Risks and Rewards

April 27, 2013 01:40 PM
Risks and Rewards

Prices drive foliar fungicide use in soybeans

Foliar fungicides were on the minds of few U.S. soybean farmers a decade ago. Today, soybeans are second only to corn in the number of acres treated, with strong commodity prices driving the increase in use.

That’s true for Trent Kuntz who farms near Oakville, Iowa. He saw a benefit from making a foliar fungicide application in soybeans even in 2012. "Under hot, dry conditions, we saw very good plant health and a real good yield response as well," he reports.

While Kuntz’s experience was positive, it is not the norm, says X. B. Yang, Iowa State University (ISU) Extension plant pathologist. He says most soybean growers see a payoff  with foliar fungicides only in the presence of yield-limiting disease pressure.

David Holshouser, Virginia Tech Extension agronomist, agrees. His research shows that their use in soybeans provides a return-on-investment only a third of the time in Virginia.

The reason: it’s difficult to predict whether foliar fungicides will pay off because the most common ones, the strobilurin products, are preventatives. This means they must be applied before disease develops.

A 3 bu. to 4 bu. per acre yield boost will likely pay for a foliar fungicide application in soybeans

"We’re applying a chemical to prevent a disease that may or may not progress to yield-reducing levels," Holshouser explains. "If there’s no disease we’ve wasted our money."

However, Holshouser adds that, given current soybean prices, farmers need less of a yield boost to make foliar fungicide applications worthwhile.

"If you can average a 3 bu. to 4 bu. per acre increase over all acres (based on April prices), then a foliar fungicide will likely pay for itself," he says.

Yang’s ISU data from the past six years shows that precipitation levels might also help farmers predict the outcome from foliar fungicide use: "When precipitation was plentiful, more than 50% of the fungicide sprays yielded an economical return and more than 70% provided a positive yield return."

Scott Rahn, Bingham Lake, Minn., says foliar fungicides are an excellent risk management tool that helps push his soybean genetics to their top yield potential. His on-farm test results during the past seven years indicate that, on average, he picks up an additional 5 bu. per acre from the  fungicide applications.

Tough to predict. For best results, Yang advises farmers to use a foliar fungicide at the R3 growth stage, when soybeans start to set pods. Applications at R5 or later rarely provide a financial benefit, he adds.

Holshouser’s recommendation varies slightly from Yang’s. "Our preliminary models tell us to not spray right at R3, but a few days later," he says.

The timing differences might be due to weather factors, Holshouser notes: "In 2012, we received just as much of a yield response from an R5 (beginning seed) application as we did from an R3 (beginning pod) application because weather conditions were just about as conducive for disease development." 

Foliar fungicide applications at R1 or earlier usually don’t provide a return on investment, except in the case of white mold control. Yang says an application at R1 is most effective because the fungus infects soybeans through dead flowers on the plants. 

Anne Dorrance, Ohio State University Extension plant pathologist, advises farmers to consider a foliar fungicide application in continuous soybeans and no-till soybeans. These fields tend to have high levels of residue on the soil surface and are at risk for early season foliar diseases.

Diseases that respond well to a foliar fungicide application include anthracnose, cercospora leaf blight, frogeye leaf spot, septoria brown spot, soybean rust, pod and stem blight. 

You can e-mail Rhonda Brooks at rbrooks@farmjournal.com.

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