Iowa Soybean Planting Rate Studies Show Unusual Results

Iowa Soybean Planting Rate Studies Show Unusual Results

Strip trial results in 2013 showed higher populations paid off in most cases with late-planted beans.

The 2014 On-Farm Network Conference was held Feb. 20 at Ames. The Iowa Soybean Association sponsors the annual event. One of the presentations focused on soybean seeding rates (plant population) and the results of strip-trials with various seeding rates in farmers' fields in recent years.

BEST BEAN SEEDING RATE?: Population studies do not find much profit from higher rates planted during the typical planting window. But if wet weather prevents planting in May or early June, higher seeding rates are an option you may want to explore.

The last time the On-Farm Network summarized its planting rate replicated strip trial results was in the April 25, 2013 On-Farm Advance newsletter under a headline that said "Higher Planting Rates Generally Not More Profitable". But the results for 2013 show that the 2013 growing season with its rain-delayed, very late planting conditions might be the exception. Following is a summary from the On-Farm Network highlighting the soybean plant population trials.


With the wet spring and delayed planting, ISA On-Farm Network researchers and cooperating farmers noticed some interesting results in their 2013 soybean population trials. Rainy weather conditions in the spring not only delayed corn and soybean planting, but likely led to fewer soybean population trials being planted as well. Of those that were planted, only two made it into the ground during the month of May, nine were spread across the month of June and two were planted in early July.

Populations used for On-Farm soybean trials are determined by the grower. While the ISA On-Farm Network researchers recommend that populations vary by at least 30,000 plants per acre between high and low population, growers decide on the actual planting rates. And sometimes, weather and soil conditions encountered at planting can have a big impact on final stands.

Soybean specialists at Iowa State University,  Purdue, the University of Wisconsin, as well as other Midwest universities have long recommended that seeding rates be bumped higher as planting is delayed beyond the preferred planting window. Higher populations help compensate for shorter plants and fewer pods than normally occur when soybeans are planted with a limited growing season remaining.


Since a large proportion of the 2013 trials were planted after the preferred planting time, how did the results of those trials match up with recommendations?

On-Farm Network trials conducted from 2009-2011 indicated that higher populations were only profitable about 40% of the time. (See the accompanying graph). Plant stand, plant spacing and other data were collected on some of the trial fields in 2010 and 2013.

When stands were reduced in both treatments due to environmental factors like cool, wet soils, the probability of a profitable yield response to the additional seed went up. Under current pricing for commodity beans and seed soybeans, it would take a full 1- bushel-per-acre yield advantage to cover the cost of the extra 30,000 seeds planted.

Another observation the On-Farm Network researchers make from these trials is, regardless of conditions or when planting occurred, current planter technology is quite effective at varying rates and hitting the target seeding rates.

While past On-Farm Network soybean population studies tend to favor the lower seeding rate, the higher seeding rate in the 2013 trials was more profitable 54% of the time. Most notable is that the higher seeding rate was profitable in every single trial planted after June 10 (see graph below).

Here's how the On-Farm Network research report sums it all up: "So it seems that April headline last spring was almost prophetic. On-Farm Network soybean population studies do not find much profit to be gleaned from higher rates planted during the typical planting window (unless you run into seed germination issues). However, if spring conditions prevent you from planting in May or early June, higher seeding rates are an option you might want to explore."

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