Research Highlights

Evaluating Harvest Weed Seed Control to Lessen Resistance and Contamination

By Barb Baylor Anderson

More than $2 billion in potential profits is lost annually to herbicide-resistant weeds in soybean production, according to a study funded by the United Soybean Board. Horseweed, Palmer amaranth and waterhemp are all weeds that can drive yields lower and treatment costs higher.

“This work (regarding herbicide-resistant weeds) profiles just one example of what can be learned from the Soybean Research & Information Network (SRIN),” says Tom Oswald, soybean farmer from Cleghorn, Iowa. “As a farmer leader who serves on checkoff boards, I am often asked by farmers what we do with the money. SRIN allows farmers to easily and intuitively look up checkoff-funded research projects for advice like never before.”  

Prashant Jha, Iowa State University associate professor and Extension weed specialist, is principal investigator for this research project funded by the Iowa Soybean Association.

“The multiple-resistant waterhemp epidemic has rendered valuable herbicides ineffective, increased weed control costs, prompted a reversion to tillage and increased use of herbicides with more adverse environmental impact,” Jha says. “This warrants implementation of multi-tactic approaches to manage resistant weed seedbanks in soybeans.” 

Jha is evaluating a relatively inexpensive harvest weed seed control (HWSC) technology he believes will have a significant economic impact on soybean production and reduce weed seed contamination at harvest. The technology has already been found effective in Australia.

“The success of HWSC relies on the propensity of annual weed species to retain seeds until soybean harvest. Then, HWSC strategies involve collection and/or destruction of weed seeds during crop harvest to minimize weed seedbank additions,” he says. “Weed seeds are intercepted by a combine and separated from the bulk crop residue and grain for subsequent management.”

Jha is testing an HWSC technology called chaff lining, which confines chaff material between stubble rows during harvest and relies on a mulch effect to prevent or reduce weed seed germination and emergence and also reduce weed seed survival through enhanced predation and microbial decay in chaff line residue. He says chaff lining technology holds the greatest promise for use in soybeans since it is relatively inexpensive with minimum combine modifications. 

“A majority of the weed seeds are in the chaff fraction of the chaff and straw that get spread out from the rear of the combine. These chaff rows are typically established by retrofitting a combine with a baffle to separate the chaff from the crop residue or straw and using a chute at the rear of the combine to collect and place chaff into 8-12-inch narrow rows,” he explains. 

Crop residues, or the chaff fraction that includes weed seeds, are deposited directly behind the combine in narrow rows at the center. That means chaff lining concentrates weed seeds to less than five percent of the field, rather than spreading them across a whole field. 

Concentration of the chaff material additionally places weed seeds in an environment unsuitable for germination and emergence, if left undisturbed. Chaff lines can then be targeted with additional weed control tactics like narrow windrow burning in fall or shielded sprayers/ herbicides the next spring. Seed destructors and narrow windrow burning are other HWSC technologies that have been successfully used in Australia.

“The process of chaff lining, by its nature, can ultimately help decrease weed seed contamination of soybean grain,” he says. 

Jha ran a chaff liner last fall and is currently evaluating the results. Observations during spring 2021 showed that although the chaff lines with weed seeds had settled down a bit over the winter, there had been minimal disturbance caused by wind or other factors. This concept will also be tested across multiple fields in Iowa this fall.

Once completed with the project, Jha hopes to better understand the efficacy of chaff lining on reducing weed germination and seedling emergence and reducing weed seedbanks while increasing soybean yield potential. He also will see how herbicide programs influence efficacy of chaff lining for mitigating herbicide-resistant weed seedbanks and soybean seed contamination.

Jha will test the weed seed kill efficacy with a Redekop Seed Destructor unit at soybean harvest.

Jha adds the chaff lining method will also be beneficial for organic farmers. “I don’t have specific information on the economics of these HWSC technologies in Australia, but I am sure returns on the investment are high four to five years after adoption when farmers see drastic reductions in herbicide-resistant weed seed banks and in herbicide use and cost,” he says.

Published: Jun 1, 2021