• Rachel Stevens

The Importance of Residual Herbicides

When we look at our price of inputs for all our crop acres, it can be tempting to try to find some ways to cut some corners. One of the first things to go is usually a robust weed management plan. I want to talk a little bit today about why residual herbicides are so critical and why you should make sure to budget them into your crop plan.



What are residual herbicides?

Residual herbicides are soil applied herbicides that kill weed seedlings as they germinate or emerge. Our window of effectiveness is usually a few weeks, and can dramatically reduce competition during this window to get your corn or soybeans off to a great start.


Why residuals?

Like we mentioned above, residuals are critical for getting your stand of corn or soybeans off to a good start without competition. This is essential for getting a healthy stand that will have a better chance during the rest of the growing season.


Residuals also provide a more flexible window before we have to get in the field for a post application. With volatile weather in the spring and early growing season, having a larger window to get in the field for applications will be extremely helpful for maintaining good weed control.


Residuals mix up the modes of action available to us. We’ve talked in past blogs about the importance of multiple modes of action, both for preventing weed resistances and fighting against resistant weeds. You can read more about modes of action here. Residuals broaden the modes available to us and ensure that we are being good managers for long term weed planning.


As a general rule, it is easier to control plants as they come up, rather than after they have emerged and are larger in size. Residuals provide excellent control up front of seedlings as they emerge that prevents us from having control issues later.


Why weed control is so essential:

A study from Iowa State found that weeds can reduce yields by about 3 bushels per acre for every day they are not sprayed or removed. One of the studies showed a 7 bushel per acre reduction for one foxtail plant for one foot of row. This can be hugely detrimental to yield and it doesn’t take much to pencil out and see the benefit of having a residual down to decrease the quantity of weeds emerging.


Another concern we have to consider is the amount of weed seed that fills our soil bank if we don’t control weeds. Some of or main weed species, like waterhemp, will produce thousands of seeds per plant and could last hundreds of years in the seed bank. Early control of any of these emerging seedlings to prevent them from creating a thousand more each, totally worth it. It is always important to consider not just the yield impacts, but the long term impacts of weed control and the decisions we make for the longevity of our cropping fields.


Use of residuals is critically important as more and more weed resistances are discovered. Residuals often have several modes of action which can help fight against resistant weeds. If you have some weed resistances in your fields, then you need to be sure to include a residual in your plan to help get control.



Resources

https://www.farmprogress.com/corn/weed-competition-can-decrease-corn-yield

https://cropwatch.unl.edu/timing-weed-control-corn-get-most-effect

https://crops.extension.iastate.edu/encyclopedia/managing-weeds-protect-crop-yields

https://agcrops.osu.edu/newsletter/corn-newsletter/2019-12/it%E2%80%99s-all-about-weed-seedbank-%E2%80%93-part-2-where-has-all-waterhemp

https://crops.extension.iastate.edu/encyclopedia/seed-production-following-mowing-summer-annual

https://www.no-tillfarmer.com/blogs/1-covering-no-till/post/5717-attack-the-weed-seedbank-to-control-resistant-waterhemp

https://eupdate.agronomy.ksu.edu/article_new/residual-herbicides-for-corn-432-1

https://emergence.fbn.com/inputs/the-importance-of-residual-herbicides

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