As weed resistance to herbicides continues to grow, with no new chemistry on the horizon, some people are asking whether herbicides are a once in a century method of weed control. “We’re at a point where herbicides don’t work as well,” says University of Minnesota Extension Educator Jared Goplen. “An annual weed becomes a perennial problem with herbicide-resistant weeds; it will take a combination of management practices with a focal point of interrupting seed production to control them.”
Any time you prepare to fight a worthy opponent, you want to learn as much as you can about that opponent. That’s what Jared Goplen, Extension Educator for Crops at the University of Minnesota, suggested corn and soybean farmers do who are fighting herbicide-resistant weeds. Goplen told farmers, crop consultants, and others at the 29th Annual Integrated Crop Management Conference in Ames in late November that they need to get to know more about the opposition—weeds.
“The first thing you need to do is identify, by field, which weeds are your biggest problem,” Goplen said. Then learn everything you can about that weed. Know when it emerges, how long it sticks around, how many seeds it produces, when those seeds are viable, and other details about its biology. That’s what you need to know so you can be confident you’re choosing the best control methods for that particular weed, and to time your treatments for that weed.”
Weeds can have very individual emergence and growing patterns, Goplen told the group, and knowing those patterns is essential to proper timing for herbicide and other treatments. He said 15 weed species in Minnesota and Iowa are confirmed resistant to herbicides. Two of those, giant ragweed and waterhemp, have very different growing patterns and need to be treated much differently.
Giant ragweed emerges early
“Giant ragweed is one of the earliest emerging weeds, with an emergence period of only several weeks,” Goplen said. “In 2013 to 2015 observations, giant ragweed began emerging in early May in southern Minnesota, and by June 11 about 95 percent of the giant ragweed plants had emerged,” Goplen said.
In fields tilled ahead of planting, delaying the tillage and soybean planting date can have a significant impact on giant ragweed density, he said. Trials conducted in southeastern Minnesota have shown that delaying soybean planting until mid-May or later can remove over half of the total giant ragweeds that emerge during the season. That compares to only 8 percent of the total giant ragweed that will be removed with early May tillage and planting.
Soybean yield potential is still over 94% of optimal with a mid-May planting date, according to long-term research conducted in Minnesota. The later planting date won’t eliminate the need for preemergence and postemergence herbicides, but will reduce the giant ragweed population needing postemergence control.
“Soybean yield potential of an early-may planting date will be close to optimal with earlier planting dates, assuming weeds are controlled,” Goplen said. “But in fields where heavy populations of giant ragweed are expected, you might consider trading a little bit of yield for more effective ragweed control, by tilling and planting just a few days later. Or, you might save that field with the worst giant ragweed potential to be the last one planted.”
Waterhemp emerges all summer
The strategy for controlling waterhemp is completely different from giant ragweed, since waterhemp emerges all summer long, from June through August. In this case, layering herbicides—a preemergence residual application at planting and then another residual herbicide application 30 days later, is more effective. “The preemergence herbicide doesn’t last long enough to control the bulk of the waterhemp that emerge from mid-June to mid-July,” Goplen said. “That second application around the first of June can extend control of waterhemp until August, when the soybean canopy can help shade out the small waterhemp plants that emerge late in the season.” Minnesota research showed as high as 95% waterhemp control with layered applications, compared to under 80% with a preemerge application only. It’s basically a split application and Goplen recommends you follow the label for amounts.
Stop weed seed buildup
The bad news on waterhemp is that one plant can produce 350,000 seeds or more per plant.
Considering that weed seeds can remain viable in the soil for years, one year of poor control can lead to significant weed control challenges for years to come. The good news is that if waterhemp isn’t allowed to go to seed, the waterhemp seedbank can be depleted by more than 99% in four years.
Research conducted in southeastern Minnesota demonstrated that the giant ragweed seedbank could be 97% depleted in two years if weed populations are managed intensively to prevent seed production. Roguing weed escapes prior to seed production can help prevent replenishment of the seedbank. Goplen said to keep in mind waterhemp can form viable seed 7 to 12 days after pollination, and seeds may mature on pulled plants if the plant pollinated before pulling.
Use other management tools
Goplen said some of the obvious management choices that help limit the spread and growth of weed populations aren’t always followed. “NOT running the combine through a weed patch will help limit the spread of weed seeds throughout the field,” Goplen said. “And mowing and otherwise managing weeds along field edges helps prevent buildup of the weed seedbank.
Other options, none of which control weeds by themselves but which can have a cumulative effect, are:
- Grow healthy crops to compete against weeds
- Use narrow rows for earlier canopy to shade weeds
- Pull out that old cultivator
- Consider small grains or alfalfa in the crop rotation