Benefits and Limitations of Residual Herbicides

May 28, 2020

The best time to take care of a problem is before it becomes a problem.  This is a proactive approach instead of a reactive approach.  It’s hard to argue against both of those statements.  This is what pre-emergence herbicides, or residual herbicides, do for us. They take care of problems before they become problems.  We all have our own experiences with pre-emerge and not all are good.  From my experience, the limitations, as well as the benefits, of pre-emerge are not always fully understood prior too using them.  Let’s dive into these. 
               Small weeds are more susceptible to chemicals and therefore easier to kill than larger weeds.  If you look at the label of most herbicides, you’ll likely find a size range for weeds controlled.  This is usually in the 2-4 inch range, beyond that range there is no guarantee the weeds will be controlled.  It’s not always possible to spray when weeds are this size due to field conditions, scout timing, and other work that needs to get done to name a few.  On top of those, some weed species can get over 4 inches in 2 days with adequate moisture and sunlight.  This is where a pre-emerge or residual herbicide comes in handy.  The roots of those weeds will hit that residual layer in the field and take that chemical up, killing them while they are small.  Another trouble with a lot of post emerged herbicides is they only kill what’s above ground the day of application.  This is also a problem with tillage, tillage only kills growing weeds.  Weeds can germinate minutes after a tillage pass and be up before the field is planted.  Some weeds will re-root themselves after a tillage pass and keep growing without skipping a beat.  Residual herbicides kill those weeds if there is enough concentration left in the soil, usually 3-6 weeks after application but is subject to environmental conditions.  Now these environmental conditions I just talked about are the limitations that come with residual herbicides.
               For residual herbicides to work they need rainfall to carry them into the soil.  When I talk to people who aren’t fans of residual herbicides, this is usually the reason why.  To make matters worse, different residual products take a different amount of rainfall to carry them into the soil, the amount required can usually be found on the label.  So, what happens to that residual herbicide as its laying on the soil surface?  Some products get broken down by the sun’s UV light, reducing the concentration and therefore reducing the ability to control weeds once it is carried into the soil, this is called photolysis. But once its in the soil, its all good right?  Well not exactly, just like nutrients in soil, some residual products move more freely than others within the soil profile.   This means that some products get carried below the root zone of the weed, reducing the ability of the product to kill said weed.  Also like nutrients, some products are more available in lower or higher ph ranges.  Some products get broken down by microbial activity or hydrolysis easier than others as well.  On top of all that, there is the limitation that all herbicides share, weed resistance.  All these limitations factor into how your residual herbicide performs. 
               There are limitations to all herbicides, pre or post applied.  When formulating a weed management plan, take into consideration these limitations.  If you look back at the history of chemical weed control, there’s a common theme of relying too heavily on a single product to do the bulk of the heavy lifting.  This is how we got to the level of resistance we’re at today.  If you want to learn more about how to use residual herbicides, give you local Allied Agronomy agronomist a call today.

This article is an opinion and not a base for making trading decisions.  The author and Allied Companies are not responsible for any trading decisions made off this article.

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