The How and Why of Preemergence Herbicide Incorporation for Row Crops

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 that emerging weeds 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 a 0.25-0.5 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.  Also, 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 roto-tiller.

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 typically will mix 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 not to incorporate too deeply.

Should all preemergence herbicides be incorporated mechanically? 

No.  Valor is an example of a herbicide that performs best when applied directly to the soil surface.  It 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.  Though the yellow herbicides will not provide season-long weed control, proper incorporation can allow them to perform at an optimum level.  For more information, read the incorporation instructions on the Treflan label.  Though not for use in peanuts grown in the Southeast, this label provides an excellent narrative on pages 8 and 9 concerning proper herbicide incorporation, particularly for the yellow herbicides (http://www.cdms.net/ldat/ld7HD002.pdf).

For crop specific preemegence herbicide recommendations, use the following publication links:

• Weed Management in Peanuts

• Weed Management in Cotton

• Weed Management in Soybeans

• Weed Management in Corn