Field Day: SIU studying crop-based strategy to suppress waterhemp

The field day featured several research projects.

Southern Illinois University graduate assistant Madison Decker discusses her study of intercrop winter wheat as a practice to suppress waterhemp in a soybean crop. Decker’s research trials were among field presentations on the University Farms near Carbondale. (Photo by Kay Shipman)

By Kay Shipman

Southern Illinois University (SIU) scientists are trying to suppress waterhemp by controlling weed seed germination in soybean fields.

Madison Decker, a weed science graduate assistant, discussed her research during this week’s field day on University Farms, Carbondale. The event, which featured several field research projects, was sponsored by SIU College of Agricultural Sciences, the Illinois Nutrient Research and Education Council (NREC) and Illinois Farm Bureau. 

“This is wonderful,” said NREC Chairman Jeff Kirwan of the research projects and field day presentations.

Kirwan, an IFB director, noted the highlighted research projects covered water quality issues, precision nitrogen applications, cover crops and gypsum as a phosphorus source. “All these are tools that southern Illinois farmers are looking for,” he said.

Decker is conducting field trials in Carbondale and at SIU’s Belleville Research Center. She is using intercropped winter wheat in soybeans to suppress waterhemp.

“Any time there is bare soil with sun, weeds can germinate, but a mulch layer will reduce weed germination,” Decker said.

Her trials involve drop spreading 1.5 million wheat seeds per acre. “We want uniform coverage; that’s why we drop spread,” Decker explained. If a farmer has to drill the wheat, she recommended “coming at it two ways to get good coverage.”

This year, the wheat was spread in April and soybeans were planted in late May. The wheat will be terminated “relatively soon,” she reported. Early results appear promising with her fields “looking like a grass field” with few weeds, she noted.

Previously, the wheat and soybeans were planted the same day with wheat terminated when soybeans reached the V5 stage. Wheat must be planted in the spring because fall planting would require early termination to prevent heading, but the dropped leaves would reduce soil cover, Decker noted.

Decker’s study includes field plots a soybean-wheat intercrop with no herbicide treatment, soybeans only with a “Cadillac” herbicide treatment, soybeans-wheat intercrop with post-emergence treatment, and soybeans with a post application.

Decker reported some of the best waterhemp control results were in soybean-wheat plots with a post application, reaching 93% and 98% control in 2017 and 2018, respectively.

“My concern is two crops (in the field at the same time) influencing yields,” Decker said. In 2017, average soybean yields differed from 57 bushels per acre in soybean only plots to 38 bushels per acre in the wheat-soybean plots, which Decker attributed to rain. In 2018, plots showed no significant yield differences among the treatments.

Decker posed several questions she hopes her research trials answer. These include: the best time to drop spread winter wheat for weed control while maintaining soybean yields; most effective time to terminate winter wheat while maximizing biomass and soybean yields; most effective residual herbicide options in an intercropped system; and current products that could be applied to soybeans to reduce stress in an intercropped system.

“I’m not advocating getting rid of herbicides. My goal is to integrate the two (intercropping and herbicides) so farmers have more tools,” Decker summarized.

Video: Illinois farmers can increase corn yields by 15-20% if the seed is oriented perfectly in the ground and the growing plant does not cast a shadow on neighboring plants.