You know the pure glory of a yield monitor hitting 300+ bushel corn or maybe 70+ bushel soybeans. But what if the monitor showed waterhemp seed numbers scattered inside the combine and spread out the back, like 15,000 to 100,000 herbicide-resistant seeds per plant?
Taming the continued onslaught of Midwest weeds that drive your weed control program—giant ragweed, marestail, waterhemp, Palmer amaranth and others—requires diligent scouting, weed I.D. and a diverse control plan focused on these herbicide-resistant drivers. Since escaped weeds add to the soil seedbank every year, tracking growing weed patches as you harvest fields this fall is a wise investment.
Iowa State University's longtime weed scientist Bob Hartzler stated during a recent webinar that reviewed weed control over the last 40 years that waterhemp truly changed how farmers manage weeds. “It is the best example of the consequences of high reliance of herbicides for weed control,” he said.
The spread of native waterhemp from wet areas near fields to taking over croplands began in the mid-1980s in Missouri and southern Illinois, then began migrating across the Corn Belt northward to North Dakota by 1998 and eastward to Ohio by 2017. As a result, Hartzler believes we’ll see greater use of heavy biomass cover crops to help suppress waterhemp and other herbicide-resistant weeds, and methods to harvest and destroy weed seeds using the combine.
Courtesy: Iowa State University
In a perfect world, you’re headed into harvest with weed patch maps by field to harvest those areas last so you don't spread weed seed beyond those patches. But, since that isn’t your scenario, logging what weeds you see and dropping a pin location can help optimize a weed control plan. “Farmers can use all kinds of scouting and tracking technology with available apps to mark locations and remember them for future years,” says Meaghan Anderson, field agronomist, Iowa State University Extension.
While harvest is the priority, watching for weed patterns in a field can provide insight on how to best tweak your herbicide program for the following year. “The first step to effective weed management is preventing new issues from coming into crop fields from borders and ditches,” says Anderson. Field edges provide opportunities for weeds like giant ragweed or Palmer amaranth to establish. Watch when opening fields and when cutting along field edges.” Some edges may have more weeds due to lower doses of chemicals used in border rows required when farmers use dicamba or Enlist products under specific scenarios.
Usually, two types of weed patches exist in a field unrelated to sprayer application: drowned-out spots and herbicide-resistant weed patches. The best advice for drowned-out areas full of weeds is to steer the combine around them. Harvesting through it spreads weed seed further to make the patch larger and carries weed seed into following fields.
“If you leave that weed patch alone to drop its seed there, you’ve contained and not spread the problem. Just take note that these areas may need special management next year due to a huge weed population,” Anderson says.
Herbicide-resistant weed patches are usually consistent and growing yearly to defy your herbicide plan. Along with taking note of their locations, Anderson advises to collect seed if your local land-grant institution offers herbicide resistance testing. “By comparing the resistance profile of that weed species to the current herbicide program, you know if a new resistance issue is building in this population,” she says.
Courtesy: Iowa State University
While waterhemp is a driver weed as Anderson scouts her central Iowa region, other farmers may need to tailor next year’s herbicide programs toward the most prevalent weeds they find at the end of this growing season. “We're occasionally hearing of species like cocklebur, Pennsylvania smartweed or velvetleaf hanging around at harvest that will contribute to weed issues next spring,” Anderson says. “Harvest is a good time to think about tweaking your herbicide program, application timing or consider other non-chemical tactics to create a better plan.”
Other tactics like narrow rows are helping soybean farmers improve weed control and yields, yet some challenges occur with white mold further north in the Midwest. Experimentation continues with narrow-row corn, but success isn’t quite there yet—unless you live in sugar beet country where all crops are on 22-inch rows.
Cover crops, especially cereal rye after corn ahead of soybeans, are becoming a game changer for weed suppression that helps reduce input costs. “Cereal rye can deliver some pretty amazing weed suppression when growers plant soybeans early, let the rye continue to grow until just before the soybeans emerge and then spray to kill the rye,” Anderson says. “Soybeans are remarkably tolerant of rye residue.”
In the future, Anderson believes the Australian’s success with destroying seeds out the back of the combine may offer a great alternative to reducing the weed seedbank. “There's a couple of seed destruction units being tested in Iowa, and they're incredibly effective against some weed species such as waterhemp. So, in the future, we're not just going to be scouting. We may actually make a final weed control pass at harvest,” she adds.