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Trading spacings? Fungicide unaffected by soybean row widths

Soybean production is a game of inches every crop season. Do growers plant in 7.5-inch rows? 15-inch rows? 30-inch rows?

The row spacing question has taken on greater significance now that Asian soybean rust is a potential threat in the United States. Fortunately for producers, Purdue University research indicates that row width has no bearing on fungicide spray coverage.

The research project was inspired by farmer inquiries, said Shawn Conley, Purdue Extension soybean specialist.

"In meetings that we held across Indiana last year, growers had concerns about soybean rust," Conley said. "One of the main questions from them was, 'Do you think we can get better spray penetration through the plant canopy by moving from 7.5- or 15-inch row spacings to 30-inch row spacings?' This past crop season we put that question to the test."

Conley and his research team conducted field studies at Purdue-owned farms in Randolph, Whitley and Jennings counties.

"We found that across all three locations there was absolutely no difference in spray penetration between 7.5-, 15- and 30-inch rows," he said.

In order to protect a soybean plant from rust, fungicide must reach the plant canopy. Achieving adequate spray coverage on the lower canopy helps the soybean plant remain healthy as it grows and puts on new leaves.

"The reason we feel there isn't a difference in spray penetration between those row spacings is kind of simple, if we think about it," Conley said. "First, ask yourself, 'Why do we plant beans in 7.5-inch row spacings?' It is done, in theory, to get the plants as equidistant as possible in order to capture more sunlight. That's generally why we see higher yields in 7.5-inch versus 30-inch row spacings. So by putting our plants more equidistant, we're actually allowing plants to have leaves all the way around the soybean canopy profile.

"When we plant beans in 30-inch rows, there's a soybean plant in front and one right behind each plant, so the only way those leaves can go is straight out. So, if we compare those two row widths, we're changing the soybean profile, which is why we really don't see a difference between 30- and 7.5-inch rows."

Purdue's research found that at all soybean growth stages, fungicide spray penetration to a depth of at least 12 inches into the plant canopy was reasonably good. Spray coverage decreased significantly at depths of 24 inches or more.

"If fungicide were applied as soon as rust began to develop in a field, as it should, penetration of 12 inches may reach the site of most primary infections," Conley said. "There seems to be no reason to move to 30-inch rows solely for the purpose of improving fungicide performance."

The soybean study also examined yield differences between the various row spacings and the impact of wheel traffic on the crop.

Overall, yields were less variable among narrower rows than wider rows.

"At our three locations, we did not see any differences between the 7.5- and 15-inch row spacings," Conley said. "In general, what we've seen in the past is a 0 percent to 3 percent yield difference between the two. However, we did see in our research a significant yield loss when we moved from drilled beans to 30-inch row spacings. Those yield losses were anywhere from 7 percent to 10 percent."

Conley and his team observed that pulling sprayers and other equipment through soybean fields once soybean pods began to develop significantly reduced crop yields.

"Another question on growers' minds is wheel track damage caused by making late-season spray applications to the soybean canopy," Conley said. "Our data showed that if we made applications on soybeans at the R1 stage or earlier -- where R1 is first flower -- we did not see any significant yield loss. However, past the R1 soybean stage we tended to see significant yield loss caused by wheel track damage -- from 1 percent to 6 percent on average.

"Those yield losses generally are based on how wide the spray booms are. On a 30-foot spray boom we're looking at a 6 percent yield loss, but with a 90- or 120-foot spray boom we're between about 1 percent and 2 percent."

Other research team members included Greg Shaner, Purdue Extension plant pathologist; Judy Santini, Purdue research analyst; and Shane Hanna, agronomy graduate student.

The Indiana Soybean Board provided financial support for the research project. Technical assistance was provided by the farm managers and crews at the Purdue Davis, Northeast and Southeast agricultural centers.

For more information about the soybean study, read "Effect of Soybean Row Spacing and Fungicide Application Timing on Spray Canopy Penetration and Grain Yield." The publication, written by the research team, can be downloaded at

Soybean production is a game of inches every crop season. Do growers plant in 7.5-inch rows? 15-inch rows? 30-inch rows?

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