The Truth About Pigweed

March 26, 2011 12:29 AM
The Truth About Pigweed

Rows of tiny seedlings fill the greenhouses at Southern Illinois University. Weed scientist Bryan Young plucks a pot and proclaims it to be Palmer pigweed. "No … wait, it is waterhemp," he says.
Side by side, the two weeds look remarkably similar in their baby stages. They should. Both hail from the same Amaranthaceae or pigweed family.

Although the plants have more distinct characteristics as they reach adulthood, the kissing cousins have both come to be considered with contempt—and sometimes outright awe—by farmers.

There are approximately 60 species recognized in the Amaranthus genus. Many are cultivated as herbs or grain around the world. As a flower, the blossoms have coaxed words from the literary likes of Aesop, Milton and Coleridge. Rock bands have found inspiration in "the never fading flower." Rich in vitamins, some species are considered important in the treatment of hypertension, high cholesterol, gout and even rheumatism.

Farmers like Steve Stevens simply get a headache and a sharp pain in the pocketbook when they think about pigweed. "Palmer amaranth is currently the biggest threat there is in Southern row crop production," says Stevens, who raises soybeans, cotton and rice near Tillar, Ark. "Those who do not adopt a zero-tolerance policy when it comes to this weed will find it quickly outcompetes crops."

Amaranthus palmeri—aka Palmer amaranth, palmer pigweed or carelessweed—is now considered problematic in much of the South, but Young says Midwest farmers should be on alert.

"Our 2010 weed surveys in southern areas of Illinois picked up a lot of Palmer, and it is almost all resistant to glyphosate," Young says. "We still don’t know if the Midwestern version of Palmer amaranth will be as aggressive, but it will definitely compete with crops."

Amaranthus tuberculatus—aka waterhemp—on the other hand, has now invaded most of the Midwestern states and can be found as far south as Texas and as far north as Canada.

Last summer, University of Illinois weed scientists Aaron Hager, Dean Riechers and Pat Tranel confirmed waterhemp as the first weed to evolve resistance to HPPD-inhibiting (Group 27) herbicides. Waterhemp had already exhibited resistance to ALS inhibitors, triazines, glyphosate and PPO inhibitors (Groups 2, 5, 9 and 14). Another case of HPPD resistance was confirmed in Iowa in 2010.

"A fifth example of resistance in one weed species is overwhelming evidence that resistance to virtually any herbicide used extensively on this species is possible," Hager says. "We’ve already discovered one waterhemp biotype that’s resistant to four different herbicide families," he says. "This is not a weed species that can be managed with one or two different herbicides. It needs a very integrated approach."

Why so ornery? The plants are very efficient at fixing carbon and well-adapted to high temperatures and intense sunlight. Palmer is more aggressive than waterhemp, setting a taproot that can extend 5'. If not killed early, Palmer pigweed can grow as tall as a small shade tree with a stem the size of a ball bat. It is not cutterbar friendly.

Both waterhemp and Palmer amaranth are dioecious species, meaning there are separate male and female plants. The male produces pollen and the female the seed. Amaranthus family members are not a bit shy about sharing their resistance genes among populations and biotypes.

One female plant can produce 500,000 to 1 million seeds per plant that continually germinate throughout the summer. Multiple flushes, an obnoxious quantity of seed and the ability to grow rapidly put it in a class of its own.

Pollen problems. Palmer amaranth is also unique because resistance in this species can travel through pollen (wind) dispersal, says Stanley Culpepper, University of Georgia weed specialist.
"No matter how effectively you keep seed out, you can’t prevent pollen movement. Once resistance arrives through pollen, the massive seed production of the female plant provides the avenue for glyphosate-resistant Palmer amaranth plants to take over the farm in as little as two years," Culpepper says. Palmer amaranth pollen can travel thousands of feet.

When glyphosate-resistant Palmer pigweed was found in Georgia in 2004, it was localized to a few fields on about 500 acres. Today resistance is spread across 65 counties, infesting more than 1.5 million acres.

"It’s possible to drive across the countryside and see who is using more than glyphosate and being timely when doing so," Culpepper notes. Overlapping residual herbicides, changing trait technologies and using as many as five to six chemistries with distinct sites of action is part of the solution.

"Mother Nature still has to cooperate. It doesn’t matter how good you are or how timely if those residuals don’t get activated by rainfall," he says.

Some Georgia farmers have moved to deeply tilling the soil to reduce pigweed seed germination, and heavy cover crops, such as rye, which provide a thick mat between plant rows, are also being deployed. Some 54% of the cotton acreage in Georgia was hand-weeded in 2010 at an average cost of $23.70 per acre. Growers averaged $63.45 per acre for herbicide controls. "We are blessed with current cotton prices, but these input costs to control one weed are not sustainable," Culpepper says.

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