Maximizing soybean yield requires farmers to plant a specific number of seeds per acre to achieve an optimal final stand, which is the density of established plants. To meet this agronomic need, the commercial standard for selling soybean seed has shifted from a measure of weight to a fixed count rather than volume. This change provides growers with the precision required for efficient planting and allows them to manage seed costs accurately based on the number of seeds they purchase.
The Commercial Standard for Soybean Units
A “unit” of soybean seed is the standardized commercial package used for sale, defined by a fixed seed count. The industry standard for this unit is typically 140,000 seeds, though some vendors offer counts like 130,000 or 150,000 seeds per unit. This standardized count ensures that a farmer always knows the precise number of seeds being purchased, regardless of the bag’s weight.
The agricultural sector adopted the seed-count unit because the size and weight of individual soybean seeds vary significantly. Previously, seed was often sold in a 50-pound bag, meaning the number of seeds inside fluctuated widely, making it difficult to calculate an accurate planting rate. For example, a unit of 140,000 seeds might weigh 50 pounds if the seed size is 2,800 seeds per pound, but the weight could increase to over 60 pounds if the seeds are larger.
Selling by count removes the guesswork from purchasing and allows growers to focus on their specific field needs. This method provides a consistent measure, essential for calculating the exact number of units needed to plant a specific acreage at a desired seeding rate. The transition to a fixed-count unit aligns soybeans with other crops, such as corn, which have been sold by count for decades.
Why Seed Size Varies
The size of an individual soybean seed is highly variable, affecting the number of seeds contained in any given weight. This variability is tracked by seed companies using the metric “seeds per pound,” which can range from approximately 2,100 to over 4,000. This measure is influenced by two main factors: the genetics of the soybean variety and the environmental conditions during the growing season.
The genetic makeup of a soybean variety determines its potential seed size, with some varieties naturally tending toward larger or smaller seeds. However, the most significant influence comes from the environment, particularly the weather conditions experienced late in the reproductive phase, known as the seed-filling period. Adequate rainfall and favorable temperatures during the R5 and R6 growth stages generally contribute to the development of larger, heavier seeds.
Conversely, environmental stresses like drought, high heat, or nutrient deficiencies during seed fill can severely reduce the size of the seeds. A period of moisture stress followed by late-season rainfall may also result in surprisingly large seeds. The “seeds per pound” figure is calculated for each specific lot of harvested seed and is printed on the tag to account for yearly fluctuations caused by growing conditions.
Translating Seed Count into Planting Rate
The fixed count of 140,000 seeds per unit is the starting point for calculating the precise planting rate needed to maximize yield. The goal is to establish a specific final stand—the number of surviving plants per acre—often targeted between 100,000 and 140,000 plants per acre. To achieve this desired final stand, a grower must first determine the required seeding rate, which is the number of seeds to plant per acre.
This calculation accounts for factors that cause some seeds to fail, primarily the seed’s germination rate and the expected field emergence rate. The germination rate is listed on the seed tag, indicating the percentage of seeds viable under ideal laboratory conditions, often around 90%. The field emergence rate is an estimate, typically 70% to 90%, which accounts for losses due to several factors:
- Planter function
- Soil conditions
- Weather
- Disease
To find the required seeding rate, the desired final stand is divided by the total expected survival percentage (germination rate multiplied by the field emergence rate). For example, if a farmer desires a final stand of 120,000 plants per acre and expects 77% total survival (90% germination multiplied by 85% emergence), the required seeding rate is 155,844 seeds per acre (120,000 / 0.77). After calculating the necessary seeds per acre, the farmer determines the number of commercial units to purchase by dividing the total seeds needed by the fixed unit count of 140,000. This method allows for an accurate and cost-effective planting plan, ensuring the correct plant density is established.