The Great Cover-Up

Adding cover crops to your arsenal helps to control Palmer amaranth.

Cover crops can smother Palmer amaranth, reducing the seed bank of the nasty weed. (Progressive Farmer image by Matthew Wilde)

The nastiest of pigweeds is threatening the livelihoods of Midwest farmers. Randy Uhrmacher is worried, but the Nebraska farmer has the problem covered.

Palmer amaranth, a hard-to-kill noxious weed native to the Southwest, has tormented row-crop farmers and slashed yields in that region for decades. Since its migration north more than five years ago, the pest is doing the same thing in this area.

Weed experts say farmers should fear Palmer amaranth. It's hardier than its cousin and look-alike, waterhemp, which is hard to kill in its own right. Palmer can reduce corn and soybean yields by 91 and 71%, respectively, according to the University of Nebraska.

"Three years ago, we had a train wreck on about 400 acres of soybeans due to Palmer," Uhrmacher admits. "We weren't the only ones. Our yields were off by 10 bushels per acre (bpa), but some (other farmers') fields, the real train wrecks, made 10 bushels per acre where Palmer took over."

Uhrmacher initially tried glyphosate and other herbicides to control Palmer amaranth with limited success.

Glyphosate-resistant Palmer amaranth was first confirmed in Georgia in 2005. The aggressive, adaptable weed also has confirmed resistance to other herbicide modes of action -- PPO inhibitors, ALS inhibitors, HPPD inhibitors, microtubule inhibitors (dinitroanilines), triazines (atrazine), metolachlor and 2,4-D.

"This weed seems to adapt to everything we throw at it," Uhrmacher says.

That was enough for Uhrmacher, who farms near Hastings, to declare war on Palmer amaranth.

Uhrmacher attacked the weed on multiple fronts. He sprayed dicamba in conjunction with planting Roundup Ready 2 Xtend soybeans that are tolerant of the herbicide, which did a much better job controlling Palmer compared to his previous program. Cover crops were added to the weed-control arsenal, and the smothering qualities of cereal rye enhanced efforts.

After cover crops are terminated in the spring, either shortly before or after planting, the deteriorating biomass creates a mat that shades, smothers or outcompetes weeds but allows row crops to grow. Rye needs to be at least 1 ½ to 2 feet tall before burndown to suppress weeds in the spring and summer, experts say.

Uhrmacher says Palmer amaranth is no longer the yield-robbing threat it once was in his fields.

"We're finding things that work, and hopefully they keep working," he says. "We're trying different herbicides, more residuals, spraying quicker and using cover crops."


Uhrmacher only seeded 200 acres of cover crops after the 2018 harvest because of wet weather and delays, less than half of the 2019 acres. Fifty acres of soybeans were planted into green, living cereal rye last spring. Corn, which was chopped for silage, was planted into the other 150 acres of cover crops.

He's sold on the practice.

"Cover crops did a better job controlling Palmer in corn and soybeans than herbicides," Uhrmacher says. "There were more weed escapes in fields without it."

If hail hadn't pummeled cover-crop acres in late August, Uhrmacher says soybean yields and silage tonnage would have easily matched or exceeded noncover acres at 70 to 78 bpa and 27 to 31 tons per acre, respectively.

"They were by far my best-looking beans until the hail hit," Uhrmacher says. "The cereal rye suppressed weeds and eliminated one pass of residual herbicide, which easily paid for it."


Limiting, or better yet eliminating, weed escapes is crucial to control Palmer amaranth, experts say. A single plant can produce hundreds of thousands or even a million seeds.

Cover crops are an effective method of weed control in conjunction with chemical options, explains Kevin Bradley, a University of Missouri Extension weed scientist and professor.

"There is no question that cereal rye biomass will help reduce waterhemp and Palmer amaranth emergence," Bradley says. "Cover crops are a great tool for weed management, but they are just one tool."

Recent studies conducted by the University of Missouri show cereal rye can slow waterhemp emergence with little to no impact on soybean stands. Bradley says farmers can expect similar results for Palmer amaranth since it's a close relative of waterhemp.

The university scientists examined waterhemp emergence after planting cereal rye, with seeding rates from 0 to 110 pounds per acre. A seeding rate of 70 pounds per acre resulted in about 50 waterhemp plants to emerge per square meter. That's compared to 120 waterhemp plants with no cover crop and 90 waterhemp plants at 30 pounds of cereal rye per acre. Results were shared at a pest-management field day in July.

Cover-crop weed-control benefits:

> Some species release chemicals from roots
or decaying residue, which can inhibit weed
seed germination.

> Altering the moisture and temperature environment in the soil surface can deter weed seed germination and emergence.

However, there can and always will be challenges associated with the practice.

The weather can prevent timely cover-crop planting and termination. The relatively short growing season in the Upper Midwest can limit the amount of cover-crop biomass accumulated.


Bob Hartzler, Iowa State University Extension and Outreach weed specialist and agronomy professor, says Palmer amaranth is definitely controllable with vigilance and proper management. It's found
in about half of Iowa's
99 counties.

"The lack of a large, established Palmer amaranth seed bank in nearly all of Iowa's crop fields makes this a winnable fight," Hartzler says. "It doesn't jump out of the ground in the spring here like in the South due to colder temperatures. That makes it easier to control early when they are 2 inches tall and not 8 inches."

Cover crops are an effective weapon against waterhemp and Palmer amaranth, he adds. "For someone struggling to stay on top of weeds, it should be something to consider."

> Follow Matthew Wilde on Twitter: @progressivwilde

Palmer Amaranth 101 (Source: University of Nebraska):

> Palmer amaranth is a flowering plant and a member of the pigweed family.

> It's an erect summer annual dioecious (distinct male and female plants) species.

> It grows 2 to 3 inches per day and can grow to 8 feet tall.

> Palmer amaranth closely resembles waterhemp, but there are differences: Palmer has a denser canopy and leaves with petioles longer than the leaf blade. Female Palmer flowers have large, sharp bracts that are painful to the touch when mature.

> Some Palmer amaranth plants have a silverfish watermark on the leaves.

> Leaves on Palmer amaranth are often clustered tightly at the top of the plant.

> Mature plants have larger and thicker stems
than waterhemp.


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