Improved genetic yield potential and management practices have helped to increase soybean yield. As new varieties enter the market, the need to identify optimal seeding rates continues to be important. Higher seeding rates can offer advantages such as earlier canopy closure and lower weed competition; however, this may not always result in greater profitability. DuPont Pioneer has conducted numerous studies evaluating variety seeding rates to help growers determine optimal rates to improve the profitability of soybean production.
In 1 study, multiple soybean varieties were compared at 6 different seeding rates at research locations across several Midwestern states. Average yield in this study was greatest at 250,000 seeds/acre; however, yields at 150,000; 175,000; and 250,000 seeds/acre were very similar (Figure 1). Soybean yield was significantly reduced at 25,000 seeds/acre but was still over 70% of maximum yield, demonstrating the ability of soybeans to adapt to reduced stands.
Figure 1. Average soybean yield response to seeding rates across 9 research locations.
The optimum economic seeding rate is always lower than the agronomic optimum, as seed cost and soybean market value are taken into consideration. This rate can vary from year to year due to variability in the soybean market price and seed costs. The optimum economic seeding rates based on a $60 per unit seed cost and a soybean market price of $10, $12 or $15 per bushel are shown in (Figure 2).
Figure 2. Optimum economic seeding rates at soybean market prices of $10, $12 and $15/bu and a $60/unit seed cost based on DuPont Pioneer studies at 9 locations in Iowa, Ill., Ind., Minn. and Neb. (Studies conducted in 30-inch rows; economic optimum seeding rates are likely greater in narrower rows).
A 3-year DuPont Pioneer study conducted at 9 Midwestern research locations showed that economic optimum seeding rates with early planting were influenced by the use of a fungicide seed treatment or fungicide + insecticide seed treatment (Figure 3).
Figure 3. Optimum economic soybean seeding rates for early planting dates influenced by seed treatments in a 3-year study. (Based on soybean market price of $10/bu. All locations were planted in 30-inch rows.)
Soybeans have a greater ability than corn to compensate for reduced stands. However, too much reliance on this ability may lead to poor stands and the need to replant in some situations. As a result, higher seeding rates may help prevent yield reductions or replanting when seedbed conditions, weather or pests are likely to reduce soybean stands.