A recent report examines the theory that weed populations are not able to develop a resistance to herbicides if they are applied in the soil instead of on the foliage.

It is well known by weed management practitioners that weed species and populations can become resistant to herbicides, sometimes even multiple types of herbicides.

But the general belief is that that weeds demonstrate resistance only to foliar-applied herbicides.

A recent report by the Integrated Pest Management Department at the University of Illinois has helped shed new light on that idea.

The author of the report allows that integrated weed management programs offer the greatest potential for long-term, sustainable solutions for weed populations demonstrating resistance to herbicides, and that frequently the soil-residual herbicides that are used in these programs can reduce the potential for resistance to foliar-applied herbicides. The report claims however that in many instances, weeds do in fact also demonstrate resistance to soil-applied herbicides.

The study looked at the effects of PPO-inhibiting herbicides on waterhemp, a member of the pigweed family that is native to Illinois and while beneficial to wildlife, can invade cultivated fields and cause allergic reactions in humans.

The study found that the majority of field-level populations of waterhemp do contain one or more types of herbicide resistance and that the number of waterhemp plants and populations demonstrating multiple herbicide resistance is becoming increasingly more common, which greatly reduces the number of herbicide options that remain effective for their control.

More importantly, the study showed that the types of waterhemp plants resistant to PPO-inhibiting herbicides are resistant to them regardless of whether the herbicide is applied to the soil or foliage.

But since application rates of soil-applied PPO-inhibiting herbicides are selected to provide several weeks of residual weed control, that rate is much higher than the rate used when the herbicide is foliar-applied.

So even though weeds can build a resistance to soil-applied herbicide, the report concluded that an integrated weed management approach that uses soil-applied and foliar-applied herbicides at labeled rates in combination with other management tactics can help slow the selection for additional resistances.