“Do we need to till or not?” Purdue University weed scientist Bryan Young often hears this question from Midwest soybean growers fighting herbicide-resistant marestail, waterhemp and Palmer amaranth.

In parts of the south, multi-herbicide-resistant Palmer pigweeds have forced growers to include or intensify tillage, Young says. Likewise, in western Kansas, glyphosate-resistant kochia has led some dryland wheat growers to resort to tillage.

But before Midwest farmers put steel to the ground to attack resistant weeds, he says, it’s important to understand weed biology. Tillage affects not only emerged weeds but also germination and weed seed banks. Tillage decisions must also balance weed control and soil conservation. Equally important, growers who resort to tillage for weed control should also adopt a diversified herbicide program.

Young says to think about weed emergence patterns and tillage timing first. Conventional pre-plant tillage is an effective way to control winter annuals such as horseweed and summer annuals that germinate early such as giant ragweed.

Keep in mind that you need more aggressive tillage for weed control than for residue management. For example, vertical tillage tools “can be a hindrance as much as a help in weed control,” says agronomist Monty Webb, Southern FS, Inc., Marion, Ill. “Often, vertical tillage injures weeds but doesn’t kill them.” While the root systems remain intact, injured weeds don’t take up herbicide well, so they are very hard to kill, he notes. For spring vertical tillage, Webb recommends a burndown first to avoid spraying injured weeds.

The biggest weed problem for Jarrett Nehring, a Murphysboro, Illinois no-till farmer, is waterhemp with three-way resistance to glyphosate, ALS and PPO herbicides. Pre-plant tillage is useless for managing these weeds, which emerge after corn. “Waterhemp is still emerging when corn is head high,” he says.

When it comes to pigweeds, pre-plant tillage “just moves the seeds around in the soil,” Young says. In fact, spring tillage can actually intensify waterhemp pressure after planting by triggering more germination. “In terms of weed emergence, tillage is like turning on a light switch,” he adds. “Pigweeds will come faster and there will be more of them.”

In Illinois trials last year, tillage performed May 20 knocked down emerged waterhemp plants but caused a six-fold spike in waterhemp emergence over the following two weeks, compared to no tillage. Tillage on June 7 sparked a 14-fold increase in waterhemp emergence. The same thing happened with Palmer amaranth.

One-time deep tillage

By contrast, deep tillage inverts the soil and buries about 80% of weed seeds below the germination zone.

Certified crop adviser Stephen Morfeld, Linn, Mo., worked with a couple of farmers who pulled out the moldboard plow to deal with Roundup- and Flexstar-resistant waterhemp. One grower selectively plowed about 15 acres of a 50-acre continuous soybean field, burying weed seeds 10 inches deep through the heaviest populations. The grower also rotated the field to corn and switched up his chemical program.

“There was very low waterhemp pressure the next year,” Morfeld says. Deep tillage combined with rotation and herbicide diversification proved effective, and the benefits continued into the following soybean year.

“If you have a bad waterhemp or Palmer amaranth problem, we know that one deep tillage operation to bury the seed can get that to a more manageable level,” says Kevin Bradley, University of Missouri weed scientist. “It’s not 100% effective, but there’s a dramatic reduction.” Missouri trials last year found that inverting the soil with a moldboard plow slashed pigweed densities five- to eight-fold compared with conventional tillage, minimum tillage or no tillage.

Earlier research from the University of Arkansas found a similar reduction, says Jason Norsworthy, University of Arkansas plant scientist. “We can take some selection pressure off herbicides by burying high weed seed populations,” he says. That allows other weed control measures to work better, too. For example, deep tillage combined with a cover crop of cereal rye cut Palmer amaranth emergence by about 93% over two years — more than double the reduction with deep tillage alone, Norsworthy says.

A last-resort tactic

Norsworthy and Bradley caution that deep tillage to control waterhemp or Palmer amaranth is a one-time-only tactic, however.

This strategy exploits two pigweed weaknesses, Bradley says: These small-seeded weeds, which are relatively short-lived in the soil, cannot emerge from lower depths. A 2001 Iowa State University study found that 89% of a waterhemp seed bank was no longer viable after being buried for four years.

“But that’s not 100%,” Norsworthy says. You can draw down the seed bank with deep tillage, “but you’re never going to eradicate resistant weed seeds.” And once resistance has evolved, “you almost never lose it.”

Running the plow every year will just bring resistant seeds back up to the germination zone, he says. “You want the seed to rot in the soil. Once you turn the soil over, you must prevent weeds from going to seed so you don’t have to flip the soil again.”

Although deep tillage can be effective, Morfeld points out that “it’s not always practical for our farming operations now. We’ve re-tooled, changed our practices.” While it’s a tactic some growers may have to consider, he warns that it should be used a last resort.

It’s especially heartbreaking to see longtime no-tillers take steel to their soil again after decades, adds Morfeld, who has no-tilled for 40 years. “You lose your no-till advantages overnight.”

Because deep tillage raises erosion risk and destroys soil structure, “We don’t recommend this everywhere,” Bradley agrees. “But for nightmare situations and landscapes that are not highly erodible, it can make really bad field more manageable.” If you decide to go this route, “Do it once, and then get religion — don’t let it get that bad again.”

No-till advantages

Because no-till leaves waterhemp and Palmer amaranth seeds in the top inch of soil where they can germinate, some farmers think no-till is the worst choice for controlling these resistant weeds.

Not so fast, Young says. Seeds near the surface are more likely to be eaten by birds, insects and rodents. Predation removal in no-till can amount to a third or more of the seed bank, Young says, although losses are highly variable. For example, cover crops provide good habitat for mice, resulting in higher seed predation.

Seed mortality is greater in the top layer of soil too, Young says.

Georgia researchers reported in 2013 that the persistence of Palmer amaranth seed was directly related to the depth of burial. After 36 months, 9% of seeds in the top half-inch of soil were still viable, while 15% of seeds buried four inches deep remained alive.

Likewise, a study on waterhemp seed persistence in Illinois from 1996 to 2000 found that more than 10% of seeds buried 2 to 8 inches deep germinated after five years in the soil, compared to 3% in the top half-inch, Young says.

“In a deep-tilled environment, you might remove about 80% of the weed seeds from the germination zone, which alleviates some of the pressure for the following year,” he says. “However, you might increase the seed longevity in the soil in the following years.”

No-till production systems are also more likely to employ diversified herbicide programs that incorporate multiple modes of effective action, Young says. That’s confirmed by the Benchmark Study, a multi-state research program carried out from 2006 to 2010 at 154 sites in the Midwest and south. The study found that soybean growers who use tillage are nearly twice as likely as no-tillers to rely on glyphosate alone. This suggests that “the most at-risk sites for developing glyphosate resistance may be tilled fields,” Young says.

“So far, we’ve left tillage out of it. We started applying fall chemicals to keep marestail from coming up,” says Nehring. In the spring, they switched their management to kill waterhemp before the crop emerged. “We’re very aggressive with our pre-plant herbicide program. We scout and then apply another residual post to keep down emergence until the canopy forms. We also maintain crop rotation so we get corn chemicals every other year.”

Young advises using pre-emergence residual herbicides and tank mixes that are effective on problem weeds, even if you achieve a clean seedbed with tillage. “Tillage can’t be a replacement for a good, sound integrated resistance management program,” he says.

Mechanical control may be the only post option for some resistant weeds

Mechanical methods may be the only way to control multi-herbicide-resistant waterhemp or Palmer amaranth that emerges after soybeans. “Inter-row cultivation won’t save the day,” says Purdue weed scientist Bryan Young, “but it can help, especially if there are no effective herbicides available.”

In the mid-south, where resistant waterhemp and Palmer amaranth have become very difficult to control with herbicides, more growers have been forced to cultivate soybeans in season, says Jason Norsworthy, University of Arkansas weed scientist.

“If we look back in Arkansas seven or eight years, we couldn’t find a row cultivator,” he recalls. Today, about half the state’s 3.5 million soybean acres are cultivated in-season because of resistant pigweeds, and about a quarter are hand-hoed to remove weed escapes and prevent additions to the weed seed bank, he says.

The cost of hand weeding depends on weed densities. In Arkansas soybeans, Norsworthy estimates an average of $29/acre, but the price can go much higher. However, if growers are proactive and keep the weed seed bank down, hand-weeding can cost as little as $5 or $10/acre, which is less than the cost of tillage or herbicide application, he says. 

Tillage for weed control: Pros and Cons

Pre-plant tillage

Inter-row cultivation

Deep tillage

Sources: Bryan Young, Kevin Bradley, Jason Norsworthy, Monty Webb, and Stephen Morfeld