The question of how many pounds of seed are required per acre is central to successful planting. The “seeding rate” refers to the amount of seed, typically measured in pounds, necessary to sow a unit of land area. This rate is highly variable, depending on the specific crop or forage species. Calculating the precise amount requires a detailed understanding of seed quality and how field conditions affect plant survival.
Understanding the Concept of Pure Live Seed (PLS)
Using the bulk weight of seed printed on a bag can be misleading because not every pound of material is capable of growing into a plant. Pure Live Seed (PLS) is a measurement designed to accurately determine the percentage of the seed lot that is viable. PLS accounts for both the physical purity of the seed and its ability to germinate.
The calculation for PLS is straightforward, requiring the purity percentage to be multiplied by the total germination percentage, which includes hard and dormant seeds. These percentages are required by law to be printed on the seed analysis tag accompanying the seed lot. For instance, a seed lot with 90% purity and 85% germination has a PLS of 76.5% (0.90 x 0.85 = 0.765).
If a desired seeding rate is 10 pounds of PLS per acre, the bulk weight needed is found by dividing the desired PLS rate by the calculated PLS percentage. For example, if the PLS is 76.5%, approximately 13.07 pounds of bulk seed must be purchased. This method ensures the grower pays for the actual amount of viable seed, not for inert matter or non-germinating material.
Standard Seeding Rates Across Different Applications
Standard seeding rates serve as the initial target before field conditions are considered. Small-seeded legumes, such as alfalfa and red clover, often require lower rates, typically ranging from 10 to 20 pounds per acre. Other small seeds like oats and wheat, when used as a cover crop, generally fall into a higher range of 60 to 110 pounds of bulk seed per acre.
Turf and forage grasses, like Tall Fescue, are commonly seeded between 15 and 20 pounds per acre for pasture establishment. Annual ryegrass typically requires a heavier rate of 25 to 30 pounds per acre. These rates are based on the assumption of good planting conditions and optimal seed-to-soil contact.
For large-seeded crops like corn and soybeans, the rate is often determined by the number of seeds per acre rather than weight, but a weight equivalent can be calculated. Field corn seeding rates typically target 32,000 to 38,000 seeds per acre. Soybeans are more flexible, with seeding rates often targeting 140,000 to 150,000 seeds per acre, which is roughly equivalent to 50 to 60 pounds per acre for an average seed size.
Factors That Modify the Final Seeding Rate
The standard rate must be adjusted according to several environmental and management factors to achieve the desired stand density. Poor soil conditions often result in lower seedling survival. In these less-than-ideal environments, increasing the PLS seeding rate by 10 to 15% is a common practice to compensate for expected losses.
The chosen planting method also significantly impacts the final rate, with broadcast seeding requiring a substantial increase over drilling. Because broadcasting scatters seed on the soil surface, leading to poorer seed-to-soil contact, a 20% to 50% higher rate is frequently recommended. Conversely, using a specialized grain drill allows for the use of the lower, more efficient end of the recommended rate range.
The purpose of the planting also drives the final rate adjustment; a fast-growing, dense cover crop for maximum weed suppression requires a higher stand density. Planting under environmental risk, such as late in the season or in dry soil, necessitates a higher seeding rate to mitigate the risk of poor emergence.
Ensuring Accurate Application and Equipment Calibration
Once the final, adjusted seeding rate is determined, the next step is ensuring the planting equipment accurately delivers that quantity to the field. This requires equipment calibration, which matches the machine’s output setting to the calculated rate. Calibration prevents both under-seeding, which results in thin stands and poor yield, and over-seeding.
For a drill or planter, calibration involves running the machine for a calculated distance, collecting the seed distributed, weighing that amount, and then extrapolating the weight to an acre. If the delivered weight is too high or too low, the gear settings, sprockets, or flow gates are adjusted and the test is repeated until the output matches the required pounds per acre.
This practical check is especially important because factors like the size, shape, and flow characteristics of the seed lot can vary. Maintaining the correct planting speed during operation is also necessary, as excessive speed can reduce the accuracy of seed placement, even with a properly calibrated machine.