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.
"Soybean reproductive stages are triggered by day length—or, more accurately, the length of the night,” Ferrie says. "The number of hours of darkness tell the plants when to start flowering [based on maturity group]. Flowering is not triggered by vegetative stages, as it is with corn.
"Corn flowers when the tassel comes out, but beans can flower anytime. When a bean plant starts flowering, it is in its reproductive stages. As it moves through the reproductive stages, it slows its vegetative growth. The earlier a plant begins to flower in its vegetative stages, the shorter it will be and the fewer nodes it will have.”
Surprising results. Knowing that, Ferrie was surprised the 200,000 population was not the clear winner in every row spacing. He also was surprised to see the 160,000 population stay strong in all but 30" rows. "That knowledge gives us something of a comfort zone if we are in a delayed planting situation with narrow rows and wind up with only 150,000 plants,” he says. "We know you don't need 200,000 plants per acre; it doesn't hurt, but there's no obvious yield reduction in narrow rows.”
Ferrie's research from 1992 to now suggests there's a potential penalty for planting in wide rows. "It doesn't happen every year,” he says. "It depends on the variety and the weather. But when soybeans are planted late, population becomes a factor. In wide rows, you may pay a yield penalty due to short plants not being able to close rows, especially if you have hot dry weather in July and August.
"In 2008, when it got late, many farmers wanted to use their 24-row corn planters and plant their soybeans in wide rows. We recommended that they use auto-steer and double back to plant 15" rows,” he adds.
"In a no-till environment, this works well because the ground holds the tractor up. In tilled ground, you will wind up planting into wheel tracks and get stunted rows. But that's not a real big issue because the soybean plant will compensate.”
If 2010 brings a normal planting season, the 1992 to 2007 data will once again apply: Expect no yield premium from planting more than 120,000 viable seeds per acre. You may have better weed control in the fields with the higher populations, but planting 160,000 to 200,000 seeds per acre won't necessarily bring a yield penalty (although it will increase your seed cost).
What about soybean populations of more than 200,000 plants per acre?
"Whenever we plant populations of 220,000 or more, depending on the planting date, we get downed soybeans,” Ferrie explains. "For us, planting 220,000 seeds per acre in early April produces too much vegetative growth.”
Ferrie advises his clients to resist the temptation to use their wide corn planter to plant in 30" rows when faced with a late-planting situation. The only exception to that piece of advice is if they are in an area where white mold is a problem and the wide rows would aid in its management.
"Roundup Ready technology makes it tempting to go back to wide rows because it makes weed control possible,” Ferrie says. "Our data suggests a yield premium for staying away from 30" rows, even with normal planting dates. Our numbers suggest the difference between wide and narrow rows at the same plant population is about 3 bu. to 5 bu. per acre.”
(Note: All of the references to plant populations in this Soybean Navigator article refer to the number of viable seeds per acre, according to the tag on each bag of seed.)
You can e-mail Darrell Smith at email@example.com