“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.