Alfalfa is a perennial forage legume grown worldwide for high-quality animal feed. Establishing a dense, productive stand is the first step toward maximizing its potential yield and quality. Determining the correct amount of seed to plant per acre is a complex calculation, as the final rate must account for variations in planting conditions and seed quality. The goal is to maximize the number of vigorous, healthy plants while avoiding the expense and competition that results from over-seeding.
Establishing the Baseline Seeding Rate
The widely accepted starting point for the alfalfa seeding rate is 15 to 20 pounds of bulk seed per acre when using a traditional drill under optimal conditions. This baseline ensures a high number of viable seeds are placed in the soil, helping the young stand compete against weeds during establishment. For maximum yield potential, the agronomic goal is to achieve an initial target of 60 to 75 viable seeds per square foot. This high initial density accounts for the natural loss of seedlings due to competition and environmental stress. A successfully established stand will typically settle at 20 to 35 plants per square foot by the beginning of the first full production year.
Key Factors Requiring Rate Adjustment
Several external variables necessitate adjusting the baseline seeding rate up or down to ensure a successful stand. Soil type and condition are major considerations; lighter, sandy soils can tolerate a lower rate, while heavier clay or loam soils may require a higher rate to compensate for potential crusting issues. Planting into a less-than-ideal or cloddy seedbed also requires an increased rate to mitigate poor seed-to-soil contact and uneven planting depth.
Moisture availability significantly influences plant competition and survival. Under dryland conditions, a lower rate may be preferred to reduce moisture competition between young seedlings, conserving water for established plants. Conversely, irrigated fields, where water is not limiting, can support a higher plant population, allowing for the use of the upper end of the seeding range. The planting method also demands adjustment; broadcasting the seed across the surface, rather than using a precision drill, typically requires a 10 to 50 percent increase in the baseline rate. This compensates for the poorer seed distribution and reduced seed-to-soil contact.
Ensuring Seed Viability and Preparation
The physical quality of the seed must be considered, as it directly impacts the effective amount planted. Two metrics on the seed tag, the Germination Percentage and the Purity Percentage, are the primary indicators of this quality. The Purity Percentage indicates the weight of the bag that is actual alfalfa seed, while the Germination Percentage estimates how many pure seeds will sprout under favorable conditions. If a seed lot has lower purity or germination, a grower must plant more bulk pounds to compensate and still achieve the desired number of living seeds per acre.
Seed preparation, specifically inoculation, is necessary for successful alfalfa establishment. As a legume, alfalfa requires a symbiotic relationship with specific Rhizobium bacteria to fix atmospheric nitrogen. Most commercially available seed is pre-inoculated, but growers must confirm the viability of the bacteria has not expired, which would necessitate re-inoculation. Some seed is sold with a coating, which adds weight, often resulting in 20 to 30 percent inert matter in the bag. This coating lowers the effective amount of raw seed being planted.
Calculating Pure Live Seed and Final Adjustments
The final and most accurate step involves calculating the Pure Live Seed (PLS), which normalizes the seeding rate based on quality. PLS represents the percentage of the seed lot capable of producing a healthy seedling. The PLS percentage is calculated by multiplying the Purity Percentage by the Total Viable Seed Percentage (including germination and any hard or dormant seed), then dividing by 100.
For example, a seed lot with 95% Purity and 90% Total Viable Seed has a PLS of 85.5%. To find the total bulk pounds required, the target PLS seeding rate is divided by the calculated PLS percentage. If 15 pounds of PLS per acre is desired, a grower would need to plant 17.5 pounds of the bulk seed (15 / 0.855) per acre. This final calculated rate requires calibrating the planting equipment to ensure the correct amount of bulk seed is delivered. Maintaining a shallow planting depth, typically between one-quarter and one-half inch, ensures the calculated rate translates into a uniform and productive stand.