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Do Soil-Applied Herbicides Applied At Below Labeled Rates Encourage Development of Weed Resistance? (06/06/19)

I received the following email last week. Dear Tom, you promote the use of S-metolachlor, Outlook and Warrant at below labeled rates for waterhemp control in sugarbeet. Granted, herbicides are applied in layers beginning at planting and through canopy closure. How can you be sure you are not encouraging weed resistance with these practices?

I received the following email last week.

Dear Tom, you promote the use of S-metolachlor, Outlook and Warrant at below labeled rates for waterhemp control in sugarbeet. Granted, herbicides are applied in layers beginning at planting and through canopy closure. How can you be sure you are not encouraging weed resistance with these practices?  Cody

The easiest way to answer this question is to move away from sugarbeet for a moment and focus on the PPO inhibitor herbicides in soybean as we know some biotypes of waterhemp are resistant to Flexstar. Let's begin by comparing ‘POST’ vs. ‘soil applied PPO inhibitor herbicides.’ A common misconception is resistance to PPO inhibitors exists only when foliar applied. Waterhemp biotypes resistant to PPO-inhibiting herbicides are resistant regardless of whether they are applied to the soil or foliage. When a PPO-inhibiting herbicide is applied in the soil, the waterhemp control level is often comparable to that of a susceptible population.

Why do soil-applied PPO-inhibiting herbicides seem to control PPO-resistant waterhemp populations that foliar-applied PPO-inhibiting herbicides do not control? Foliar-applied PPO-inhibiting herbicides are applied at rates required to control weeds present when the application is made. Rates for soil-applied PPO-inhibiting herbicides are calculated to provide several weeks of residual weed control and thus are much higher than what is needed to control weeds that are present when they are applied. The higher rates overcome the mechanism of resistance to PPO-inhibiting herbicides because the magnitude of resistance is relatively low. If a waterhemp population contains a mix of PPO-resistant and sensitive individuals, a soil-applied PPO-inhibiting herbicide will control both resistant and sensitive plants for a period of time after application. As the herbicide degrades, a threshold concentration is reached at which the sensitive individuals are controlled, but the resistant individuals survive.

Sugarbeet are a challenge since they do not tolerate group 15 corn or soybean herbicide rates. Thus, we reach the threshold quicker since we begin with less than full rates and is the reason for implementing repeat (at least 2 and preferably 3) doses of soil applied herbicide applied at 17 to 21-day intervals. Our idea is to replenish the supply of chloroacetamide herbicide as it degrades to the threshold level. We continue this strategy until the canopy closes and weed seeds no longer germinate and emerge.

A weed management plan considers the crops and herbicides applied to the field across years. Consider diversifying herbicides across crops and years. To date, resistance to group 15's has been strongest in fields where metolachlor was extensively used and demonstrates the strongest resistance to that product and less so to other group 15's. Where possible, use a group 15s in combination with another class of chemistry.

Tom Peters

Extension Sugarbeet Agronomist

NDSU & U of MN

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