices for more comprehensive weed control, such as:

  • Reevaluating planting dates
  • Considering cultivation
  • Diversifying rotations


Ever run into the former high school wallflower at a class reunion who now mimics a flashy NBA star? In the weed world, that’s waterhemp.

Why Waterhemp?
Waterhemp used to just line banks near streams and rivers. If this small-seed broadleaf did meander into your crop fields, tillage would bury it. No more. These days, less tillage ensures this shallow soil dweller has ample germination and emergence.

Season-Long Emergence
This emergence – which has started as early as late March in Illinois – can continue all through July, says Pat Tranel, University of Illinois weed scientist.

Late Flusher
Late-season flushes separate waterhemp from other weeds. A 1996 Iowa State University study showed that most wooly cupgrass emerged May 10-20. Peak waterhemp emergence didn’t start until June 24.


  1. Use full rates of preemergence residual herbicides as close to planting as possible.
  2. Follow with overlapping contact and residual postemergence herbicides.
  3. Apply multiple effective sites of action to help manage multiple-resistant weeds.

1 Million
The seeds one waterhemp plant can make with no competition, such as in a prevented-planting case.

It pays to control weeds early. That’s because 1% is the soybean yield loss that results when control occurs on 6-inch waterhemp with infestations of fewer than 10 plants per square foot.

Waterhemp is the league scoring champ when it comes to corn and soybeans. It infests row crops all the way from Louisiana to North Dakota. It’s particularly thick in Iowa and Missouri, infesting 90% or more of soybean acres.  

Palmer Amaranth

That’s some wingspan. The seed head can reach up to 3 feet long!

Palmer amaranth is an aggressive, invasive weed native to the desert regions of the southwest U.S. Facing a drought? That won’t slow Palmer. Given it originates from the desert, it’s drought-resistant.  

Palmer is akin to a towering center who grows every game. It grows nearly 2 inches per day. It’s also as prolific a scorer as a 3-point shooting guard.

The cheapest way to manage a weed is to prevent it from getting established in a field, says Iowa State University’s Bob Hartzler. He encourages farmers to hand-weed, especially the female plants, before it is established.

Why is Palmer a challenge? It’s already knocked out two main defenses. Most populations resist glyphosate and ALS-inhibitor herbicides. If you let Palmer amaranth play out all season, yield loss could be up to 91% in corn and 79% in soybeans.

Double Team
It’s easy to confuse Palmer and waterhemp seedlings, says Hartzler. Waterhemp has a shorter petiole and longer, narrower leaves than Palmer. The best trait to differentiate seedlings is the presence of petioles longer than the leaf blade on Palmer. Once they have flowered, the sharp bracts on female Palmer are a dead giveaway.

Diversity of weed management must expand beyond herbicides, says Mike Owen, Iowa State University Extension weed scientist. Repeated use of the same chemical tools selects for resistance. Instead, try a combination of cultural practices and chemical tools for control.

Harvest infested fields last. Otherwise, your combine will spread Palmer seeds. Flag areas where you spot Palmer amaranth, says Russ Higgins, University of Illinois Extension educator. When it’s in the introduction stage, there’s still an opportunity to eradicate it before it becomes a widespread threat, he says.

Palmer infests row crops from Louisiana to Ohio to North Dakota. In a 2015 survey, it was on 60,000 acres in Kentucky, according to University of Kentucky’s J.D. Green. Nearly one third of Illinois counties have documented cases of Palmer.


Lack of crop diversity and no-till spur this winter annual’s spread.

Marestail is number one, but not in a good way. It was the first glyphosate-resistant weed confirmed in the U.S. when it surfaced in Delaware in 2000. Since then, it’s moved at full speed into other areas like the Midwest.

Fall forward
Marestail rosettes often appear in the fall in the northern U.S. A fall application of 2,4-D or dicamba plus chlorimurin or metribuzin products offers the best residual control, says the University of Missouri’s Kevin Bradley.

Preemergence herbicides are a must! Larger weeds are tough to kill. Limited postemerge corn options exist, and there are even fewer in soybeans, say University of Nebraska weed scientists.

Springing back
Spring and summer flushes of marestail south of I-70 in the Corn Belt are becoming more common. This likely is signaling a change in plant biology, due to herbicide selection and changes in crop rotation.

Why not rye?
A cereal rye cover crop planted after corn, soybean, or wheat harvest can nix fall marestail emergence and aid control. “A glyphosate and Fierce preemerge provides nearly 98% control of marestail at planting,” Bradley says. “Take out the cereal rye, and the same treatment provides just 40% control.”

Glyphosate-resistant marestail migrated to the mid-South after its 2000 Delaware discovery. It now infests 90% of Kentucky soybean acres. It’s since moved to the Midwest and now plagues an estimated 30% of Iowa’s soybean acres.

Giant Ragweed

This old fencerow and ditch dweller now infests crops

Its 17-foot maximum height would make it any team’s starting center. Typically, though, it’s 1 to 5 feet taller than the field’s crop. Giant ragweed biotypes exist that have resisted ALS inhibitors (Pursuit) in seven states and EPSP synthase inhibitors (glyphosate) in 12 states. There are confirmed giant ragweed biotypes resisting both herbicide action sites in Ohio, Minnesota, and Missouri.

A single plant can produce an estimated 1 billion pollen grains during its lifetime. Its pollen triggers more than allergies; it creates cross-plant pollination potential. This creates genetic diversity that creates more herbicide-resistance potential.

Why is it a problem?
Giant ragweed used to dwell in undisturbed areas like fencerows and drainage ditches. Since the late 1980s, it’s moved into cropland. It’s now prevalent in cropland due to:

  • Crop rotation. It surfaces more when corn is rotated with soybeans vs. continuous soybeans.
  • Tillage. Buried giant ragweed seed escapes surface weed predators like insects and mice.
  • Stem-boring insects. They enable large giant ragweed plants to survive glyphosate applications.

The most effective herbicide programs combine preemergence and postemergence herbicide treatments with two or more herbicide sites of action. Scouting after the first post trip can pinpoint escapes.

Giant ragweed infests more acres in the eastern Corn Belt. In Kentucky, for example, giant ragweed infests 40% of soybean and corn acres, Still, infestations occur farther west, with infestations estimated at around 10% in Iowa, mainly in north-central regions of that state.

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