Determining the correct volume of seed to plant across a given area, known as the seeding rate, is foundational to successful crop production. This rate is typically expressed in pounds of seed per acre, representing the total bulk weight applied. Optimizing the seeding rate directly influences the final plant stand density and maximum yield potential. Planting the correct amount minimizes costly seed waste and prevents under-seeding, which leads to increased weed competition. Precise calculation requires accounting for the actual quality of the seed, not just simple weight measurements.
Defining Seeding Rate and Pure Live Seed
The Bulk Seeding Rate refers to the total weight of the seed mixture applied to one acre. This bulk weight often contains inert material, such as dust, chaff, broken fragments, and non-viable seeds. Relying solely on bulk weight can lead to significant errors in the intended plant population, resulting in too few or too many established plants.
To ensure planting accuracy regardless of seed lot quality, the concept of Pure Live Seed (PLS) is used. PLS is a standardized metric quantifying the percentage of material that is the desired seed type and capable of germinating. Seed companies test each lot to determine the percent purity and percent germination, which are required for the PLS calculation.
The PLS percentage is calculated by multiplying the percent purity by the percent germination, then dividing by 100. For example, a seed lot with 85% purity and 75% germination has a PLS percentage of 63.75%. This means only 0.6375 pounds of every bulk pound purchased is capable of producing a healthy seedling. Using PLS guarantees the same amount of viable seed is planted per acre, regardless of seed lot quality.
Calculating Target Seeding Rate
Calculating the required bulk pounds of seed per acre begins by setting a specific biological objective. Agronomists determine the desired final plant population (plants per acre) needed for optimum yield. This target stand must then be adjusted upward to account for expected emergence mortality, since not every viable seed sown will successfully establish a plant.
The next step involves determining the physical properties of the seed lot, primarily the number of seeds contained in one pound. This figure varies widely between varieties and lots, and is typically provided on the seed tag. If unavailable, a sample must be weighed and counted to calculate the seeds-per-pound figure or the thousand kernel weight (TKW).
The core calculation converts the desired number of viable seeds per acre into the final bulk weight using the PLS percentage. The formula is: Seeding Rate (lbs/acre) = [(Desired Seeds per Acre) / (Seeds per Pound)] / PLS %. For example, targeting 1,250,000 viable seeds per acre with a lot containing 15,000 seeds per pound and 95% PLS requires a staged calculation.
The raw weight needed is calculated by dividing the desired seeds per acre by the seeds per pound, yielding 83.33 pounds (1,250,000 / 15,000). This raw weight is then divided by the PLS percentage (0.95) to account for seed quality, resulting in a bulk seeding rate of approximately 87.7 pounds per acre. This final figure represents the actual weight the planting equipment must deliver to achieve the targeted plant population.
Key Environmental and Crop Variables
The calculated seeding rate serves as a baseline that must be refined based on site-specific factors influencing seedling survival. Environmental conditions, soil properties, and the crop’s intended purpose all introduce variability in the expected establishment rate. Growers must adjust the calculated rate to compensate for anticipated losses or gains in seedling emergence.
Soil type and condition significantly affect the required rate due to differences in moisture retention and soil crusting potential. In heavy clay soils prone to crusting, or sandy soils with poor water-holding capacity, the seeding rate may need to be increased to compensate for lower emergence rates. Conversely, highly productive soils with optimal tilth and fertility support higher seedling survival, allowing for a slight reduction in the initial seeding rate.
Moisture availability is a primary determinant of final stand success, especially in non-irrigated environments. In drought-prone areas, high seeding rates can lead to excessive competition for limited soil water, potentially reducing overall yield. A reduced rate may be beneficial in these scenarios, while fields with ample moisture can benefit from a higher rate.
The crop’s intended use also dictates density requirements. Forage or silage crops benefit from higher plant density to maximize biomass. Grain production requires a slightly lower density to allow for individual plant development, such as tillering. Planting outside the optimal window, especially late, often requires a small increase in the seeding rate to offset increased pest pressure or reduced growing time.
Calibrating Equipment for Accurate Application
Once the final bulk seeding rate is calculated and adjusted, the mechanical delivery of the seed must be verified. Calibration is the process of setting planting equipment (e.g., a seed drill or broadcast spreader) to accurately dispense the exact calculated weight over the target area. Without this step, even precise mathematical calculation is meaningless if the machine is applying a different rate.
A common calibration method involves a catch-and-weigh test, simulating planting over a small, measured distance. The drive wheel is rotated a predetermined number of times, and the seed delivered by the metering mechanism is collected and weighed. This collected weight is then extrapolated to determine the current pounds-per-acre rate the machine is applying.
This mechanical test is often a trial-and-error process, requiring adjustments until the collected seed weight is within a tight tolerance of the target rate. Factors can alter the flow rate through the metering system, necessitating a physical calibration for each new seed lot. These factors include equipment wear, seed size variability, and seed treatment coatings.