Agronomist outlines natural pigweed controls

Pigweed, also known as Palmer Amaranth, is a pesky pain to passels of farmers.

But that weed and others are just part of the game to Dale Strickler, agronomist for Green Cover Seed, of Bladen, Nebraska. He spoke at Soil Health U, hosted by High Plains Journal, Jan. 22 and 23, in Salina, Kansas. 

“If you’re not experiencing some mental anguish, your brain’s not growing,” read a quote from an unknown author he flashed on a big screen in a meeting room at Tony’s Pizza Events Center.

Strickler, of Belleville, Kansas, displayed photos of pigweed in a field that was sprayed six times with Roundup herbicide.

“This is a jungle,” he said. “Now Roundup doesn’t kill anything, and if we use dicamba-tolerant crops and (after) constant sprays of dicamba, soon we’re gonna have dicamba (herbicide) resistance.”

Pigweed is a resilient weed, Strickler said, with a “fibrous tap root that’s very good at extracting water from soil. Pollen from pigweed trails for miles. One resistant pigweed can create resistant (offspring) miles away. It outgrows everything, has a different method of photosynthesis.”

Plowing is not a solution, he said, and shallow tillage promotes pigweed emergence.

“The deeper you bury the seeds, the longer they live in the soil,” Strickler said, “and you can be sure that today’s pigweeds plowed under will be a problem for your grandchildren.” 

But while Palmer Amaranth is “an amazing plant,” he said, “those weeds have a kryptonite,” referring to natural solutions of control.

“Pigweed seeds are tiny and they contain very little energy, and they have to have a burst of light to germinate. If you have a thick mulch cover, they’re going to find it very difficult to survive,” Strickler said. “Pigweeds can only grow three-quarters of an inch before they need light. They’re very intolerant to shade, and they have to get to sunlight quick or they will die.”

Pigweeds need nitrogen in the nitrate form, he said, so using a winter cover crop that takes up the available nitrogen in the field “can starve pigweeds out.”

If the next crop is soybeans, Strickler said, “They make their own nitrogen, so they don’t care.”

If the rotation calls for corn, sorghum, wheat or another crop that needs nitrogen, he said, producers can choose a legume cover crop, such as hairy vetch.

“It uses nitrogen, but also fixes nitrogen, takes it out of the atmosphere and puts it into protein. When this is killed and left as a mulch,” Strickler said, “protein from the hairy vetch cover crop will release nitrogen slowly throughout the growing season as it decays. A starter fertilizer close to the row can provide nitrogen needs for the crop until the mulch begins to decay.”

Another strategy to solve the pigweed problem, Strickler said, is a 7.5-inch row spacing.

“It results in fewer weeds than 30-inch row spacing,” Strickler said. “Shade from a crop is the best herbicide you can invent.”

He advocates rotating to alfalfa and mowing weeds before they go to seed.

“It eliminates 90%,” Strickler said. “Alfalfa is a good competitor.”

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Regarding tillage, he said, “I haven’t seen a weed yet that’s resistant to iron, but it often takes at least three trips to kill every weed in a field.”

And tillage is very damaging to soil structures and reduces water holding capacity, Strickler said, by 50,000 gallons per acre compared to no-till soils. V-blades to undercut the unwanted vegetation, he added, are one of the tillage tools least destructive to soil structure.

For those using chemicals for control, he warned, “Post-emergent contact herbicides will not kill weeds over four inches (tall). Contact killers only kill weeds they touch.”

Tim Unruh can be reached at [email protected].