Evaluate the Fungicide Factor

January 7, 2010 06:00 PM

Think applying a fungicide will bump your soybean yield 10 bu. or 15 bu. per acre? Well, it might, if you have a significant amount of disease, such as frogeye leaf spot. But what about a preventive application in the absence of serious disease, and what if soybeans are selling for only $6 a bushel?

"Knowing when to spray a fungicide on soybeans is not as simple a decision as it is with corn,” says Jason Webster, Practical Farm Research director for Beck's Hybrids. "I spend a lot of time evaluating not only whether to spray a fungicide but, more importantly, when to spray.”

The results of two 2009 trials offer food for thought as you plan your 2010 soybean program. The trials were conducted by Farm Journal Associate Field Agronomist Missy Bauer and by Webster for Beck's Hybrids.

Both studies show the importance of economics when deciding whether to spray fungicides on soybeans. "In 2009, we had one location with a 16-bu. response to fungicide,” Bauer says. "That was on irrigated soybeans. But most of our responses were 3 bu. to 5 bu. per acre.” (See "Dry-Year Yield Response,” page 52.)

"Many farmers think fungicide is the next thing that will bump yield to a whole new level, and there's a lot of hype about adding 10 bu. to 15 bu. per acre,” Bauer continues. "In reality, fungicide helps in minor increments.

"Economics should drive your decision. During six years of studies, we have averaged a 3.6-bu.-per-acre yield increase with fungicides,” she says.

Cost of the fungicide and application averaged $26 per acre ($19 for the product and $7 for application) in 2009. With $10 soybeans, you need 2.6 bu. to break even, so a 3.6-bu. yield increase equals $10 per acre. But if soybeans were only $6 per bushel, the application wouldn't pay, Bauer says.

"Of course, if you were making an insecticide application anyway and applied a fungicide at the same time, you would eliminate the application charge and lower the break-even point,” she adds.

Larger yield increases do occur, Bauer says. They result from heavy pressure of diseases, such as frogeye leaf spot, as opposed to baseline disease levels that are always present. But if you wait to spray until you can see heavy disease pressure, the yield benefit will not be as great as if you had made a preventive application.

Complicating the question, Bauer's study indicates a varietal response to fungicide application. In one plot, conducted in cooperation with The Andersons, where she compared 22 soybean varieties, yield increases ranged from 0.3 bu. to 5.7 bu. per acre (see "Not All Varieties Respond to Fungicides,” page 52). One variety showed no response, and eight varieties actually lost yield.

"The differences may have resulted from the absence of disease or because some varieties have more disease tolerance,” Bauer says. The plot was planted in 30" rows, so disease incidence may have been lower than in narrower rows and with less air circulation.

In another trial, conducted in cooperation with The Andersons' Litchfield, Mich., Farm Center, Bauer recorded a yield response where she did not expect it. That plot received only 1.7" of rain from June 21 to Aug. 16, scattered among seven rainfall events, so yield was limited because of drought. "The soybeans were very short—less than 20" tall,” Bauer says.

Quadris, a fungicide, was applied alone and in combination with Warrior insecticide at the R3 stage of growth. At 29.5 bu. per acre, the Quadris plot outyielded an untreated check by 4 bu. per acre (see "Dry-Year Yield Response,” above). Where insecticide was applied along with the fungicide, the plot produced 30.9 bu. per acre, or 5.4 bu. more than the untreated check.

"You would not expect a yield response from a fungicide under these conditions,” Bauer says. "But because these plants were under drought stress, disease probably affected them more than it normally would, especially after rain began to fall later in the season.

"I think the additional yield increase from the insecticide application resulted from some residual spider mite control,” Bauer continues. (The plots had already been sprayed for spider mites earlier in the season.)

Four-state study. For the study conducted by Beck's Hybrids, Webster involved more than 30 farmers in Illinois, Indiana, Ohio and Kentucky. The study compared fungicide applications from the R2 through R4 growth stages. Webster monitored leaf wetness,
humidity, temperature and rainfall. Headline, a preventive fungicide treatment, and Stratego, a preventive and curative fungicide, were applied.

In Webster's study, the cost of the fungicide averaged $16.40 per acre and the cost of application averaged $5.50 per acre, for a total cost of $21.90 per acre. With soybeans valued at $9.55 per bushel, the break-even yield increase was 2.3 bu. per acre.
The average gains from applying fungicide at the R2 growth stage were 2.4 bu. per acre—barely inching out break-even. With applications at the R3 stage, the average response was 2.2 bu. per acre. Applications at R4 only averaged 1.4 more bushels per acre—so the R3 and R4 applications produced a negative return.

The study shows how important weather data can be in deciding if and when to apply fungicides. From 2006 through 2008, Webster recorded weather data at Downs, Ill., building a threshold model for spraying based on leaf wetness.

"Until 2009, we received the best response from fungicide applications when soybean leaf wetness levels averaged near eight hours of the day,” Webster says. "We were hoping the study could help build a model showing when or if a producer should make a fungicide application.”

But the 2009 study showed that temperature as well as moisture must be taken into account. "Overall, even though moisture levels were very high—with average leaf wetness at 9.8, 10.8 and 11.3 hours per day—in most trial locations, disease pressure was very low, as a result of the very cool summer temperatures,” Webster says.

"We need moisture and temperature for leaf diseases to thrive. This is when fungicides may have the best opportunity to offer higher yield responses,” he summarizes.

In two locations—one in Ohio and one in Illinois—where weather was dry, fungicide applications during the R2 and R3 stages had no effect on yield. But when rain began to fall during the R4 stage, a fungicide increased yield by about 2.5 bu. per acre in one trial and 3 bu. per acre in the other.

"I'm advising farmers that, once the R2 stage has occurred and they are experiencing dry conditions, they generally need to wait for more favorable weather during the R3 and R4 growth stages,” Webster says.

Farmers who don't have their own weather stations might consider combining county-level weather data with their own rainfall figures to help determine whether spraying fungicides is likely to pay, he suggests.

In summary. More research on weather and fungicides is needed, says Webster, who hopes to continue his study if the participating farmers are willing.

"Compared to soybeans, applying corn fungicides is easier,” Webster says. "With corn, we have a period of application timing from full tassel to brown silk. Soybeans have a much larger application window.

Under ideal growing conditions, soybeans can reach a new growth stage every 10 days. This means that from R2 to R4, we could have an application window of 30 days.

"Varieties respond differently, as well. We need to know how tolerant soybean varieties are to leaf disease,” Webster adds.

"Our fungicide trial results don't generate a lot of excitement,” says Farm Journal Field Agronomist Ken Ferrie. "They show you may hit a home run, but usually the yield increase from fungicide application is 2 bu. to 4 bu. per acre.

"When we consider whether to apply a fungicide, we monitor the weather and consider the environment, such as beans on beans, and the susceptibility of each variety to disease. We don't know when soybean rust is going to come rolling through, and you don't want to be caught by surprise. So don't write off fungicide treatment because you tried it once and only got a small yield increase.

"Make it a routine to scout soybean fields for disease, so you know what's going on,” Ferrie concludes. "As a bonus, you'll catch insect infestations before they become a problem and while there's still time to treat.”

Fungicide Results

From 80 trials conducted during a six-year period in southern Michigan, Indiana and northwest Ohio, Missy Bauer recorded these average yield responses:

2004 – 4.0 bu. per acre
2005 – 3.8 bu. per acre
2006 – 3.1 bu. per acre
2007 – 2.9 bu. per acre
2008 – 3.1 bu. per acre
2009 – 4.8 bu. per acre

Six-year average: 3.6 bu. per acre

"You always have some base level of disease,” Bauer explains. "The yield response to a fungicide will be based on that level—usually about 3.5 bu. or so. Greater yield increases may result if you have a high level of disease.”

How Fungicides Boost Yield

If you tally a yield response from a soybean fungicide, most of the increased yield usually results from larger soybeans, explains Missy Bauer, Farm Journal Associate Field Agronomist. In one study in Coldwater, Mich., in 2009, it required 2,842 beans from an untreated check plot to make a pound, compared with 2,685 beans from a plot treated with a fungicide.

"Larger seeds alone usually increase yield by 4 bu. to 5 bu. per acre, and sometimes by as many as 8 bu. or 10 bu.,” Bauer says.

"We expect a fungicide to increase yield,” says Ken Ferrie, Farm Journal Field Agronomist. "But for seed producers, its effect on seed quality, including uniformity of seed size, is equally important.”

Treating seed with fungicides. In addition to foliar fungicide applications, farmers must also consider whether to treat seed. "Foliar fungicide treatments protect yield, and seed treatments protect the stand,” Ferrie says.

Seed destined for poorly drained fields, where water molds, such as pythium, are a frequent problem, are prime candidates for a planter-box fungicide treatment, Ferrie says. "Some growers plant higher populations in those fields to compensate for stand loss,” he says. "But if they get a year when all the plants survive, they'll wind up with too high a population.”

Once present in the soil, water molds never go away, Ferrie points out. Most of them need 48 hours of saturated soil to infect plants. "If you have fields that fit that description, it makes sense to plan to treat that seed with a fungicide, rather than trying to guess how much to overplant,” he says.

You can e-mail Darrell Smith at dsmith@farmjournal.com.

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