Soybean production is a game of inches every crop season. Do growers plant in 7.5-inch rows? Fifteen-inch rows? Thirty-inch rows?
The row spacing question has taken on greater significance with Asian soybean rust now a potential threat in the United States. Fortunately for producers, Purdue University research indicates that row width has no bearing on fungicide spray coverage.
Shawn Conley, Purdue Extension soybean specialist, his research team conducted field studies at Purdue-owned farms in Randolph, Whitley and Jennings counties in Indiana. "We found that across all three locations there was absolutely no difference in spray penetration between 7.5-, 15- and 30-inch rows."
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 says. "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."
Spray penetration most effective of at least 12 inches
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 says. "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 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-inch row spacings," Conley says. "In general what we've seen in the past is a 0% to 3% yield difference between the two."
Conley explains the team did see a significant yield loss when moved from drilled beans to 30-inch row spacings. Those yield losses were anywhere from 7% to 10%.
Conley and his team observed that pulling sprayers and other equipment through soybean fields once soybean pods began to develop significantly reduced crop yields.
Wheel track damage minimal at R1 stage
"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% to 6% 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% yield loss but with a 90- or 120-foot spray boom we're between about 1% and 2%."
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 online at www.agry.purdue.edu/ext/coolbean/PDF-files/Final_Report_05.pdf .