Re-calibrating the drill between seed lots is necessary to hit the target seeding rate and plant population — especially if seed lot thousand seed weights (TSWs) vary by more than one gram. It will also help identify any equipment issues that may contribute to inaccurate seeding rates.
To calculate an appropriate pound per acre seeding rate for each seed lot, use this formula:
Seeding rate (lb./ac.) =
[9.6 x desired plant density (plants/ft2) x TSW (grams)] ÷ estimated seed survival (%, expressed as a whole #).
For example, if TSW is 4 grams, desired plant population is 10 plants per square foot and estimated seed survival is 50%, then the formula goes like this: (9.6 x 10 x 4) ÷ 50 = 7.7 pounds per acre. If that rate is higher than you wish to seed, you will need to take steps to improve the rate of seed survival.
Seed survival is an estimate. Factors that can affect the survival of a seed include seeding depth, disease, seed-placed fertilizer rates, insects and frost. A true way to measure survival is to conduct plant counts once the crop has reached at least the 2-leaf stage. Marking that area and doing another count in the same area in the fall just after swathing can also be beneficial to determine if any mortality or additional emergence occurred after the spring count.
Once the appropriate seeding rate has been determined, check that seeding equipment is properly calibrated to deliver the desired pounds of seed per acre. Consult the operator’s manual for recommended calibration instructions, or follow this procedure:
1. Measure out 100 feet.
2. Collect the seed from several drill runs over this distance. Weigh the collected seed.
3. Calculate the seeding rate using this formula: Grams of seed ÷ number of drill runs collected ÷ row spacing (inches) x 12 = seeding rate (lb./ac.)
For example, if the grower collects 18.75 g from 5 drill runs and row spacing is 9”, then the actual seeding rate is 18.75 ÷ 5 ÷ 9 x 12 = 5.0 lb./ac.
Adjust the seeder and recalibrate until the target seeding rate is achieved. Then record the drill settings, TSW and seeding rate.
For more detail, click here to download the factsheet Plant Populations for Profitability and see below for tables that give a seeding rate based on TSW, target plant stand and estimated seed survival.