How Long Will Your Residual Herbicide Actually Last?
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.
How to Use Residual Herbicides
Residual herbicides control weeds by root, shoot and seed absorption, and persistence varies between products. With this in mind, selecting the correct herbicide to match the target weed’s emergence pattern is important.
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.
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.
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.
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.
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.
Here are a few popular herbicide products and potential product pairings as recommended or required based on each product label:
How to Order Inputs from FBN.com
Copyright © 2014 - 2023 Farmer's Business Network, Inc. All rights Reserved. The sprout logo, “Farmers Business Network”, “FBN”, “FBN Direct” are trademarks, registered trademarks or service marks of Farmer's Business Network, Inc.
Paraquat and Atrazine are restricted use pesticides.
FBN Direct products and services and other products distributed by FBN Direct are offered by FBN Inputs, LLC and are available only in states where FBN Inputs, LLC is licensed and where those products are registered for sale or use, if applicable. If applicable, please check with your local extension service to ensure registration status. Nothing contained on this page, including the prices listed should be construed as an offer for sale, or a sale of products. All products and prices are subject to change at any time and without notice. Terms and conditions apply.
ALWAYS READ AND FOLLOW LABEL DIRECTIONS. It is a violation of federal and state law to use any pesticide product other than in accordance with its label. The distribution, sale and use of an unregistered pesticide is a violation of federal and/or state law and is strictly prohibited. We do not guarantee the accuracy of any information provided on this page or which is provided by us in any form. It is your responsibility to confirm prior to purchase and use that a product is labeled for your specific purposes, including, but not limited to, your target crop or pest and its compatibility with other products in a tank mix and that the usage of a product is otherwise consistent with federal, state and local laws. We reserve the right to restrict sales on a geographic basis in our sole discretion. You must have a valid applicator license to use restricted use pesticides. Please consult your state department of agriculture for complete rules and regulations on the use of restricted use pesticides, as some products require specific record-keeping requirements.
All product recommendations and other information provided is for informational purposes only. It is not intended to be a substitute for consulting the product label or for specific agronomic, business, or professional advice. Where specific advice is necessary or appropriate, consult with a qualified advisor. Neither Farmer's Business Network Inc. nor any of its affiliates makes any representations or warranties, express or implied, as to the accuracy or completeness of the statements or any information contained in the material and any liability therefore is expressly disclaimed.