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Late Planting Changes the Rules

August 29, 2009
By: Darrell Smith, Farm Journal Conservation and Machinery Editor

The right variety can help minimize weather-related risks. Field conditions, yield potential, lodging, maturity group, herbicide program, grain composition and disease and pest issues are all part of the equation.

A soybean crop begins with placing seeds in the soil—but how many seeds per acre and in what row width? After 15 years of replicated test plots, Farm Journal Field Agronomist Ken Ferrie thought he had the answers—and he did, for planting at the normal time.

But 2008 replicated research shows that late planting knocks standard row-width and population recommendations into a cocked hat.

Plots planted in 2007 on the Bob Kuntz and Rod Wilson farms in central Illinois echoed the previous 15 years' results. During those 15 years, the soybeans were planted in either late April or early May.

On the Wilson farm, 120,000 viable seeds per acre had the highest yield in 10" and 15" rows and was only 1 bu. per acre lower in 20" rows. Only in 30" rows did 160,000 seeds per acre yield more. In none of the row widths did 200,000 seeds per acre produce the highest yield (see chart on page 20).

On the Kuntz farm, 120,000 seeds per acre produced the top yield in 10" and 15" rows. In 20" and 30" rows, 120,000 seeds per acre tied with 160,000 seeds per acre for the top yield (see chart on page 22).

"On the Kuntz and Wilson farms in 2007, we can't build a case for high plant populations,” Ferrie says. "However, it does get a little fuzzy whether 120,000 is enough in 20" and 30" rows. Based on 15 years of data, 20" and 30" rows did not raise any yield concern, but we could always see more scattered weeds in the wide-row plots.”

Things changed in 2008. Now consider the results for 2008, when wet weather delayed soybean planting until the end of June. On the Wilson farm, 200,000 seeds per acre produced the top yield in 30" rows. Planting 120,000 seeds per acre carried a yield penalty in 10", 15" and, especially, in 30" rows (see chart on page 20).

On the Kuntz farm, in 7½" rows and in twin rows (7½" apart on 30" centers), the top yield was a tie between 160,000 and 200,000 seeds per acre. In 30" rows, 160,000 seeds per acre produced the highest yield. Dropping to 120,000 seeds per acre in all three row widths carried a significant yield penalty.

Why were higher populations more successful in 2008, and why did the lowest population yield less? "From 1992 until 2007, we were following a normal planting cycle, planting soybeans on time,” Ferrie says. "Based on that research, it doesn't look like you can push yield by pushing population. But in 2008, when we planted in late June—more like double-cropping than full-season soybeans—that changed.

"Dealing with late planting, we have to throw out the window all of our knowledge from 1992 to 2007,” Ferrie continues. "The reason is late planting results in a short soybean. To offset that, we have to push population. In late planting, lower populations work against us. The results don't suggest that higher populations are where it's at; the 200,000 seeds per acre plantings didn't run away from 160,000. But the low populations paid a penalty in yield.”

The reason the lower populations yielded less in 2008 has to do with the nature of the soybean plant.

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FEATURED IN: Farm Journal - September 2009
RELATED TOPICS: Agronomy, Soybean Navigator

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