Preemergence herbicides control germinating weeds, which minimizes early-season competition with crops to keep yield potential higher. If weeds emerge with the crop, there is a high potential to have a yield reduction, even if the weeds are effectively managed later. Below are factors that influence the effectiveness of preemergence herbicides and what can be done to avoid control failures and unintended crop injury.
Timing
Preemergence herbicides can be applied preplant (fall or spring), at planting, or shortly after planting before the crop emerges. Some preemergence herbicides can be applied to emerged crop plants, but you should always refer to the specific label to ensure the application will not result in crop injury. Select preemergence herbicides can be applied later in the growing season along with postemergence herbicides to control later-emerging weeds.
Rainfall
Many preemergence herbicides do not control emerged weeds nor stop the germination of weed seeds, only germinated weed seeds. At least a one-half to 1 inch of rain is needed to move the preemergence herbicide into the soil layer where weed seeds are actively germinating; this rainfall is commonly referred to as “activating rainfall.” Ideally, preemergence herbicides should be applied before a rain event. If activating rainfall is not present within 7 days of the application of the preemergence herbicide, scouting will determine if an early postemergence herbicide application is necessary. Incorporation via tillage can also mimic a rainfall event by mixing the herbicide into the soil profile. If the crop has already been planted, a rotary hoe can be used to incorporate the herbicide without harming the crop plant/seed.
While an activating rain fall is critical, continuous moisture after the application is critical as well. If there is adequate moisture after the activating rain, the herbicide stays available to the weed by remaining in the soil water. If conditions are dry after the activating rainfall, the herbicides can bind tightly to soil colloids and not be available to control the germinating weeds. A preemergence herbicide application that endures a period of dry conditions is usually not a lost cause. When an activating rainfall event occurs, the herbicide can be moved to the soil layer to control later germinating weed seeds (Figure 1).
If a preemergence herbicide has been applied at an early preplant timing and conditions have been dry, a burndown herbicide application may be necessary to control emerged weeds at the time of planting. Fall-applied preemergence herbicides usually move into the soil layer where the weed seeds are germinating due to the melting snow in the spring. However, monitoring the snow melt and scouting if weeds have germinated will likely be necessary.
Rate
Preemergence herbicide rates are based on soil types. Coarse texture (sandy) soils usually require a lower rate than fine texture (loam or clay) soils. Coarse soil usually has less areas for the herbicide to bind compared to a fine texture soil; when the herbicide binds to a soil colloid, it becomes unavailable to control the germinating weed. Be sure to select a rate based on the field to be sprayed; adjust the rate according to other fields and in-field variation. Maximum labeled rates (within reason for the soil type and texture) can help overcome adverse precipitation (both dry and wet) and there will be more herbicide to contact the germinating weed seeds. However, crop injury, inadequate weed control, and carryover injury to subsequently grown crops can occur if inappropriate rates are selected.
Herbicide selection
Ideally, preemergence herbicides from different groups (modes of action) should be applied. Applying two different herbicides increases the spectrum of weed control. Many preemergence herbicides will provide either good broadleaf or grass weed control; including both herbicides ensures more weed species will be controlled. Different combinations of preemergence herbicides may have to be utilized in different fields depending on the weed species present. Additionally, applying two different herbicides reduces the selection pressure on herbicide-resistant weeds. Another important aspect of using different herbicide groups in the preemergence application is different chemical characteristics. Some herbicides can move into the soil profile with less water (for example, metribuzin, Callisto - mesotrione, and Outlook - dimethenamid) or more water (for example, atrazine, Prowl – pendimethalin, and Zidua - pyroxasulfone). Utilizing herbicides with different chemical characteristics can also hedge against environmental conditions, such as precipitation. The combination of herbicides with different chemical characteristics can also influence the time of residual control. Many preemergence herbicides are still effective for controlling weeds resistant to herbicides applied postemergence, for example, glyphosate and acetolactate synthase (including Accent, Harmony, Pursuit, and others)-inhibiting herbicides). However, if only a single preemergence is applied, this could select for weeds resistant to the herbicide applied. When applying both unique herbicides, the chance of selecting for a weed resistant to an individual herbicide is reduced.
While many herbicides have already been purchased, refer to the latest SDSU Extension Pest Management Guides for a full list of preemergence herbicides that can be used in various South Dakota crops. The 2023 Weed Control field Test Data is also available to determine what herbicides are effective on select weed species.
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