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Importance of pre emergence herbicide programs

By Staff | Apr 16, 2020

-Farm News file photo Early control of weeds, such as lambsquarter, shown here, with use of a pre emergence herbicide helps to get a jump start on weed control for the season.



A series of daily webinars were made available last week in collaboration of extension specialists from Iowa State University and the University of Minnesota. Last Thursday, Lisa Behnken, University of Minnesota Extension crop educator took to the airwaves to present benefits and guidelines to using a pre emergence herbicide program.

Knowing and understanding your weeds

Behnken, said it is important to know and understand what weeds are out in your fields.

“Know what they are resistant to, what do you know about the biology? Looking at corn planting goal of April 15, some of those weeds basically start to emerge when we want to put our corn in. Tillage helps work the field and take out some winter annuals, but it doesn’t take out many flushes unless we get delayed in our planting, but in general, with corn, we need that pre emergence herbicide that is going to control those tough weeds that are emerging with corn. We need that herbicide to take out that competition,” she said.

For soybeans, Behnken, said if we are on an early planting schedule, tillage doesn’t help to control many of the weeds, so a pre emergence program is needed to help control the early emerging weeds.

“If we think about tillage as another tool for us, adjust that planting date a bit for those soybeans to the middle of May, then we can take out a good flush of these early emerging weeds, then lay our pre emergence down so it helps us out later in the season with a next flush of weeds.”

Choosing the right herbicide

When it comes to choosing the right herbicide, Behnken, advises knowing the strengths and weaknesses of that herbicide.

“Think again about what resistance problems we may have out there. Maximize the rates of the soil and rank your fields in the priorities they are. Knowing that population what you are dealing with will help you make a better choice for a pre program,” she said.

Behnken said she realizes many producers are looking at ways to trim budgets, but if all possible, to still try to maximize the rates of your soils types.

“My best advice to that is prioritize your fields,” she said. “Know what fields have the bigger problems. That’s where you may need to focus the dollars, maximize those rates to get a good head start. You may have fields where you have done a really good job or have few problems. That might be the place where you, again, put those as a lower priority and be able to reduce your inputs on those herbicides. We want to take those early flushes out to keep the fields clean so our crops have the greatest advantage.”


Behnken said because we can see differences each year with the weather and how those pre emergence programs work to pay attention to the performance, which weeds are coming through and how soon they are coming through so you can have timely post applications.

“Monitor the fields to see how that pre emergence program worked for you so you can time that post and be effective for the season long,” she said. “Most of our herbicides want small weeds. If they start to get bigger, it is more difficult for the post program to get that job done. Get the small seeds early enough so you have complete control of the season. Pay attention to the size and get that post program on as soon as you can.”

Evaluating your success

“Are you happy with the program? Or did you have some escapes? Did you map those fields so you know where to put your attention next year? Pay attention to the products so you know how they perform for you,” said Behnken.

In the end, Behnken said it is a good idea to evaluate the success of your pre emergence programs – keep track of what worked, what weeds came through and look at that program the entire year.

“We may have some misses. If possible, map your fields, take some notes so you know how to plan for next year,” he said.


Behnken said one of her concerns that has stemmed from visiting with producers over the winter is due to some of the newer technologies in weed control now available, some are considering skipping their pre emergence program.

“They are looking at technologies and thinking they can get by without it and put it on a post. My message today is don’t. Don’t stop making the investment in the pre emergence programs because they are too valuable to your entire weed control program on your farms,” she said. “Keep it (pre emergence) in your plans, make sure you get those pre emergence programs on for any of the technologies. To be successful, all of them need that pre emergence program. They all recommend that is your starting block to that foundation. Do not stop. Continue to have that in your plans.”

Behnken said proper weed control, starting with a pre emergence herbicide will give your crop the best advantage for getting your corn and soybeans out of the ground ahead of the weeds.

“It lays a residual herbicide that is going to work for you, giving you that weed control you need to give that crop that advantage and it buys you some time that is very critical in June when you are getting a lot of things done. It widens that window before you need to get that post program on,” she said.

A pre emergence program will essentially give your post emergence program an advantage.

“It helps it do a better job,” Behnken said. “It lowers the density of the problem weeds. You may not get all of them, you may not get 90 to 100%, but what if you take out 60 to 70% of a problem weed? Then you post program doesn’t have as much to take out, evening the playing field.”


Behnken also took the opportunity to talk about a very problematic weed waterhemp.

“This is moving into the post program, but, we start with our pre, we have to remember that waterhemp is a different beast out there and for some dealing with palmer amaranth in their fields, these are long, entire season emergers,” she said. “We have to pay attention to what is happening with them. Our best approach is that we layer those group 15 herbicides that will give us residual for the entire season to control that waterhemp.”

Behnken said the target for an additional residual application should be about 25 to 30 days after planting.

“Our goal is to have that working for us for the season,” she said. “It may not be complete, but this is still our best program for taking care of, and working a good program for waterhemp throughout that season.”

Studies have shown there is a 25%-plus better control of those late season weeds when layering those group 15 herbicides, Behnken said versus just utilizing them for a single application as a pre emergence.

“We are gaining a great increase in our control by using a layering concept,” she said.

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