How Weeds Spread in Corn Fields
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Taking notes of the weeds you see from the combine seat each fall offers a glimpse into weed challenges next spring. It also provides you with important data to how weeds may be spreading in fields and/or contaminating clean fields.
Rodrigo Werle, a University of Wisconsin-Madison (UW-M) weed scientist, says step one is taking good notes of weed locations and infestation levels to understand how weeds move in and between fields. “With this information, you can be more strategic about herbicide programs and rates in weedy areas and change harvest order to combine the weedy areas last.”
Watch Headlands And Borders
Observing weed movement begins from the headlands (turn rows) and field borders. “These areas often don’t get a full herbicide dose, and the corn crop is slower to develop and canopy, so that’s where most weed problems begin and grow,” Werle says.
While water, wind and birds can spread weed seeds, the majority is spread by growers running weeds through combines. In addition, fall tillage increases the spread of weed seed and buries them for future emergence.
“I’d see little waterhemp pockets on headlands and field edges. Once a combine went through those areas, the next year, you’d see strips through the fields,” he says. “Two years later, waterhemp was present across entire fields. We’ve seen such patterns of weed spread in research fields, and farmers tell me similar stories of their fields.”
Werle has watched herbicide-resistant waterhemp spread throughout Wisconsin since 2018. Over the years, only the toughest waterhemp plants survive, and each plant releases an average of 25,000 seeds annually.
The biology of waterhemp allows the female plant to retain 85% to 90% of its seed until harvest, unlike many other weeds that drop seed before the combine rolls through. As the combine chews through waterhemp, a lot of seed goes airborne out the back with crop residue. Many weed seeds ride along within the combine, waiting to dislodge throughout the field and into other fields.
Alter Harvest Order
You can reduce weed spread by becoming more proactive at harvest and paying close attention to weed species, location and density within each field. Mapping these by field each year helps track the weed spread.
Werle recommends harvesting weedy areas last to reduce the weed spread. He understands the time crunch of harvest, but this is a critical step to help contain herbicide-resistant weeds.
After recognizing problem areas, extra control measures next spring can begin to reduce the size of the problem. “Build a strategy such as using higher preemergence herbicide rates in those areas, and maybe an extra postemergence pass along headlands and borders,” he says. “We’re also seeing a very promising weed reduction strategy when achieving good spring biomass growth from cover crops.”
Reduce Weed Seed Longevity
Considering it took years for herbicide-resistant weeds to get established, it’ll take longer to reduce an infestation. Seeds of small-seeded species like waterhemp, Palmer amaranth and others can remain viable four to five years in the soil weed seed bank. Research shows that some weed seeds can last longer, like 17 years for lambsquarters and 10 to 12 years for velvetleaf. Yet, giant ragweed only lasts two years in the soil.
While only a fraction of the total weed seed bank germinates every year, this can change depending on how tillage is used. Small-seeded weeds only germinate in the top ½-inch of soil, while large-seeded weeds like giant ragweed can easily germinate from two to three inches deep.
Clean The Combine
The final method to reduce the spread of weed seed is cleaning the combine. Research shows that approximately 150 pounds of biomass remains in the combine after harvest.
Unfortunately, finding all the weed seed hiding spots and removing the seed is difficult. Check out this video by UW-M that details the correct combine clean-out process to limit the spread of weed seed.
Werle and UW-M colleague Dan Smith understand this difficulty first-hand as they hold combine clinics each year before harvest. “Of the 14 combines we examined by our group before harvest, only one, a new combine, didn’t have any weed seed in it,” he says.
Due to the combine effectiveness as weed seed carrier, Werle hopes more farmers will take 20 to 30 minutes between fields to at least blow out the front end of the combine. This Iowa State University video shows a between-field combine clean-out procedure.
“We’re really concerned about stopping the spread of these tough weeds we struggle to control—waterhemp, Palmer amaranth, common and giant ragweed, and others—because spraying [herbicides] alone won’t solve it,” he says.
In times of razor-thin profit margins, no grower wants to spend more input dollars on weed control. By following these strategies to limit the spread of your toughest herbicide-resistant weeds, you can control your future weed control costs better.
More Information
- VIDEO: How to Clean a Combine to Limit the Spread of Weed Seed
- Weed Seed Movement via Combines (2019-2020 Case Study)
- Weed Seed Management at Crop Harvest
- Between-field Combine Clean-out
Content provided by DTN/The Progressive Farmer
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