Mitigate weed pressure with preemergent herbicides

As weeds gain resistance to the most popular herbicides on the market, starting the year with a clean field is a farmer’s first line of defense. Herbicides applied before weeds emerge are an effective addition to a weed management plan.

“The evolution of herbicide resistance is a lot more simple than a lot of people make it out to be,” says Kip Jacobs, a technical service manager for UPL. “If the weed doesn’t come up and it doesn’t put seed on, there is 0% chance of it passing potentially herbicide-resistant traits on to the next generation. A weed’s only goal is to grow up and put seed on to create that next generation. As long as we can prevent that from happening, we can control the issue.”

The use of preemergence herbicides was more common before the introduction of glyphosate, which changed the way farmers looked at weed control. As overreliance on the herbicide has caused its effectiveness to wane, more farmers are looking at weed management techniques of the past.

“What’s old is new again,” says Chris Wharam, a district manager for Wilbur-Ellis. “With the resistance to glyphosate emerging in a number of weed species in the Corn Belt, growers are in a position where they’re having to use additional products postemerge or try to kill those weeds before they even emerge from the soil surface. That’s where preemergence herbicides come into play.” 

Bill Johnson, a weed science professor at Purdue University, estimates about 60% of farmers use residual herbicides on soybean fields, while at least 80% of cornfields are treated with residual herbicides. 

Who Can Benefit?

Most herbicide-resistant weeds are prolific seed producers, so almost every farmer could benefit from a preemergence application. 

“As the weeds we’re trying to kill, like waterhemp, get bigger and more mature, that’s when their defense mechanisms really start to become amplified and more efficient at not being controlled by herbicides,” Jacobs says. 

Killing the weed before it emerges also reduces pressure on the postemergence herbicide and can minimize competition for the crop by up to 90%.

“The more we can take away pressure on our postemergence herbicides, the longer it’s going to be before we have a resistant-weed problem,” Johnson says. “Everybody would benefit, but typically the fields that see the most benefit from residual herbicides are fields that have high weed densities or fields that have weeds that are simply not effectively controlled by the post herbicides that the grower has been using.” 

In corn, preemergence herbicides can be effective against grasses and small-seeded broadleaves, while in soybeans, they primarily target broadleaves.

Fields with past weed issues as well as fields surrounded by problem fields can benefit from additional herbicide applications.

“These prolific seed producers can produce tens or hundreds of thousands of seeds,” Wharam says. “Even if a resistant weed is not an issue on a particular farm due to cultural practices or a history of rotating chemistries, the neighbor could [have a weed problem]. It’s a very good practice to rotate active ingredients and classes of chemistry to mitigate future development of resistance.” 

Application Considerations

A clear understanding of weed pressure in each field is key before implementing any weed management strategy. Crop rotation and geography should also be considered before choosing which herbicide to apply. 

“It’s very important to consider what the crop rotational restrictions might be,” Wharam says. “In the geography I’m familiar with, we have a number of crops that are grown outside of corn and soybeans. Some of these residual or preemergence herbicides provide weed control over a window of time. If it’s got a very long residual, that can actually result in concerns of planting certain crops the following year.” 

Farmers also have several options when it comes to applying preemergence herbicides. 

• Impregnate fertilizer: While not a common option, adding a layer of weed protection with dry fertilizer can be done in both the spring and the fall, and provide efficient control.
• Incorporate preplanting: A goodoption for photo-degraded herbicides, incorporating the product into the soil before planting can protect the longevity of the weed control.
• Apply post-planting: The most common option is applying the herbicide between planting and emergence. 

“Historically, we’ve recommended applying residual herbicides really close to the planned day of planting to get the most benefit out of them,” Johnson says. “With a weed like waterhemp, which has a long emergence pattern, we’ve seen some benefit 

to putting some of the residual out at planting and some of it with our early posttreatment. In corn, we’ve also evolved to a system where we’re putting out about two-thirds of the residual at planting and maybe another third with the postemerge treatment to spread out the length of residual protection.” 

Timing herbicide applications to the right weather conditions can impact their effectiveness as well. Most require some type of moisture to activate. 

“If a grower puts the preemergence herbicide out and it doesn’t rain for days, weeks, or even months, they may not get a strong activation,” Wharam says. “There are many factors that influence how much moisture will be readily available to the herbicide including soil type, soil texture, organic matter, soil pH, and the chemical properties of that particular herbicide. There’s a complex relationship there to get the most out of a preemergence herbicide application.” 

Farmers can also consider adding a soil adjuvant to their preemergence herbicide. These adjuvants can hold a higher percentage of that active ingredient in the top few inches of the soil, which could boost effectiveness. 

“Preemergence herbicides kill emerging or germinating weeds by acting on either the roots as they’re coming out of the seed or by acting upon the shoots,” Wharam says. “By using soil adjuvants to hold that herbicide in a higher concentration in the upper 2 inches of the soil profile, you can really extend the window of time that herbicide is active.” 

Follow-up Management 

Evaluating fields through scouting in the weeks following a preemergent application can help farmers decide on follow-up management, including further treatments. A check strip can be an effective way to monitor the success of the herbicide throughout the season. 

“I don’t know that a lot of folks do that, but sometimes you can simply find a spot in the field that is difficult for the application equipment like behind a telephone pole,” Wharam says. “Make notes and observations as to the weed population in that area that didn’t get treated, then compare it to the portions of the field that did get treated.”  

Preemergence applications should be part of a comprehensive weed management plan rather than the only tool used. 

“Weed control today is best handled as a full systems approach, including all potential tools like pre- and postemergence herbicides, crop rotation, tillage practices, and rotation of modes of action in chemical products,” Wharam says.

Resistance Grows in Seed Corn

Weed management is handled differently in fields of seed corn compared to grain corn. To protect the seed production, less aggressive herbicide products are often used.

“In a seed-corn field, a farmer is not going to do anything super aggressive that could possibly cause phytotoxicity,” says Kip Jacobs, a technical service manager for UPL. “That’s often why you see escapes and a buildup of weed pressure in seed production.” 

Repeated applications of less aggressive products, especially to weeds that have already emerged, has led to resistance problems in many seed-corn fields. 

“When we look at a lot of the peer-reviewed articles investigating resistance mechanisms, almost every one of those populations came from a site with some period of seed corn production,” Jacobs says. “It speeds up the resistance process because being nice to the crop means we’re also being nice to the weeds.”

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