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Successful forage establishment requires that seed be planted at the recommended density. Planting lower than the required rate will result in thin stands with increased weed problems and lower yields. On the other hand, planting at a higher than recommended seed rate will significantly increase seeding costs. Calibration becomes more important as the cost of the seed increases.
Calibration charts can be found on most seeding equipment and they provide a good starting point. However, variations in seed size, weight, purity, and coatings, and performance of seeding equipment can cause large discrepancies between chart settings and actual seeding rates. Therefore, it is critical to know how much seed is actually being metered out for any given combination of variety, seeder, and field condition.
Forage Seeder Calibration at a Glance

To calibrate forage seeding equipment, two things must be known: (1) the area covered, and (2) the amount of seed metered out. Seeding rate is calculated using the following equation:
Seeding rate = amount of seed ÷ area covered
Measure effective seeding width of equipment. Effective seeding width is determined by measuring the width that seed is actually being dropped for droptype seeders such as a Brillion or Gandy (figure 1). For grain drills (figure 2), the distance between disk openers can be measured and then multiplied by the number of disk openers plus (+) 1. Spreading width for spinner and pendulumtype seeders (figure 3) varies with seed type. Consult the owner’s manual for effective seeding width for specified materials.
Depending on the seeding equipment, distances for calibration can vary. Use the following equation to calculate the area covered for an arbitrary distance:
Area covered = seeder width (ft) x travel distance (ft)
(acres) 43,560 (sq ft/acre)
In some cases it is desirable to calibrate equipment based on a fraction of an acre. This allows the seeding rate to be calculated by multiplying the seed collected by a constant factor. Table 1 provides equations for calculating the distance that must be traveled for 1/10 acre, 1/25 acre, 1/50 acre, and 1/100 acre.
Table 1. Calculating the distance that must be traveled for fractions of an acre.  

Fraction of an acre  Equation  Multiplication factor 
1/10  Travel distance = 4,356 sq ft ÷ seeding width (ft)  10 
1/25  Travel distance = 1,724 sq ft ÷ seeding width (ft)  25 
1/50  Travel distance = 871 sq ft ÷ seeding width (ft)  50 
1/100  Travel distance = 436 sq ft ÷ seeding width (ft)  100 
Collection: Place containers under seed drops and collect seed for a known distance, traveling at the same speed you will travel in the field to be seeded. Special collection containers can be purchased or fabricated. For grain drills, seed tubes can be disconnected and plastic bags held in place by rubber bands to catch seed (figure 4). If possible, calibrate the seeder in the field that will be seeded. This will take into account any variation due to seedbed conditions.
Difference: Place a known amount of seed in the seeder and travel a known distance at the same speed you will travel in the field to be seeded. Weigh the remaining seed. The seed distributed will equal the starting amount minus the amount remaining. Seed can easily be removed from seeding equipment with a shop vacuum. Make sure to clean out the vacuum before and after use.
Run out: Place a known amount of seed in the seeder and travel at the same speed you will travel in the field to be seeded until the seed has run out. Multiply this distance by the seeder width. This method assumes even distribution of seed within the seed box and consistent feed out as the seed runs out. Of the three methods for determining the amount of seed used, this is the least precise and should only be used for larger areas.
Note: When weighing the seed, always tare the scale for weighing container OR subtract out the weight of the weighing container.
Area covered (acres) = (15 ft x 200 ft) ÷ 43,560 sq ft (1 acre) = 0.069 acre
Seeding rate = 0.56 lb ÷ 0.069 acre = 8.12 lb/acre
Area covered (acres) = (10 ft x 100 ft) ÷ 43,560 sq ft (1 acre) = 0.023 acre
Seeding rate = 3.22 lb ÷ 0.023 acre = 140 lb/acre
Area covered (acres) = (8 ft x 650 ft) ÷ 43,560 sq ft (1 acre) = 0.12 acre
Seeding rate = (10 – 7.5) ÷ 0.12 acre = 20.83 lb/acre
Travel distance (ft) = 4,356 sq ft (1/10 acre) ÷ 12 ft = 363 ft
Seeding rate = 10 lb x 10 (multiplication factor) = 100 lb/acre
Area covered (acres) = (12 ft x 1,195 ft) ÷ 43,560 sq ft (1 acre) = 0.33 acre
Seeding rate = 15 lb ÷ 0.33 acre = 45.45 lb/acre
Wheel circumference = 2.5 ft x 3.1416 = 7.85 ft (It can also be measured)
Wheel turns to equal 0.10 acres = 4,356 sq ft (0.10 acre) ÷ (7.85 ft x 10 ft) = 55.5 turns
Seeding rate = 1.2 lb ÷ 0.10 acre = 12 lb/acre
Travel distance = 871 sq ft (1/50 acre) ÷ 12 ft = 72.6 ft
Seeding rate = 0.7 lb x 50 (multiplication factor) = 35 lb/acre
Useful Conversions and Formulas  

1 pound (lb) =  16 ounces (oz) 
454 grams (g)  
1 kilogram (kg) =  1,000 grams (g) 
2.205 pounds (lb)  
1 foot (ft) =  0.3048 meter (m) 
1 yard (yd) =  3 feet (ft) 
0.9144 meters (m)  
1 acre (A) =  43,560 square feet (sq ft) 
4,840 square yards (sq yd)  
0.405 hectare (ha)  
1 hectare (ha) =  10,000 square meters (sq m) 
2.47 acres (A)  
Circumference of a circle = diameter x 3.1416 (Π)  
Area of a square or rectangle = length x width 
Reviewers; Ozzie Abaye, associate professor, crop and soil environmental sciences, Virginia Tech; Scott Baker, Extension agent, agriculture and natural resources, animal science, Virginia Cooperative Extension Bedford County Office; Cynthia Gregg, Extension agent, agriculture and natural resources, animal science, Virginia Cooperative Extension Brunswick County Office; Robert “Bobby” Grisso, Extension engineer, biological systems engineering, Virginia Tech; Garry Lacefield, Extension professor and forage specialist, University of Kentucky Research & Education Center, Princeton, Ky.; David Reed, Extension specialist, tobacco, Southern Piedmont Agricultural Research and Extension Center, Blackstone, Va.; Ben Tracy, associate professor, crop and soil environmental sciences, Virginia Tech; Carol Wilkinson, director, Southern Piedmont Agricultural Research and Extension Center, Blackstone, Va.
Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
December 10, 2009