Years ago I had my first exposure to weeds that had become resistant to herbicides. As my agronomy center’s first responder to complaints about underperforming herbicide applications I was stumped as to why a single weed species had survived.

Conversations with the grower led me to believe he was equally perplexed about why his herbicide program had failed.

As he said, “I rotate crops religiously; I never plant the same crop two years in a row.”

Rotation is a good measure to ensure that a field doesn’t harbor populations of insects, disease and weeds that could become an issue to the succeeding crop.

Further discussion with the grower identified the potential problem for the weed escape. Although he had rotated to a different crop every year he had persisted in the practice of utilizing herbicides with the same mode of action. Both the soybean and the corn herbicides being used employed the ALS mode of action – acetolactate synthase.

Resistance is defined as a change in the sensitivity of a pest population to a pesticide, resulting in the failure of a correct application of the pesticide to control the pest. Resistance can develop when the same pesticide or similar pesticides with the same mode of action are used again and again.

As I crop-scout and even as I drive around my area I’m concerned by those fields that are clean and have overall good weed control – but one weed species penetrates the crop canopy. It’s suspicious to me when there is only one weed species apparent. I know giant ragweed and certain annual grass species are late-germinating and may escape early weed sprays.

But with species like marestail or waterhemp that have been documented to have developed resistance to herbicides, a red flag should be raised. The grower needs to focus on controlling and stopping the spread of those weed species.

I’ve heard the excuse from some growers who have said Roundup is cheap; we’ll just use increased rates. Unfortunately if a weed is resistant to reduced rates it probably is resistant to greater rates as well.

In my crop observations from many years ago I noted that weeds can be introduced to new fields with equipment. Canning companies were responsible for moving wild proso millet from one contaminated field to a new one when the harvest equipment started and shook seed around the driveway.

In the 1080s I heard from many growers that they never had velvetleaf until they bought a load of corn from Minnesota. Feeding corn silage from velvetleaf-infected fields spread seeds with the manure to many other fields. And it’s now well-established throughout Wisconsin.

Pesticide resistance is not unique to weeds. The genetic modification of crop species has provided a boom to crop production and yields while at the same time reducing the amount of pesticides applied to crops.

An example is the introduction of the bacterial protein Bt – Bacillus thuringiensis – to corn in the 1990s. It was released with the warning that misuse could accelerate resistance of caterpillar pests. The Bt gene genetically modified organism was released with a strategy for slowing resistance by planting the crop with a specific management strategy.

Pest resistance is here. Keeping it under control is up to the growers respecting it and managing to slow it. Resistance management, crop protection and pest management depends on it.

Tim Boerner is a career agronomist who has worked in many regions of Wisconsin. Currently serving farmers in northwest Wisconsin, he's advising growers on Integrated Pest Management techniques, writes Nutrient Management Plans, works on composting projects, and employs GPS soil and data collection for precision-ag implementation. He has a passion for clean-water advocacy.