Multi-year soybean population study in Michigan’s Thumb

Soybean yields plateau at higher seeding rates.

Michigan State University Extension educators in Michigan’s Thumb area studied soybean populations in multiple on-farm studies over a six-year period, including 2009, 2011, 2013 and 2015. These studies represented a total of eight site years. In 2009, 2011 and 2013, seeding rates ranged from 80,000 to 240,000 seeds planted per acre in increments of 40,000 seeds. In 2015, seeding rates ranged from 80,000 to 160,000 seeds planted per acre in increments of 80,000, 100,000, 130,000 and 160,000. Soybeans were planted in 30-inch rows.

See the table below for actual population and results for individual years. In determining per acre seed costs, a 140,000 seed count bag price of $70 was used. As you can see, at that price, each 20,000 unit increase in seeding rate resulted in a $10 increase in per-acre costs. 

Soybean population results from 2009 to 2015
Planted Pop. X 1,0002009201120132015AveragePer acre seed cost 
Actual Pop. X 1,000Yield (Bu/A)Actual Pop. X 1,000Yield (Bu/A)Actual Pop. X 1,000Yield (Bu/A)Actual Pop. X 1,000Yield (Bu/A)Actual Pop. X 1,000Yield (Bu/A)
80 71.5 36.8 72.8 50.4 86.5 49.6 73 65 76 50.5 $40
100 No data No data No data No data No data No data 92 65.8 92 65.8 $50
120 121 43.2 117.6 52.3 109 52.6 No data No data 115.9 49.4 $60
130 No data No data No data No data No data No data 122 67.9 122 67.9 $65
160 136.5 43.9 132.8 52.1 128.6 53 151 69.4 137.2 54.6 $80
200 200.5 44.8 180 52.8 149 50.8 No data No data 176.5 49.5 $100
240 242.5 48.8 216 50.9 163.7 51 No data No data 207.4 50.2 $120

Below is a graphical representation of the data above. As you can see, soybean yield plateaus at about 130,000 seeds per acre. At lower seeding levels, incremental yield increases rapidly and levels off at 130,000 plants per acre.

Soy Yield Response to Plant Population

Soy yield response to plant population.

It should be noted that these studies were conducted on high-yielding, well-tiled, finely-textured soils in the Thumb. The Michigan Soybean Promotion Committee provided funds that made these studies possible.

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