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Trading spacings? Fungicide unaffected by soybean row widths

Agriculture.com Staff 03/14/2006 @ 9:36am

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."

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