Delayed Planting, Seeding Rate, Seed Quality Issues For State’s Soybean Growers to Mull Agri-View: Crop News

Planting date is one of the most important and least expensive ways to impact soybean yield. Unfortunately, most Wisconsin growers are frustrated by how little influence they have as to when the beans get in this spring.

Research at the UW Arlington Ag Research Station has shown an average yield loss of 0.4 bushels per acre per day when planting is delayed past the first week in May, report UW-Madison soybean specialist Shawn Conley and outreach specialist John Gaska. It’s a fairly straight line downward. Last year, soybeans planted May 8 yielded about 67 bushels. Planted the last couple days of the month, the beans yielded around 10 bushels less at Arlington. When they weren’t planted until the second week of June last year, they yielded just over 50 bushels.

Yield loss in delayed plantings can be attributed to decreased pod numbers. Pod number is the most important yield component in March, April and early May planting dates. While in late May and early June plantings, seed mass plays an equally important role in determining yield, say these UW experts.

Thus, they note, there’s good news - delayed planting of soybeans can partially compensate for reduced pod number per acre through increased seed size.

Conley and Gaska advised growers early last winter to read seed tags and “check the germ” in 2008, because they had concerns about soybean seed quality. At planting time now, they’re again reminding growers to check the percent germination of all their soybean seed lots and adjust seed rates accordingly.

In a normal year, growers can assume 90 percent of the live soybean seed they plant will emerge. To estimate stand population, do this math: Seed rate X percent germ X percent expected emergence = estimated stand.

Say you drill 180,000 seeds per acre of 94 percent germ seed and assume 90 percent emergence. Your estimated stand will be 152,280 plants per acre. However, if you plant 80 percent germ seed, your estimated stand will only be 129,600 plants per acre.

“Under most environmental conditions, 129,000 plants per acre would produce 100 percent yield potential,” they note. However, if you don’t achieve your assumed 90 percent emergence rate due to poor early-season growing conditions (which this year wouldn’t be out of the question…), then you rapidly approach stands where yield loss may occur.

This year, Conley says he’d be “extremely cautious” about planting beans into cold wet soils. “The questions regarding seed quality and vigor, coupled with the lack of replant seed make me a little leery this year to really push our soybean plantings,” he states.

Further, it takes soybeans longer to emerge than corn. Corn, on average, takes about 125 growing degree units (GDUs, base 50 degrees), where as soybeans take about 145 GDUs (base 50 degrees). Across planting dates, UW specialists have seen a range of seven to 30 days for soybeans to emerge. (A soybean plant is considered emerged when the cotyledons get above the soil surface.)

Planting date also greatly impacts the percentage of seeds that emerge, as noted by work at Purdue University in Indiana. Last year, percent germination at a March 27 planting date was just over 60 percent. With April 10 planting, germination was pushing 70 percent. On an April 30 planting date, percent germination was just over 85 percent. However, on May 9 in this Purdue study, it was well over 90 percent, and even higher from a June 1 planting date, only to decrease some when planting stretched out to June 7. However, germination percentage was still well over 90 percent. The story was different in 2006; there a definite environmental impact on planting date and year on emergence across 2006 and 2007, Conley points out.

Seth Naeve, University of Minnesota soybean agronomist, also thinks it’s important to focus on some “underlying principles of populations and yield.”

“Soybean plant populations do not create yield, yet maximum yields require sufficient populations,” he says.

Soybean stands need to be large enough to maximize light interception throughout the growing season and provide an abundance of “fruiting sites,” i.e. leaf axils, so pod set can be maximized, he explains. More plants allow more “potential” places for seed to set and mature. Thus, the minimum plant stand at harvest to maximize yield is the critical number to strive for, he reminds, adding that initial seeding rates help determine springtime stands. Those spring stands, in turn, help determine the number of plants that’ll ultimately bear seed and produce yield.

“Many years of studies related to hail injury have taught us that soybean stands of 100,000 plants per acre or more at harvest are sufficient for maximizing soybean yields. While most of this research has been conducted in southern Minnesota and Iowa,” that number is likely to be only “slightly conservative” for producers farther north (both in northern Minnesota and Wisconsin).

“Again, seeding rates serve only to establish this minimum stand. Increasing seeding rates provides returns only when stand establishment is poor,” he reports. “Increased seeding rates can serve as ‘insurance’ against poor stand establishment due to cold soil, or crusting, but when severe injury occurs (such as hail), replanting is rarely averted by small increases in seeding rates,” Naeve says.

Research in Minnesota looked at five seeding rates from 50,000 to 150,000 live seeds per acre, planted in 30-inch rows, at five southern Minnesota locations last year. It confirmed earlier work showing very low seeding rates under good conditions lead to maximum yields. Only the lowest seeding rate of 50,000 provided significantly lower yields than the higher rates. “In fact, seeding at a mere 75,000 live seeds per acre in these trials maximized soybean yield,” Naeve points out.

That’s significant considering that since 1998, weed control costs have been shifting more heavily onto the seed in the form of “tech fees” related to glyphosate resistance, he notes.

“Now soybean seed costs tend to be larger than soybean herbicide costs. This has led many producers to consider reducing seeding rates,” Naeve reports.

“If final stands drive yield potential, what affects final stands?” he poses, answering his own question: seeding rate, seed quality, soil type, soil conditions at planting and throughout emergence, stand establishment and late-season growth.

Higher seeding rates result in a greater percentage reduction in fall stands due to inter-seed and inter-plant crowding. This effect, he says, is accentuated in wide rows where inter-plant distances are much smaller. Seeding equipment plays a big role in determining emergence and final stands. If seed is distributed poorly within the row and/or vertically in the soil, you’ll have fewer plants at harvest (relative to seed planted).

How does row spacing impact seeding rate? In a 12-site-year study, Minnesota researchers found no interaction between row spacing and plant population.

Naeve says “narrow rows receive no greater yield benefit from higher seeding rates than do wide rows.

“We have seen a five-bushel-per-acre increase when moving from 30 to 10-inch rows,” he continues. About half of the yield boost was from 30 to 20-inch rows, and half was seen between 20 to 10-inch rows.

“Therefore, 22-inch rows should provide about a two-bushel advantage over 30 ninch rows, but are probably at a two-bushel disadvantage when compared with 15-inch rows,” he remarks.

Naeve says growers don’t need to plant narrow-row soybeans at a higher rate than those planted in wide rows. “Again, planter type will affect seeding rate needed to achieve minimum required stands. Grain drills and air seeders require higher seeding rates n not due to their narrow row arrangement but because of poor linear and vertical distribution of seed,” says this Minnesota soybean specialist.

Under ideal conditions, he says soybeans (in southern Minnesota) should be planted at about 140,000 live seeds per acre. In central and northwestern Minnesota, harvest stands of 125,000 to 150,000 plants per acre are needed for maximum yields n due to shorter-stature beans with fewer total nodes. Increased seeding rates are needed the farther north one is. Thus, soybean maturities point producers to reasonable seeding rates: Group II n 140,000 live seeds per acre; Group I n 150,000; Group 0 n 160,000; and Group 00 n 170,000.

Naeve adds a few “cautions,” noting that recommendations are independent of row spacing and based on live seed. Carefully examine germ rates on seed tags, he, too, advises.

He says his seeding rate suggestions are also based on excellent-to-ideal planting conditions. Planting into cold and/or wet soils may require increased seeding rates, he notes. Seeding into heavy clay will likely require stepped-up rates, compared to lighter sandier soils.

Seeding with planting equipment that distributes seed poorly either linearly or vertically will require greater seeding rates. “Likewise, greater seeding rates will be required when soybeans are planted at an excessive speed,” Naeve adds of a tendency growers are apt to have this year. “High rates of speed through the field decrease the precision of both linear and vertical seed placement.”

Soybeans planted in high pH areas prone to iron deficiency chlorosis may also benefit from higher seeding rates. That benefit is “amplified” when beans are in wider rows (22 to 30-inches), he remarks.

Finally, Naeve says that “due to the myriad of variables affecting final stands, producers should utilize liberal seeding rates until they clearly understand how their planting equipment, planting date, soil type and spring conditions affect their actual plant populations.”

“This will require taking stand counts in all fields each year,” he notes, adding, however, that the “payoff will be an eventual fine-tuning of seeding rates that reduces input costs, while providing sufficient plant population “cushion’ to avoid replant situations in those years when Mother Nature does not cooperate.” (Growers certainly don’t wonder what year that might be…)