Small soybean plant (copy)

Whether working in medicine, engineering or farming, today’s technological advances seem almost miraculous. Research and development have given us some astounding new products, and among these are the pre-emergent herbicides.

Like little guards standing patrol, a pre-emergent herbicide determines which tiny seedlings pass through from soil to sunshine.

Corn or soybeans – “Yep, you’re through.” Waterhemp – “Not so much.”

“Basically what they do is they’re preventing germinating weed seedlings from becoming established,” said University of Minnesota Extension educator Liz Stahl. “They’re either inhibiting the growth of the roots, the shoot or both.”

Pre-emergents have no effect on seeds, so they don’t work on weed seeds that are lying dormant in the soil. They also may not work as well on weeds that germinate from seeds lying deeper in the soil.

But, how do pre-emergents inhibit the growth of roots or shoots?

That’s where the “mode of action” comes in. The mode of action for pre-emergents affects something in the weed that makes things go wacky.

Dual Magnum, for example, acts as a fatty acid inhibitor. Other pre-emergents can inhibit lipid synthesis.

“It’s like a specific process or enzyme in the plant that is impacted,” Stahl said.

One of the ways corn or soybeans are not injured is because these crops often metabolize the pre-emergent herbicide differently than the targeted weeds.

“Corn or soybean may not have a target-site that’s sensitive to the herbicide,” she said. “There are a lot of different processes that differentiate crops from weeds – but again, it depends on the chemistry because some products do have a higher potential than others to cause some crop injury under certain conditions.”

It is recommended to put down a pre-emergent at planting to control waterhemp. Because waterhemp seeds can continue to germinate well into the season, applying another pre-emergent with a different mode of action about 30 days after planting can help keep this challenging weed in check. Following the label is always required with herbicides.

Selecting the fittest

Pre-emergent herbicides can be a very effective tool in weed control. What happens though, is weeds like waterhemp have tremendous genetic diversity. Maybe one in a million, or one in a billion seeds carries a genetic makeup or trait that results in a plant that can survive a particular herbicide. The resulting plant might be injured by the herbicide, but able to recover and produce potentially thousands of seeds that go into the seedbank.

Over the next couple of years, seeds from the resistant plant germinate, and if the same herbicide is used again and again, more plants will survive, each potentially producing thousands of seeds.

It can take as little as three years for a weed population to shift from one that was controlled by a herbicide to a resistant population if nothing else is done to control those weeds and prevent them from going to seed.

This happened with waterhemp.

“Waterhemp has been around for a long time, but it really didn’t become a big problem until we started using Pursuit extensively in the past,” Stahl said. The ALS inhibitor (Group 2) worked really well at first, but we quickly selected for populations of waterhemp that were resistant to that key chemistry. Now most of our waterhemp plants are resistant to ALS herbicides.”

Even though reliance on ALS products like Pursuit declined, resistance to ALS herbicides is now in the genetic makeup of most waterhemp seeds in the fields, making Pursuit non-effective on waterhemp into the foreseeable future, she added.

Farming practices also help select for a certain weeds, Stahl said.

“When you reduce tillage, for example, you see a shift in your weed species as well over time,” she said. “Probably more grasses – especially in no-till – because those are smaller seeds and they emerge near the surface.

“Whatever you’re doing, you’re selecting for the weeds that like the environment you’re creating – plus the chemistries play a big role too.”

That is why, she said, it is so critical to diversify weed management strategies, and to not rely solely on herbicides to manage weeds.

Read the label

Reading and following the label for pre-emergent herbicides is the law, as well as a best management practice.

A great tool for farmers and the ag community is the new “Take Action” 2020 Herbicide Classification Chart. A Quick Start Guide is also available that shows how to use the herbicide classification chart.

The left half of the chart classifies herbicides by their mode of action. The right half of the chart lists specific herbicides or premixes by their sites of action.

Using the chart, farmers can set a goal of using more than one effective mode of action on their worst weeds in a given year. It can be easy to double up on the same site of action if this chart isn’t used.

“We are dealing with so much more resistance now that we need to be aware of modes of action and sites of action and mix things up so we can have the most effective system in the long run,” said Stahl.

Posters, booklets and methods to download the information are available at There’s also information on webinars and more at this United Soybean Board sponsored site.