Widespread Failure of PRE Herbicides (AKA Failure to Rain)

A broad spectrum of broadleaf weeds and grasses are emerging where pre-emerge herbicides have not received rainfall or irrigation.

A broad spectrum of broadleaf weeds and grasses are emerging where pre-emerge herbicides have not received rainfall or irrigation.

As I walk peanut fields, it becomes clear that we are “back to the drawing board” with our weed control programs. Lack of rainfall has resulted in failure to incorporate pre-emerge herbicides into the weed seed emergence zone. Dr. Jason Ferrell, UF/IFAS Weed Scientist,  recently shared some timely information on herbicide incorporation at Panhandle Ag. Some key points he included:

Why does incorporation improve herbicide performance? For a pre-emergence 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. The commonly used herbicide, Valor, cannot be mechanically incorporated, this product must be incorporated by rain or irrigation.


Daily rainfall during the month of March at SVAEC in Live Oak, FL. Peanut planting began in earnest following rainfall on April 4th and 5th.

Peanut planting began in earnest in North Florida following the area-wide rainfall on April 4th and 5th. Pre-emergence herbicides applied after planting failed to receive any moisture. Most areas in Columbia County received less than 0.10″ of rainfall finally on May 4th, which may be of some help for recently planted fields. However, we are seeing strong weed emergence where peanuts were planted without irrigation to incorporate herbicides. This is typically a situation where we  rely on paraquat for a “burndown” application. However, farmers are very concerned about the combination of burndown applications and severe drought stress. At this time, with no rainfall in the forecast, it  looks like we are “darned if we do, and darned if we don’t.”



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