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YOUR CORN might not be the only hybrid you find in your field. While corn breeders have been making strides since they first hybridized corn back in the '30s, Mother Nature has been doing her part, too. She's been quietly letting weeds exchange pollen and create their own hybrids.

Two of the weeds that hybridize easily are waterhemp (Amaranthus rudis) and smooth pigweed (Amaranthus hybridus). But why should you care if you have waterhemp-pigweed (or waterpig? pighemp?) in your fields? Well, just as corn yields skyrocketed with hybridization, hybrid weeds could benefit from the same process and possibly bring herbicide resistance along for the ride.

In addition to waterhemp-pigweed hybrids, researchers are studying other possible problem hybrid combinations, including common and giant ragweed, wild and perennial sunflowers and members of the marestail family.

Already hybridized

Chances are you've been dealing with some sort of hybrid weed already, says Patrick Tranel, weed scientist at the University of Illinois. He's been working on weed hybrids since 1997, when he tried to classify the differences between common and tall waterhemp.

“It's impossible,” Tranel says. “If you could find pure versions of each, there would be differences. But now, out in the field, they're really the same species.”

Tranel's work naturally progressed to other weed hybrids. Because waterhemp and pigweed are cousins, he decided to see if herbicide resistance could transfer between these species.

The simple answer is, it can. But it doesn't happen often. Here's why: Pigweeds are monoecious, meaning they have both male and female parts on the same plant, and waterhemp is dioecious, meaning a plant is either male or female, not both.

“It's a simple matter of who gets there first,” Tranel says. “Since pigweed's flowers are pretty tightly packed and it can self-pollinate, it usually does.” In studies with pigweed planted adjacent to rows of male waterhemp, around 3% of the progeny were hybrids.

For the reverse — pigweeds pollinating female waterhemp — nearly 30% were hybrids, mainly because there wasn't much waterhemp pollen with which to compete.

“However, just because you're forming a hybrid doesn't mean there will be gene exchange,” Tranel says.

Waterhemp-pigweed hybrids are nearly sterile, so they produce only a few seeds. Given the thousands — or even hundreds of thousands — of seeds that waterhemp and smooth pigweed can produce, this seed-producing disparity puts weed hybrids at a real disadvantage. However, the few seeds that a hybrid manages to produce usually germinate and grow into very healthy plants.

“These ‘second-generation hybrids’ are often fully fertile and may look just like waterhemp, or just like smooth pigweed,” Tranel says. “So next time you see a smooth pigweed plant in your field, consider that it may have waterhemp genes — maybe even waterhemp genes for herbicide resistance.”

Herbicide resistance

The fact that glyphosate resistance has been confirmed in smooth pigweed's cousin, Palmer amaranth (Amaranthus palmeri), which is sometimes called Palmer pigweed, is making weed scientists nervous.

“The biggest concern for us right now is Palmer amaranth,” says Mike Burton, weed scientist at North Carolina State University. He points out that researchers are in the process of testing whether biotypes found in parts of the state are resistant to glyphosate. “We're very worried about how widespread it might already be in North Carolina and the possibility of the exchange of genetic material with other Palmer populations and pigweed species, since we already have ALS [acetolactate synthase]-inhibitor resistance [in Palmer amaranth],” Burton says.

If the two pigweed species — Palmer amaranth and smooth pigweed — can trade genetic material with each other, or with populations of other herbicide-resistant weeds, such as waterhemp, Burton is concerned that farmers could end up dealing with weeds that have resistance to several herbicide families. Remaining herbicide options would be limited. And more selection pressure would be placed on weed populations to adapt and overcome the remaining effective herbicides.

Weed scientists are not certain how weed hybridization and the existence of weed species with resistance to multiple herbicides will play out. (See sidebar for details on where resistant weeds have been confirmed.) They are sure that they need to know more. In addition to investigating hybridization of waterhemp and pigweed, researchers are studying hybridization of giant and common ragweed, perennial and common sunflowers and several species of the horseweed/marestail family.

The marestail scenario

Marestail has the dubious honor of being the first glyphosate-resistant weed found in the U.S. Because the species is prolific, and because it readily outcrosses, scientists are concerned about what hybridization with related species could imply.

Conyza canadensis [marestail] is an outcrossing species,” says Mike Owen, weed scientist at Iowa State University. “It has the perfect flower; it is self-compatible; but it will also accept outside pollen from other plants of the genus. It's an incredibly prolific seed producer of wind-disseminated seed (up to 250,000 seeds/plant). And it has a variable germination habit; it'll germinate in the fall or early spring, even though we call it a winter annual.”

So far, Owen's research has shown that glyphosate resistance in marestail is transferred as a single semidominant gene, not unlike what growers are used to seeing with ALS-resistant weeds.

“If a resistant marestail plant pollinates a glyphosate-sensitive marestail plant, all of the progeny are going to be glyphosate resistant,” Owen says. “When outcrossing occurs, changes tend to happen very subtly over time. But it results in weeds that are better adapted and creates the next bad actor we have to address.”

Marestail's genus, Conyza, has hundreds of members. And with the tendency for many of them to accept pollen from close relatives, the potential for outcrossing is something that has Owen and others taking notice.

Ian Zelaya, a post-doctoral researcher who worked with Owen, was able to make laboratory crosses between glyphosate-resistant marestail and highly glyphosate-sensitive dwarf fleabane, another member of the Conyza genus. His research also demonstrates the tendency for those resistant progeny to be even more resistant than their parents. Although Owen won't speculate about what that increased resistance means, he says it does warrant more scrutiny.

“It demonstrates very clearly that these traits can move around,” Owen says. “While for now it's more of a scientific anomaly, we could indeed see some new weeds. And it's possible that we could evolve a new glyphosate-resistant weed — extremely remote, but possible.”

Slowing the spread

Weed scientists recommend rotating herbicide chemistries to avoid buildup of resistant weeds. But with more farmers choosing Roundup Ready corn to go with their Roundup Ready soybeans, glyphosate is becoming a bigger player on more acres on both sides of the rotation. (For information on why weed resistance is becoming more common, and what can be done about it, see “Weed resistance gains,” page 34.)

“Everyone thinks weed management is simple and that glyphosate is simple to use. But that's just not true,” Owen says. “As we narrow the selective forces imposed upon a weed population to one herbicide, it's not surprising that Mother Nature will find a way to overcome it. The bigger issue is not herbicide resistance; it's protecting economic potential. The good news is the answer to both issues is one and the same.”

That answer is to manage weed selection pressure by choosing preemergence or residual herbicides to supplement glyphosate applications, Owen says.

“Every time you use a product, you're selecting,” says Aaron Hager, weed scientist at the University of Illinois. Waterhemp has become resistant to multiple herbicides in Illinois. In fact, Hager works with one farmer who has triple-resistant waterhemp — a population resistant to ALS and protoporphyrinogen oxidase (PPO) inhibitors, as well as to triazines. That farmer is concerned about resistance to glyphosate, his only remaining post option in soybeans, so he's now using soil-applied residual herbicides prior to glyphosate, Hager says.

By studying how weeds work and which genes are transferred, scientists hope to better understand their enemy rather than just throw new herbicides at them, Tranel says. Besides, there aren't many new herbicides coming down the pipeline.

“The scary thing about resistance is that it's not an on/off switch,” Hager says. “It's a gradual buildup over time, but when it's really significant enough for the farmer to notice the herbicide didn't do a good job controlling these weeds, there's a pretty good chance he's already suffered a yield loss.” And in upcoming years, weed-control options will be more limited.


Scientists have confirmed these weeds can survive a normally lethal dose of a herbicide.

IN ORDER for resistance to be “proved,” scientists must follow rigorous steps outlined by the International Survey of Herbicide-Resistant Weeds. They are required to document, and then reconfirm, that a plant is able to survive and reproduce after receiving a normally lethal dose of a herbicide, and that it passes that ability on to the next generation. In addition, it must be shown that resistance has an impact in the field.

These rigorous protocols provide assurance that weeds aren't randomly declared resistant without sound science. Because fulfilling these requirements can take several seasons, those weed escapes in your fields could be resistant weeds even if scientists in your state haven't confirmed the existence of a resistant biotype. That said, here's a list of resistant weeds and the states where they've been confirmed:


Glyphosate-resistant biotypes have been confirmed in Arkansas, California, Delaware, Indiana, Kentucky, Maryland, Mississippi, Missouri, New Jersey, North Carolina, Ohio, Pennsylvania and Tennessee. Marestail biotypes with multiple resistance to glyphosate and acetolactate synthase (ALS) inhibitors were confirmed in Ohio in 2003.


Resistance to ALS inhibitors has been confirmed in Illinois, Iowa, Kansas, Michigan, Missouri, Ohio, Oklahoma and Wisconsin. The weed also has confirmed resistance to Photosystem II inhibitor herbicides in Illinois, Iowa, Kansas, Missouri and Nebraska. Two states have confirmed dual resistance to ALS and Photosystem II inhibitors — Illinois and Kansas. Illinois also has waterhemp biotypes with triple resistance to ALS, Photosystem II and protoporphyrinogen oxidase (PPO) inhibitor herbicides.

Smooth pigweed

Resistance to Photosystem II inhibitors has been confirmed in Delaware, Illinois, Kentucky, Maryland, Massachusetts, New Jersey, New York, North Carolina, Pennsylvania, Virginia and Wisconsin. It's also ALS-inhibitor resistant in Delaware, Illinois, Kentucky, Michigan, Ohio and Virginia.

Palmer amaranth

Resistance has been confirmed to ALS inhibitors in Arkansas, Georgia, Kansas, North Carolina and South Carolina. It has Photosystem II inhibitor resistance in Georgia, Kansas and Texas and glyphosate resistance in Georgia. According to Mike Burton, weed scientist at North Carolina State University, it's suspected to have glyphosate resistance in Mississippi, Missouri, North Carolina and Tennessee.

Ragweed species

Resistance to Photosystem II inhibitors, ALS inhibitors and, in 2004, glyphosate has been confirmed in several states. Common ragweed has been confirmed to be resistant to Photosystem II inhibitors in Michigan and New York; ALS inhibitors in Illinois, Indiana, Michigan, Minnesota and Ohio; and glyphosate in Arkansas and Missouri. Giant ragweed has been confirmed to be resistant to ALS inhibitors in Illinois, Indiana, Iowa and Ohio.

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