Soybean production is a game of inches every crop season. Do growers plant in 7.5-in. rows? Fifteen-in. rows? Thirty-in. rows?

The row spacing question has taken on greater significance with Asian soybean rust now a potential threat in the U.S. 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, says Shawn Conley, Purdue Extension soybean specialist.

"In meetings that we held across Indiana last year, growers had concerns about soybean rust," Conley says. "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-in. row spacings to 30-in. 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-in. rows."

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

The soybean study also examined yield differences between the various row spacings and the affects 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-in. row spacings," Conley says. "In general what we've seen in the past is a zero 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-in. 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 says. "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."