And waterhemp makes seven — seven as in the number of weeds in the Mid-South with documented resistance to glyphosate herbicide.
Glyphosate-resistant waterhemp was confirmed in early June by Delta Research and Extension Center weed scientist Vijay Nandula, from seed collected in 2008 from a field of Roundup Ready soybeans on a farm in southern Washington County, near Hollandale, Miss.
The seed were planted and subsequent offspring screened for resistance to glyphosate. For comparison, Nandula acquired susceptible waterhemp seed from Missouri.
“We monitor it to see if it goes to seed,” Nandula said. “We usually screen the first generation, spray them with the labeled rate of the herbicide at a given growth stage and let them go to seed. After we cross them, we need to show that the resistant gene is transferred into the next generation. If it’s not transferred, then you don’t have as much of a problem.”
Resistance was transferred to the second generation waterhemp plants, and Nandula documented a five-fold resistance to glyphosate. If you’re counting, that’s 19 glyphosate-resistant weeds confirmed in the world, 10 in the United States, seven in the Mid-South and five in Mississippi.
Waterhemp, or Amaranth tuberculatus, is closely related to Palmer amaranth, noted Jason Bond, DREC weed scientist and Delta Farm Press contributor. In fact, the two are often confused for one another. Both are dioecious, meaning they have male and female plants. Waterhemp and Palmer pigweed frequently cross pollinate with one another.
This makes chasing down resistant biotypes in the Mid-South a bit like shooting at a moving target, say weed scientists. Open pollination, which is characteristic of dioecious plants, “facilitates moving genes around,” noted DREC weed scientist Tom Eubank. “A lot of genetic information gets exchanged in a dioecious plant versus a self-pollinating species like morningglory.”
“It’s rare that you see a true waterhemp, or a true Palmer amaranth. You end up with weeds with characteristics of both,” Bond said.
“More often than not waterhemp will tend to have a red stem, but you see a lot of Palmer amaranth that has a red stem as well,” Bond said. “Palmer amaranth will have a very long petiole, 1.5 times to 2 times as long as the leaf blade, where waterhemp tends to not have as long a petiole. But once you get out in a field, those aren’t very reliable characteristics.”
Like Palmer amaranth, waterhemp is a fast-growing plant, although they do tend to be bushier and more shrub-like than Palmer amaranth. Waterhemp also produces a tremendous amount of seed, as much as 250,000 per plant.
Weed scientists note that Palmer amaranth is starting to pop up in heavier-textured soils, including rice soils, perhaps as a consequence of cross pollination with waterhemp, which typically prefers heavier-textured soils.
As in glyphosate-resistant Palmer amaranth, there are few chemicals that provide adequate control of glyphosate-resistant waterhemp. In tests Nandula conducted on postemergence treatments on emerged waterhemp, only 2,4-D and Callisto provided better than 90 percent control. “Most glyphosate-resistant Palmer amaranth and pigweed species are also resistant to ALS-inhibiting herbicides,” Nandula said. “They were already observed to be resistant to ALS herbicides before the arrival of glyphosate-resistant crops.”
Recommendations for control of waterhemp are the same as they are for Palmer pigweed, Bond said. “In cotton, it’s going to be residual herbicides before or at planting. In corn, it’s going to be atrazine and Dual Magnum along with the bleaching herbicides like Callisto and Laudis. Those are going to work as well on waterhemp as they do on Palmer amaranth.”
“Postemergence options are very limited in cotton, corn and soybeans,” Eubank added. “Even if you get 90 percent control, you’re talking millions of seeds potentially falling on the ground, leaving 100,000 plants emerging. That’s not acceptable. It’s going to take multiple herbicide treatments like we used to do years ago.
“I talked to a farmer recently who had been farming for 60 years. He said he felt like he was going back to the 1970s and 1980s, when pigweed was the hardest thing to control. He’s thinking he’s going to have to go back to plowing and putting out the residuals.”
Eubank said there are only a few post-emergence options for control of waterhemp in soybeans, including Flexstar and Blazer or Cobra. “But the weeds have to be really small for those chemicals to control them.”
Eubank said waterhemp “has the capacity to be just as severe as Palmer amaranth. In my history working with waterhemp, it’s always been a little harder to control than Palmer.”
Glyphosate-resistant waterhemp has also been documented in the Midwest, noted Nandula.
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