Preemergence herbicides are a powerful way to prevent weed infestations before they start. In our current climate of glyphosate and ALS-resistant weeds, a good preemergence program is more valuable than ever. However, method and timing of application dramatically impact the efficacy of many preemergence herbicides. To improve their reliability, preemergence herbicides should be incorporated, this is particularly important for the yellow herbicides such as Prowl, Sonalan, and Treflan. So here are a few items that should be considered prior to making the application.
Why does incorporation improve herbicide performance? For a preemergence herbicide to work, the weed seed must germinate in the presence of the herbicide. Since most weeds do not germinate on the soil surface, the herbicide must be mixed into the soil so the emerging weeds can absorb the herbicide immediately upon germination. If the herbicide is applied to soil surface, the weed seed may germinate below the herbicide zone and emerge without harm. Additionally, many soil applied herbicides will degrade quickly in the presence of sunlight. Mixing with soil will protect the herbicide and greatly increase persistence and duration of weed control.
Is incorporation essential? Yes. A herbicide must be incorporated (or activated) for weed control to occur. This can be done using tillage equipment, irrigation, or rainfall. In a dryland system, if at least 0.5 inch of rain is not predicted within 5 to 10 days of application, mechanical incorporation will be essential to achieve weed control.
How should a herbicide be incorporated? As stated previously, the purpose of incorporation is to concentrate the herbicide in the zone where weeds germinate. We have already stated that most weed seed don’t germinate on the soil surface, but neither do they germinate from several inches deep. Therefore, deep incorporation dilutes the herbicide in the soil profile instead of concentrating it in the germination zone. The best way to incorporate a herbicide is with minimal disturbance from a field cultivator or rototiller.
Can I incorporate with a disc? A disc can be used, but careful attention to depth of the implement is essential. A heavy disc can cover ground quickly, but have the tendency to cut the herbicide several inches into the soil, moving much of the herbicide away from the germinating seedlings. Additionally, a single pass with a disc can also incorporate the herbicide in streaks directly below the turning blades, rather than distributing it evenly. If a disc is used, two passes (each angled across the other) will help distribute the herbicide more evenly. Beware to not incorporate too deeply.
Should all preemergence herbicides be incorporated? No. Valor herbicide performs best when applied directly to the soil surface. It too requires rainfall or irrigation for activation, but should not be incorporated with tillage.
Allowing the crop to emerge in a weed-free setting is essential to obtaining top yields. The yellow herbicides, in particular, benefit from light incorporation immediately after application.