The size of seed within a crop species can vary considerably. Both genetics (variety) and environmental conditions during growth can influence seed size. Most commercial soybean varieties have about 2,600 seeds per pound, but some specialty soybeans may have as few as 1,700 or as many as 5,000 seeds per pound. Seed of the same variety can vary from one season to the next or between two production locations due to variability in soil, weather, pest infestion, etc. Because of seed size variability, seeding rates should always determined on a seeds per acre basis rather than a weight or volume basis.
A quick calculation can demonstrate the problem with basing seeding rates on volume or weight. You have two soybean varieties that you planted at a 1 Bu/A rate (60 lbs/a). One had a seed weight of 2,200 seeds per pound; the other, 3,000 seeds per pound. The larger seeded variety (fewer seeds per pound) would be planted at 132,000 seeds per acre, while the smaller seed was planted at 180,000 seeds per acre. This is over a 30% difference in seeding rate between the two varieties.
We generally purchase crop seed on a weight basis (corn, which is generally sold as 80,000 seed units, is a major exception). Therefore, the crop producer must be able to calculate the amount seed needed using the intended seeding rate (seeds/acre) and weight (seeds per pound) of the seed being purchased. The following equation is used for this purpose. It is also useful for monitoring seed drop during the planting operation.
Pounds of seed per acre = seeding rate / seed weight
where seeding rate is in seeds per acre and seed weight is in seeds per pound
1. You will be planting 50 acres of a soybean variety at 150,000 seeds per acre. The seed size for this variety is 2,750 seeds per pound. How many 50 lb. bags of seed should you purchase for this variety?
Pounds of seed per acre = 150,000 seeds per acre / 2,750 seeds per pound
= 54.5 pounds per acre
Seed needed = (50 acres * 54.5 pounds per acre) / (50 pounds per bag)
= 55 bags
2. You are planting wheat at 30 plants per foot of row with a 7.5 inch row spacing. The weight of the seed you are planting is 12,500 seeds per pound. After planting 10 acres, you have used 700 pounds of seed. What is your actual seeding rate relative to your intended seeding rate (actual as a percent of intended)?
Seeding rate (seeds per acre) = 30 plants per foot of row * 43,560 ft2 per acre * (7.5 inches/ 12 inches per foot)
= 816,750 seeds per acre
Pounds of seed per acre = 816,750 seeds per acre / 12,500 seeds per pound
Intended seeding rate for 10 acres
= 10 acres * 65.34
= 653.4 lbs
Actual seeding rate as a percent of intended seeding rate
= 700/653.4 * 100