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. The ultimate length of time it will remain effective could actually cause problems for you with future crop production.
Here are some common questions we’ve gotten 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. This measurement varies greatly between herbicides, from days to years, and is dependent on several factors:
- current cropping system
- soil type
- soil pH
- 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 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 increase 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.
The label is the law.
By following the herbicide label directions you will get the best performance from the herbicide used. The herbicide label is the law. The herbicide label information was developed after years of thorough testing and meeting government requirements. Always read and follow label use instructions.
ALWAYS READ AND FOLLOW LABEL DIRECTIONS. It is a violation of federal and state/provincial 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.