Deep tillage buries seeds of weeds that can't be killed by herbicides

COLUMBIA, Mo. -- Deep-six weed seeds to control pigweeds and other herbicide-resistant pests in soybean fields.

"I'm not advocating a return of moldboard plowing," said Kevin Bradley, University of Missouri weed scientist. "Burying weed seeds with deep tillage does provide one more tool in dealing with resistant pigweeds."

Bradley spoke at Pest Management Day at MU Bradford Research Center, July 16. He was preparing crop producers for extreme controls needed for Palmer amaranth and waterhemp, both herbicide-resistant weeds.

"A return to tillage can help kill resistant weeds. Pigweed seeds like waterhemp and Palmer amaranth germinate only in upper soil layers, so they thrive in no-till fields," Bradley said. "Without soil turnover, weed seeds remain at shallow levels."

In a new study, Bradley's graduate student Jamie Farmer compares season-long weed seed germination under deep tillage, conventional till, minimum and no-till.

In the first year, no-till left 95 percent of weed seeds in the top inch of soil. Deep tillage left only 18 percent in the same soil level.

Bradley described deep tillage as a tool for use in extreme cases, the nightmare of out-of-control resistant weeds. That's when resistant Palmer amaranth overshadows the soybean field, sharply reducing yields.

When control is gone and resistant weeds take over, then bury the seeds.

The second part of the control strategy is to never allow resistant weeds to set seeds again. Kill resistant weeds before they set seed.

"Palmer amaranth and waterhemp control will take management of the seed bank in the soil," Bradley said. Deep tillage, which he called extreme tillage, can be an option once it infests a field. Then growers can return to no-till with more careful management after that.

Pigweed seeds buried 6 inches or more won't germinate. Only those near the surface sprout and survive. After being buried four to five years, the seeds die.

"Don't till the soil again, bringing buried seeds back up," he warned.

Over recent years, herbicide control won out over tillage by moldboard plowing. With these tough species, it's something to consider again, Bradley said.

Bradley continues to repeat his message to alternate use of a variety of modes of action in herbicides. Constant use, year after year, of the same herbicide led to development of weeds that can't be killed by the overused herbicide.

In any weed population there are mutant weeds that can't be killed by the herbicide used. With no competition, those few weeds thrive and multiply. The whole population of that species becomes a resistant variety.

Bradley has preached switching modes of action for years. However, the message failed to resonate with users.

At another stop on the weed tour, Bradley showed the enemy, known as Palmer pigweed or Palmer amaranth.

He'd pulled the exhibit weed from a soybean field in the Missouri River bottom, "within sight of the Capitol dome."

"Note, this weed is taller than I am and the base is thicker than my wrist," Bradley said. He called it a monster weed.

"It is very aggressive, more competitive than waterhemp," he said. Each plant produces about 500,000 seeds.

"Palmer amaranth is our No. 1 weed to watch in Missouri and the Midwest right now," Bradley said. "It's probably too late to lock the barn door; this weed is out there more than we know. Most farmers don't even know they have it."

Bradley said he sees Palmer amaranth in fields more than he's comfortable with. But he also found it in flowerpots at shopping malls and landscape beds around homes.