How low can seed rate go?

Growers working with the Iowa Soybean Association’s On-Farm Network conducted replicated strip trials for the past two years to study how various seeding rates affect yield.

Comparing different seeding rates and their plant populations over several years will, hopefully, let growers determine the optimum seeding rate for their fields without converting an entire field at once.

The 2010 soybean trials compared two different planting rates — a high and a low — in at least three pairs of alternating strips across the field. Farmer Dennis Friest of Radcliffe set up four soybean plant population trials in the 2010 growing season.

Key Points

Iowa farmers are participating in trials comparing reduced bean seeding rates.

Growers have traditionally planted higher bean populations for various reasons.

With higher seed costs, growers are interested in cutting back on seeding rate.


He lowered his planting rate by 20,000 to 25,000 seeds per acre. The intent was to compare two planting rates that were 10,000 to 15,000 seeds per acre lower and higher than the grower’s normal rate. The average low rate across all trials was 137,000 seeds per acre, and the average high rate was 30,000 seeds per acre higher.

Bean seeding rates compared

Variable-rate planters were used on some trials, or growers made two trips across the field with the different planting rates. The plant populations were ground-truthed by making stand counts in both the high- and low-rate strips in most fields in July and August to verify populations. Some trial sites had large differences between the target, or planted seeding rate, and the established stand after the plants grew.

At some sites, the scouted stand in the low-rate strips was well under 100,000 plants per acre. Yield response to the higher planting rate was larger in the fields where the low-rate strips had an actual stand count of less than 100,000 plants per acre. Row spacing varied in these trials. Most trials (22 of the 37) were planted in 30-inch rows. Eight trials were in 15-inch rows, and seven trials were in 7.5- or 10-inch rows.

“The target low planting rate had no effect on the observed yield differences, accounting for only 10% of the total variation in the soybean yield response,” says Peter Kyveryga, senior research associate for the On-Farm Network.

Friest adds, “I really wasn’t surprised with the results of my trials. I’ve heard that when populations are down around 100,000 seeds per acre, you don’t have any real yield loss. But I didn’t want to believe it.” In his case, yields from both the high and low rates were similar. He saw the savings in seed cost without sacrificing yield.

What the trial results show

In the 2010 trials, a quarter of the trials had a yield difference of less than 0.2 bushel per acre, and the other trials had a difference of under 1.6 bushels per acre. “The yield variation among the trials was about 10 times larger than the variation within a trial. This ratio was twice as much than that for the population trials on corn,” says Kyveryga.

There are many different factors that could impact the yield differences between the two soybean planting rates. The effect of local rainfall and planting date found no major effects on yield differences. Row spacing had a small effect, with a smaller yield difference in narrow-row planting rates. But the target low and high planting rates were about 15,000 seeds per acre for narrow row spacing (less than 30-inch rows) than for 30-inch row spacing.

Precision planter technology

“The new planter technology is very reliable,” says Friest. “Over the past four years, I’ve invested in precision planting. It gives me better accuracy of seed spacing in the row, and you can see a definite improvement in the uniformity of plant spacing.”

Similar on-farm evaluations were done during the 2009 growing season in 31 soybean trials across Iowa. The overall results were similar to what was observed in 2010. “In 2009, the average yield difference was about 1 bushel per acre, while the two target average planting rates were slightly lower [about 6,000 to 7,000 seeds per acre] than in 2010,” says Kyveryga. “These analyses are preliminary, and we will focus more detailed analyses on spatial variability in yield differences, which may shed light about the feasibility of variable planting rates in both corn and soybean crops.”

Friest says, “The real payback for me is being able to use the On-Farm Network to analyze the things that I’m doing. There are several pieces to the puzzle. Getting the data is one thing, but analyzing the data is where the real value is, and that’s where the On-Farm Network comes in.”

Birchmier writes for the Iowa Soybean Association’s On-Farm Network.

Where to go for more information

Differences between soybean seeding rate and established stand have varied greatly in the On-Farm Network trials. The yield difference seen in the majority of the trials, while not economic, suggests that most growers are planting at a high-enough seeding rate to provide some buffer for stand loss. Conditions with poorer germination or establishment would likely result in bigger yield differences. For more information, go to www.isafarm
net.com
and select “Strip trial summaries” under the “Publications” tab.


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ZEROING IN: “There are several pieces to the soybean seeding rate puzzle,” says Dennis Friest, a Hardin County farmer and longtime participant in the Iowa Soybean Association’s On-Farm Network trials. University studies have shown that a harvest population of 100,000 to 125,000 plants per acre is optimum.

This article published in the April, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.