The spread of multiple-resistant weeds to herbicides threatens Iowa’s crop production system. In order to slow the expansion of resistance, greater diversity in weed management is necessary.
Weed specialists with Iowa State University Extension and Outreach were hoping for a highly visual display of weed identification and weed control measures at this year’s now canceled Farm Progress Show. Although those displays will not be held, the team is actively working on its outreach related to weed identification and weed control.
Proper identification first step
Bob Hartzler, professor in agronomy and ISU Extension weed specialist, is in the process of creating a directory of weed identification fact sheets that are available online and feature high-resolution photos and descriptions of common weeds found in Iowa.
The identification sheets began as a project for a class Hartzler teaches at ISU, but they’re quickly becoming popular for farmers as well. About 40 weed sheets are available, and the goal is to reach more than 100. The descriptions also offer some historical and unusual information about weeds, that people may not already know.
“I try to convince people that weeds are more interesting than just a plant that needs to be killed,” Hartzler says.
In addition, the crops team at ISU Extension is working with the ISU Integrated Pest Management Program to produce a series of videos that help explain weed identification.
When farmers know the exact weeds they’re dealing with, they can choose the best control strategy and the best timing.
“The best control strategy differs from weed to weed,” Hartzler says. “A different weed can respond differently to a different control strategy.”
Cover crops and weed control
ISU weed specialists are also studying the benefits of cover crop biomass as a method of weed suppression for waterhemp early in the growing season. As biomass increases, cover crops have a dramatic effect at suppressing waterhemp, as well as reducing herbicide use.
Cereal rye has the best potential to suppress weeds because it accumulates more biomass than other cover crop species. A study that was done for the Farm Progress Show shows an incremental decrease in waterhemp based on the density of rye.
Prashant Jha, ISU associate professor and Extension weed specialist, is leading a team doing field studies that indicate cereal rye biomass of about 5,000 pounds per acre at termination can significantly suppress waterhemp emergence in soybeans, and reduce the size and density of waterhemp at the time of exposure to postemergence herbicides.
Harvest weed seed control
Jha is also studying various late-season tactics to control and eliminate weed seeds especially for species such as waterhemp that has evolved resistance to multiple site-of-action herbicides.
Chaff lining is a method that guides the harvested chaff into narrow bands as it flows out the back of the combine at harvest, which greatly reduces the spread of weed seeds across the fields and contains weed seeds in smaller spaces. The harvester or combine is modified with a baffle that separates the chaff (containing the majority of weed seeds) from the straw. The chaff is directed into narrow central bands using a chute at the rear of the combine.
Weed seeds in the chaff are subjected to decay, and burial of small-seeded weed species such as waterhemp in the chaff will potentially result in reduced emergence in the subsequent growing season. High application rates of herbicides or shielded sprayers can be used to selectively control emerged weeds in those narrow bands in the field.
Destructor shows promise
Another option is equipment that grinds and destroys the weed seeds, referred to as a weed seed destructor. Commonly used in Australia to fight herbicide resistance, this technology collects the weed seeds as they pass through the combine and mechanically destroys them through a grinding process (high-impact mill).
Harvest weed seed control is still in the research stages and some of the equipment can be costly, but Jha is hopeful that using a chaff liner is a relatively cheap option with minimum modifications to the combine and will gain more traction in the U.S. These tactics to capture weed seeds will be effective for weed species such as waterhemp that can retain more than 90% of seeds at the time of soybean harvest.
“It’s hard to say what form of harvest weed seed control will be adopted for use by farmers, but I’m confident that within five to six years, we’re going to see it done,” Jha says.
The overall goal is to reduce weed seed banks with a reduced reliance on herbicides. This has the potential to save farmers money and reduce the long-standing issue of herbicide resistance while preserving existing herbicide chemistries for long-term use.
Kick is an ISU Extension communications specialist.