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Focus on Weed Seed Bank Management

It’s a way to curb the spread of herbicide-resistant weeds.

If you’re looking to halt future herbicide-resistant weeds, focus on the seed bank.

That’s the message Bob Hartzler, Iowa State University (ISU) Extension weeds specialist, told those attending this week’s ISU Integrated Crop Management Conference. Herbicides have and still remain the main way to manage weeds. 

Still, Hartzler advises a shift to also minimizing the weed seed bank. Increasing resistance to current herbicides and the fact that no new herbicide sites of action are coming down the pike from industry means farmers need to refocus efforts on weed seed bank management.

“Thirty year ago, every farmer’s goal was to minimize the size of the weed seed bank,” says Hartzler. “Herbicides were not as effective as the ones we have today. Now that we have highly effective herbicides, farmers will see escapes, and say, ‘Well, I can deal with those next year.’ ”

Those escapes, though, can contain herbicide-resistant weed biotypes that can come back in full force over several years to create a major weed headache. Using steps like rouging stray plants before harvest can help halt future resistant weed infestations. 

Other matters speakers at the ISU meeting discussed included the following. 

If you’re looking to terminate a cover crop, herbicides are the way to do it, not tillage. “While we can terminate a cover crop with tillage, it is not a good practice,” says Mark Licht, an ISU Extension agronomist. Disking a tough and fibrous root system of a cover crop does not leave an ideal seedbed, he says. 

You aren’t alone this fall. It’s been a tough harvest in the Midwest, as farmers in many areas have dodged rain and mud to harvest their crops. Dave Hooker, a University of Guelph research agronomist, says that tens of thousands of acres of Ontario corn have been impacted by high mycotoxin levels. “Being around the Great Lakes and the (accompanying) high humidity, we have had lots of Gibberella ear rot in corn,” he says. 

Soybean varieties that resist soybean cyst nematode (SCN) are a good management tool, but they don’t work as well as they used to. Over 95% of SCN-resistant soybeans share the same source of resistance—PI88788. As with weeds, using the same source of resistance over and over again leads to SCN races resisting these varieties. That means farmers need to use integrated approaches that couple SCN-resistant varieties while considering use of seed treatments and rotation to nonhost crops. “I liken it to monitoring blood pressure,” says Greg Tylka, an ISU Extension nematologist. “We have had high blood pressure in the form of SCN but have not had to check it because resistant varieties were taking care of it.” No more. As with a high-blood pressure patient, soybean farmers need to sample their fields for SCN and use an integrated approach to managing it, he says. 

There’s good news on the Palmer amaranth front in 2018 in Iowa. Populations exploded in 2016, when pollinator mixes containing Palmer amaranth were planted in Iowa. 

In 2018, though, this pugnacious pigweed spread to just two new counties in Iowa. “There is no evidence of a rapid spread,” says Hartzler. 

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