A common misconception is that resistance to PPO inhibitors exists only to those that are foliar-applied.

In reality, biotypes of waterhemp that are resistant to PPO-inhibiting herbicides are resistant regardless of whether they are applied to the soil or foliage.

However, 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?

“Because the dose of the soil-applied herbicide is high enough to overcome the resistance mechanism, at least for a while,” Hager explained.

“Foliar-applied PPO-inhibiting herbicides are applied at rates to control the weeds that are 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 still controlled, but the resistant individuals survive. This provides a selective advantage for the resistant individuals and may partially explain the increasing frequency of PPO-resistant waterhemp populations in Illinois.

Selection for herbicide resistance for any herbicide takes place each time it is applied. However, the overall intensity of selection for resistance to a specific herbicide or mechanism-of-action family is reduced when multiple, different tactics are used to control the weed population.

 “Not every individual waterhemp plant is herbicide-resistant,” Hager concluded.

“An integrated weed management approach that applies soil- and foliar-applied herbicides at labeled rates in combination with other management tactics can help slow the selection for additional resistances in individual plants and populations.”
 

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