Most growers with more than 20 years under their belt recall planting soybeans in narrow rows with a drill at populations of 225,000 to 275,000 seeds per acre. The high seed rates were a way to control weeds and secondly, drills literally pour seed instead of precisely placing it in the furrow.

However, when Roundup Ready® seeds came on the market and weed control became much easier growers moved to planting soybeans in either 30-, 20- or 15-inch row spacing. At the same they begin dropping populations to 200,000 to 180,000 to 160,000 and now to 140,000. Of course, it has long been known that a final stand of 100,000 plants, evenly distributed at harvest, will maximize yield. The key is making sure you have 100,000 pod-bearing plants at harvest. And given that some soybean seeds don’t produce a plant due to soil conditions, pests or diseases, growers overplant to guarantee they have 100,000 to 110,000 plants at harvest.

There are a few good reasons why seeding rates have dropped.

The next advance in seeding rates will be variable rate seeding (VRS). Research the past decade shows there is an opportunity for growers to optimize their seed investment with variable rate seeding planting. And today’s planters come with electric or hydraulic drives that can be coupled to rate controllers and software to vary population.

With corn, you plant more seed in areas of high productivity and less seed in marginal areas. VRS for soybeans is the inverse—higher seeding rates in areas of lower productivity and lower seeding rates in areas of higher productivity. The reason is that in lower productivity areas you need more plants to generate the pods that produce yield or in other words there is more plant attrition leading to declining plant stands that require more seeds to compensate for seed or seedling loss.

For soybeans both agronomic and economic seed rates need to be considered. The agronomic seeding rate; be it 120,000, 140,000 or 160,000 is selected based on the soil conditions, environment, historical stand establishment, management level, weed control and disease management. This rate is determined based on experience, risk acceptance and advice from agronomists and seed dealers. The economic seeding rate is the population that maximizes yield while reducing seed cost. This second approach requires adoption of the best management practices.

A comprehensive seeding rate study1 completed in 2018 confirmed that soybean seeding rates should increase in areas of lower productivity and decrease in areas of higher productivity. The key is determining an average seeding rate and then what percent to increase it in marginal soils and what percent to decrease in high productive soils, yet limit the number of population levels to no more than 5 rates while at the same time incorporating a grower’s level of risk tolerance. Some growers may error on the high side with more population and some will error on the low side with less population.

The machine technology exists to variable rate seed soybeans and the strategy is to plant more in marginal areas and less in productive areas. Combine machine capabilities with soil type, yield history, agronomic practices and grower experiences to develop variable planting maps and you’re on your way to both optimizing yield and maximizing profitability in your soybean operations.

To read the full report by Adam Gaspar “Soybean Seeding Rate – Past, Present, and VRS Future click here.