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.
- January 2010