Herbicide Efficiency - How Long Will Your Residual Herbicide Actually Last?

LeRoy Toohey

Feb. 19, 2019

Herbicides — commonly known as weed killers — can work their weed-controlling ways for varying amounts of time. The length of time that an herbicide is effective is known as its residual.

Herbicides with long-ranging effectiveness that are applied after planting but before crop (and weed) emergence are called residual herbicides, referring to their longevity.

Weed Resistance and Residual Herbicides

With weed resistance on the rise, residual herbicide usage is taking on increased importance. Using residual herbicides with several Modes of Action (MOAs) can increase the probability of acceptable weed control by giving the crop a longer time to emerge and reducing resistance pressure on post-emergence herbicides.

Residual herbicides can be especially important in fields that already have herbicide-resistant weeds. These residual herbicides are selective, meaning they control certain weed species while leaving the crop being grown unharmed. They should be applied before weeds emerge. 

[READ: Common Herbicide Mode of Action Groups for Weed Management]

How to Use Residual Herbicides

Residual herbicides control weeds by root, shoot and seed absorption, and persistence varies between products. Therefore, selecting the correct herbicide to match the target weed’s emergence pattern is important. 

It is often necessary to apply an additional residual herbicide at post emergence, such as glyphosate or atrazine, to adequately control certain weed species that have an extended germination period.

[READ: Choosing the Right Post-Emergence Herbicide Applications for Your Farm]

Residual Herbicide Impacts on Replant and Future Crop Rotation 

You should also consider replant options and rotational restrictions when selecting a residual herbicide. 

Replant options generally come into play following a weather event that destroys the existing crop. Rotational restrictions refer to future plantings or planned crops to be grown. 

Make sure that the residual herbicide you choose meets your future plans for the land where you plan to use the herbicide. You can view hundreds of detailed herbicide labels here.

[WATCH: Replant, Delayed Plant, and Prevent Plant 101 - What You Need To Know Webinar]

How Long Will Residual Herbicides Actually Last? 

When many farmers are planning their weed control program, they start by thinking about how they can apply a herbicide that will provide good weed control for the entire growing season. These herbicides are said to have “residual” weed control, meaning that the herbicide remains active in preventing weed growth long after its initial spray application. 

But just how long is "long after?"

There are a few complex variables that can impact how long your residual herbicide will be active. Here are a few common questions we hear often from farmers about how long their residual herbicide will be active and effective:

How Long Will My Herbicide Be Active In The Soil?

The residual activity of a herbicide is commonly referred to as its half life, which is defined as the time required to dissipate one half of the applied herbicide. A residual herbicide will have activity in the soil anywhere from days to years and is dependent on several factors including the current cropping system, soil type, soil pH and environmental conditions.

For example, several herbicides have a half life that increases dramatically in drought years compared to wet years. The rotational crop response to each herbicide and crop species susceptibility to each herbicide can vary significantly.

What Do I Need To Know About Chemical Carryover?

A herbicide that lingers in the soil for an extended length of time (past the time you need it) could cause major problems in crop rotation plans — this is called “carryover.”

Every herbicide label has information concerning any carryover issues associated with it. Be sure you know the potential carryover of the herbicides you want to use when you’re developing your weed control program.

[READ: Decoding Ag Chemical Labels]

Why Does Soil Adsorption Matter?

Soil adsorption — that’s adsorption, not absorption — occurs when the herbicide applied to the soil becomes chemically bound to solids and renders itself unavailable for plant uptake, as well as leaching and microbial degradation.

By definition, adsorption occurs when atoms, ions or molecules from a gas, liquid or dissolved solid adhere to a surface. This is important because, where crop production is concerned, soil type regulates soil adsorption. This means that your soil type can impact how plants get access to the chemicals you apply.

A few things to keep in mind about soil adsorption: 

  • As organic matter and soil clay content of the soil increase, so does herbicide adsorption; this is due to the chemical reactivity and binding sites increasing in number.

  • Wet soils adsorb lower amounts of herbicides because water fills many of the binding sites.

  • As soil pH decreases, the soil has less positive charged particles to fill the binding sites which allows herbicide soil adsorption to increase.

  • Herbicides that are highly water soluble do not adsorb to the soil very well and can be subject to leaching. Also, low organic matter and coarse textured soils boost the leaching probability.

What Causes Herbicides To Break Down?

Microbial degradation is the breakdown of herbicides by bacteria, algae and fungi living in the soil.

These microbes use the herbicides as a food source and are herbicide specific, which means that the repeated use of a specific herbicide will likely result in shorter residual weed control due to a population buildup of the microbes that feed on that herbicide. 

A few things to keep in mind about microbial degradation and herbicide breakdown: 

  • Soils with higher organic matter favor microbe growth, while pH extremes hinder microbe activity. 

  • Soil temperature and soil moisture also regulate microbe activity. Chemical decomposition of herbicides increases with warmer soil temperatures and as soil pH decreases.

  • Some herbicides decompose when exposed to sunlight, and will require immediate incorporation into the soil to prevent loss.

[READ: Resistant Weed Control - Things Every Grower Should Know]

Good Record-Keeping Is Key

By observing and recording as much information as possible around your planned herbicide applications, you can make an educated guess at your residual herbicide efficiency.

The information you collect is also a valuable tool to estimate any herbicide carryover issues for the following year’s crop.

By following the detailed herbicide labels, which were developed after years of thorough testing and meeting government requirements, you will get the best performance from the herbicide used. Remember, the herbicide label is the law.

Find the Right Herbicides for Your Operation 

FBN Direct® has the diverse array of herbicides you need to proactively prepare against weed pressures and keep your operation running smoothly. With detailed product labels, transparent pricing, savings opportunities, and similar product references available for each product, FBN Direct provides the information you need to make an informed decision on your herbicide strategy this season. 


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LeRoy Toohey

Feb. 19, 2019