Since 2006, planting rates of 90,000, 120,000, 150,000, and 180,000 seeds per acre have been planted in 12 irrigated soybean fields on 30-inch rows. Prior to this research, most of these producers usually planted 160,000-180,000 seeds/acre. The 90,000 low rate was determined based on UNL research recommending not to replant a hailed soybean stand if at least 90,000 plants/acre remained in the field.

In 2008, cooperating producers used these same rates to plant soybeans at five sites with 20 replications. Planting dates ranged from April 29 to June 3. In the end, there was little difference in percentage stand and yield among the four planting rates (see Table 1). The 120,000, 150,000, and 180,000 yields were statistically the same (only a 0.3-bushel difference between the 120,000 and 150,000 rates) and were significantly better than the 90,000 seed-per-acre plots; however, note that the 90,000 plot yielded only 1.7 bu/ac less than 150,000 plot. All data was statistically analyzed to determine the yield differences due to the various treatments.

The findings are similar to the 2006 and 2007 studies. In 2006, yield results ranged from 65.5 bu/ac at 90,000 to 67.4 bu/ac at 180,000. In 2007 yield results were 59.4, 59.6, 59.4, and 60.2 bu/ac for 90,000, 120,000, 150,000, and 150,000 respectively with no statistical difference.

Most likely, these results are indicative of soybean’s ability to compensate for reduced populations. Figure 1 shows increased plant branching at lower populations compared to less branching at higher populations. This was observed in all fields regardless of variety. Also observed in 2008, were two additional nodes/plant at the 90,000 population compared to the 180,000 population. Nodes are important as flowers, pods, and ultimately yield are produced from them.

A dryland field in Nuckolls County also showed interesting results. This field was hailed at the cotyledon stage, so planted populations of 100K, 130K, and 160K became average actual stands of 74,417; 89,417; and 97,917 plants per acre. August rains in 2006 helped deliver yields of 38.6, 40.6 and 42.7 bu/ac, respectively. 

Rates for Drilled Soybean:  

In 2006, one drilled field in irrigated conditions in Fillmore County yielded 68.4 bu/ac, 66.6 bu/ac, and 67.2 bu/ac for planting rates of 150,000, 175,000, and 190,000 seeds per acre respectively. Another study in 2006 conducted by the Soybean Feed Grains and Profitability Project in a rain-fed field in Lancaster County showed a slight but significant yield advantage to drilling soybean at a rate of 152,500 seeds per acre compared to 115,000 seeds per acre. Yield for the higher seeding rate was 56.8 bu/ac compared to 56.0 bu/ac with the lower seeding rate. When using grain drills and reducing soybean populations, variable seed spacing and seed depth within a drilled row can be an issue for soybean emergence. This is why a population increase for drilled beans is often recommended.

Recommendation: Plant Soybeans at 120,000 Seeds/Acre

Based on three years of consistent research results, UNL specialists recommend reducing planting populations from an average of 160,000 seeds/acre to 120,000 seeds/acre in 30-inch rows. This reduction of 40,000 seeds per acre results in a savings of $10.66 to $18.57 per acre based on seed costs of $40-65 a bag. For three years producers were able to achieve a 90% stand and have not seen a statistical yield variance from 150,000 or even 180,000 seeds/acre. With soybean seed costs increasing, reducing soybean planting populations is another way producers can survive high input costs of crop production.