Deciding the right seeding rate is one of the most influential factors for increasing soybean profitability, as seed cost is one of the most expensive inputs.

Soybean seeding recommendations, row spacing, and planting date are tied together. The final number of seeds per linear foot of row decreases as row spacing narrows. For example, at a target population of 105,000 plants per acre and 85 percent germination, 30-inch rows will need twice the number of seeds per linear foot as 15-inch rows -- 6 vs. 3 seeds per linear foot (Table 1). Seeding rates will need to increase at later planting dates to compensate for the reduction in the growing season (more plants are needed to increase early light interception and biomass production).

The environment also exerts an influence on deciding the final seeding rate. Dry and hot conditions require fewer plants to maximize yields; while favorable environments need higher seeding rates to capture the maximum yield potential. Under high-yielding irrigated environments, the final seeding rate should be greater than 160,000 seeds per acre (assuming high % emergence) with a final plant population close to 150,000 plants per acre.

In recent years, a summary from 21 on-farm strip trials and 5 replicated experiment station studies in Kansas prepared by Kraig Roozeboom provided an opportunity to revisit current soybean recommendations. Most of the studies were performed in dryland environments (23 out of 26, with 3 studies under irrigation) and under no-till systems. All were in central and eastern Kansas counties: Butler, Harvey, Nemaha, Republic, Riley, Saline, and Shawnee.

As related to final field establishment, the current recommendations assume 80% emergence. Emergence in the studies ranged from less than 50% to 100%, illustrating the importance of knowing just how many dropped seeds will produce plants in each situation (Fig. 1). Studies that have compared planters and drills indicate that the 80% estimate is not far off for planters, but emergence for drills is usually closer to 65%. There is tremendous variability around both of these averages, but it illustrates the need to drop more seed per acre if field emergence is less than the 80% assumed for the current recommendations.

Figure 1. Percentage of field establishment and final seeding rate.

The primary conclusion from the summary of soybean seeding rate studies was that the optimum number of seeds per acre seemed to be highly dependent on the yield level attained at each location. Table 2 depicts the soybean seeding rate summary stratified by yield range.

Table 2. Recommended soybean plant density and seed spacing

A) Low yielding environments (test average <30 bushels per acre):

Yields were maximized at plant populations of less than 80,000 plants per acre. Optimum final plant population was achieved around 70,000 to 75,000 plants per acre (Fig. 2). Thus, if we assume 80% emergence (as presented in Figure 1), then the optimum seeding rate for this environment will range from 85,000 to 90,000 seeds per acre.

Figure 2. Optimum plant population, final plants per acre, for “low” yielding environments across Kansas, less than 30 bushels per acre.

B) Medium-low yielding environments (average ranged from 30 to 40 bushels per acre):

Yields were maximized with final plant populations around 75,000 to 80,000 plants per acre, presenting an evident plateau in maximum yield as the number of plants per acre increases beyond 80,000 plants per acre (Fig. 3). Seeding rates ranging from 90,000 to 95,000 plants per acre were required to achieve these final plant populations (assuming overall 80% emergence).

Figure 3. Optimum plant population, final plants per acre, for “medium-low” yielding environments across Kansas, ranging from 30 to 40 bushels per acre.

C) Medium-high yielding environments (average ranged from 40 to 50 bushels per acre):

Yields were usually maximized at populations of 105,000 to 120,000 plants per acre. The break-even point for the association between yield and plant population was set at around 120,000 plants per acre (Fig. 4). Increasing population above 130,000 plants per acre did not increase yields. Considering an average 80% field establishment, optimum seeding rate for this yield environment was 140,000 seeds per acre.

Figure 4. Optimum plant population, final plants per acre, for “medium-high” yielding environments across Kansas, ranging from 40 to 50 bushels per acre.

D) High yielding environments (test average above 50 bushels per acre):

The highest yields, under irrigation, were achieved with 105,000 plants per acre (or close to 130,000 seeds per acre with 80% emergence) (Table 2). There were relatively few experiments with yields in this range, so this may not represent a typical response. However, it does illustrate the tremendous ability of soybean plants to adjust the number of pods (and seeds) per plant to available resources. Other studies have shown that, given favorable growing conditions, yields of 80 to 90 bushels per acre can be achieved with 100,000 to 120,000 plants per acre.

Another series of studies funded by the United Soybean Board was conducted in 2012 and 2013 across the Midwest and Mid-South (including Kansas) to examine high-input soybean production practices. Initial results have shown that maximum yields were obtained between 100,000 and 165,000 seeds per acre across all nine states. In the southern states (Kansas, Kentucky, and Arkansas), seeding rates between 130,000 to 170,000 seeds per acre were needed to obtain maximum yields. This response was consistent across production systems regardless of whether they included a large number of yield-enhancing treatments (seed treatments, fungicides, growth promoters, etc.) or not.

Always take into consideration the yield potential for that environment when deciding the final soybean seeding rate. Yield potential is primarily defined by the weather conditions (before and after planting), genetic potential, soil type and supplemental fertility program, and use of best management practices for producing the crop (proper weed, insect, and disease control from planting until harvest). This summary allows confirming that the current recommendations are adequate, with the possible exception of extremely high-yield situations, which may require roughly 150,000 plants per acre to maximize yield. Using seeding rates higher than those recommendations seldom reduced yield, but did increase seeding cost.

For more information, see Kansas Soybean Management 2015, MF-3154, available online at: http://www.ksre.ksu.edu/bookstore/pubs/MF3154.pdf