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New, resistant pigweed hybrid confirmed in MississippiNew, resistant pigweed hybrid confirmed in Mississippi

New glyphosate-resistant pigweed hybrid found in Mississippi.From pastures to row crops.How did it happen? What are the implications?

David Bennett

March 13, 2014

8 Min Read
<p>A SPINY AMARANTH/Palmer hybrid discovered in a Mississippi cotton field. Notice the flower head with male flowers between zero and 3 inches and fuzzier female flowers between 3 and 5 inches.</p>

At this point, the confirmation of a new glyphosate-resistant weed is unlikely to raise eyebrows. However, nonchalance is hardly the suitable reaction when considering the implications of what USDA-ARS researchers have uncovered in a Mississippi cotton field in Lafayette County.

It turns out a weed that is commonly found in pastures, spiny amaranth, has made the leap into a row crop environment and is finding the new surroundings comfortable. It has done this through hybridization with the ubiquitous resistant Palmer pigweed found throughout the South.

In mid-March, Stoneville, Miss.-based researchers Bill Molin and V.K. Nandula spoke with Delta Farm Press about the field circumstances, the science behind the hybridization, and management options. Among their comments:

On the hybridization…

Nandula: “Everyone is likely familiar with (herbicide-resistant) Palmer amaranth and the havoc it has caused across the Southeast. The mechanism of resistance — the way by which the weed can withstand a herbicide application — is through ‘gene amplification’, i.e., multiple copies of the target enzyme of glyphosate, 5-enolpyruvylshikimate-3-phosphate synthase, are functional. Thus, glyphosate phytotoxicity is diluted, so to speak.

“In this case, we’re talking about a second weed, also an amaranth, commonly called ‘spiny amaranth.’ It’s actually not a very widespread weed, typically limited to pastures and other non-row crop areas.

“Back in 2011, (Mississippi State University weed researcher) Jason Bond brought us some plants that were suspected of being resistant and had traits like spiny amaranth. They came from a cotton field in Lafayette County. These plants were taken to seed. Plants generated from this seed turned out to be resistant to glyphosate at a 1X rate.

“There are several things to keep in mind. Palmer amaranth is a dioecious plant — that means male and female flowers appear on different plants. Spiny amaranth, meanwhile, is monoecious — the male and female flowers are not only on the same plant but also on the same flower stalk.

“Both spiny and Palmer amaranths produce extensive amounts of pollen and so it is likely that if these species were growing close to each other there may be some cross pollination. We have evidence that indicates it was a resistant Palmer female that was pollinated from a spiny plant. That’s probably how the spiny amaranth plants got the (herbicide) resistant gene.

“When glyphosate resistance is transferred to a hybrid, glyphosate now becomes the agent that selects for resistant plants. The seed we got from the 1X rate-surviving plants were replanted and sprayed again. We wanted to make sure the resistance gene had been passed on to another generation, which turned out to be the case. That would point to a real problem.

“Amaranthus weed species — and there are four or five others, like waterhemp — are all known to cross with each other. The most prevalent crosser is Palmer amaranth.

“When you have so many crosses, so many hybrids, it could lead to a buildup of resistance and a bad management problem.”

On moving from pasture to cotton field…

Molin: “Spiny is normally a relatively short pasture weed. Now, the hybrid that we’re studying is being found in row crops. So, the hybrid now has the ability to enter into new environments.

“It also has Palmer genes that have changed its look. It’s actually grows taller, like Palmer, so it’ll be more competitive on top of the resistance. It’s just one more weed that producers must be aware of. It could mean greater scouting, greater expense, more problems in keeping it out of row-crop fields.”


Your predictions? The possibilities of where this could go?

Molin: “With the formation of the hybrid, it can survive in farmers’ fields. It’s doubtful that regular spiny amaranth would be there.

“Also consider that one male Palmer plant in the middle of a field is incapable of producing seed. A single plant of this spiny hybrid is perfectly capable of producing seed. The prognosis is not good — this could turn out to be a problem just like Palmer.”

You’ve been at this since 2011?

Molin: “Yes, it takes time. You have to get the suspect plants, get seed, grow it out in the greenhouse and proceed through the testing.

“We’ve been back to the field a second time and collected plants this year. We found several that were resistant. So, we know this is a year-to-year problem. How quickly that problem will expand, we don’t know.

“Part of the issue with this is the seed are very small. It’s easy to move the seed from field to field on farm vehicles.

“One of the things we’re now doing is collecting seed from neighboring fields to see if the resistance and hybridization has spread. We don’t have that information, yet but at least in the one field population the resistance is being propagated.”

Has the population in that field gone up? Remained steady?

Molin: “We haven’t seen it during the cropping year so we’re still unsure of what’s exactly going on.

“The next phase of this from the field aspect is to collect soil samples this spring to be tested for the presence of the hybrid spiny. In the summer, we’ll return to the field and see what plants are up and how the grower is handling them.

“One of the things we noticed last fall was a lot of spiny at the entrances of other fields. We collected some of that and it’s in the process of being tested. The research into this is in the early stages and it’s fortunate it was caught fairly early.”

As researchers, the hybridization probably didn’t surprise you, right?

Molin: “There have been several studies — mostly with other Amaranthus weeds like waterhemp — showing hybridization under controlled lab or greenhouse conditions. Those studies show that ALS-chemistry resistance can be transferred from species to species.

“However, we didn’t expect that the resistance would transfer from Palmer to spiny. We didn’t know it would then move into an agronomic field. That was certainly a surprise.”

On the threat of resistant waterhemp…

Nandula: “Historically, Palmer amaranth is a dry-area weed while waterhemp prefers more swampy conditions. Waterhemp used to be limited to the Midwest. But because of the changing crop rotations pushed by commodity prices and custom harvesting equipment moving so much around the country, these weeds have spread all over the place. These weeds are being found in regions where they’ve never been seen before.

“One waterhemp population found in Illinois is resistant to four different herbicide families. When you have gene transfers, imagine pollen carrying resistance to four herbicide chemistries producing a brand-new hybrid. The results of these gene transfers really complicate weed management.”

Currently, the spiny hybrid is only resistant to glyphosate?

Molin: “We only tested for glyphosate. We’re most interested in glyphosate-resistance at this time. We’re preparing to see if there’s also resistance to Staple. However, if the Palmer transfers resistance to ALS chemistries — a big ‘if’ — that would be a problem in cotton with Staple. In soybeans, though, that resistance would mean a whole host of products would be removed from a grower’s control options.

“In 2013, the field with this resistant hybrid was converted from cotton to soybeans. So, there was obviously a change in the herbicides used. That’s another reason to check for more resistance.”


Recommendations? Treat it like a normal pigweed?

Molin: “Yes, treat it like you would a normal pigweed. Catch it early — two inches — and follow the recommendations from MSU for control of Palmer. If a grower uses the MSU recommendations for Palmer control, they should be able to control this spiny hybrid.”

How much greenhouse work is left before you have answers to your other questions?

Molin: “We should have those in another couple of months. That should go fairly quickly.

“However, everyone knows our ditch banks are simply loaded with pigweeds. Because we found this on the field’s edge, there’s a real concern that equipment moving in and out of the field can move seed into another field.”

Nandula: “(The spiny hybrid) can act as a back-up to the resistant Palmer already there.”

Has word filtered out on this yet? Are producers already looking for this?

Molin: “That’s part of the purpose for this article. We want growers to be aware this has happened.

“While it was predicted for other species, we didn’t expect it with spiny. A pasture weed is now being found in row crops. It may turn out to act very much like Palmer.

“Spiny amaranth has spines about a centimeter long. It’s a miserable plant to have in a pasture. Some of the spiny that have come out of the row-crop field also have the spines. So, that’s another characteristic that makes this an undesirable plant to have around.”

If farmers find something that looks suspicious do you want them to contact you?

Molin: “They can call me but should also alert the county agent. That would be great so we can identify any new populations and see where it has moved. We need to study this thoroughly.” ([email protected]; [email protected])

“Obviously, Palmer can hybridize with spiny. But it can also hybridize with waterhemp. If that happens, it would be a really serious development. If producers suspect they’ve got a spiny or waterhemp hybrid, let us know. We would like plant samples, as well.

“Of course, they also need to contact their Extension agents to make sure they have a proper control program in place. We must reduce the impact of these weeds.”

Anything else?

Nandula: “In Mississippi, we have lots of Palmer amaranth resistant to glyphosate. Some is also resistant to Staple, an ALS. However, we haven’t found any PPO resistance.”

About the Author(s)

David Bennett

Associate Editor, Delta Farm Press

David Bennett, associate editor for Delta Farm Press, is an Arkansan. He worked with a daily newspaper before joining Farm Press in 1994. Bennett writes about legislative and crop related issues in the Mid-South states.

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