## Calculating Seeding Rates

by Keith Brown
Divide County North Dakota Extension Agent

Seed Counts to Determine the Number of Seed per Pound: It is getting more common to see recommended planting rates given in terms of plants per acre or plants per square foot rather than in specific pounds or bushels per acre.  In order to determine seeding rates based on desired plants per acre it is necessary to know the seed count of the seed lot or how many seeds there are in a pound. Seed counts can be requested from many seed labs when submitting seed for germination tests, but if you didn't do so, it is not a difficult process to determine seed counts.  Basically all you need is access to a fairly accurate scale.

The first step is to count out a known number of seeds.  One hundred seed is generally works well. The second step is to weigh the seed. The third step is to calculate the seeds per pound. To do this, you'll need to do some math.  If the scale weighs in lbs., simply divide the number of seeds weighed by the weight in lbs. to get seeds per pound. For example, lets say we are trying to determine a seed count for Cruiser peas.  We counted out a hundred seeds that weighed .0457 lbs.  One hundred seed divided by .0457 lbs. would equal a seed count of 2188 seeds per pound.  If the scale weighs in ounces, the formula would be 16 divide by the weight in ounces times the number of seeds weighed.  For example, if same 100 pea seeds weighs 0.73 ounces, we would have a seed count of 2192 seeds per pound (16 divide 0.73 ounces times 100 seeds).  If the scale weighs in grams, the formula would be 453.6 divided by the gram weight of seed times the number of seeds weighed. For a grain scale the formula to use would be 7,000 divided by the weight of the seed sample in grains times the number of seed weighed. Once the number of seeds per pound is known, the number of pounds per acre to seed can be determined and adjusted for germination percentage and estimated seed mortality to obtain the desired plant population.

Determining Seeding Rate Based on Desired Plants per Acre

Information needed:

1.         Desired seeding rate in plants per acre

2.         Seed count - seeds per pound

3.         Percent germination

4.         Estimated seedling mortality - There are generally emergence losses that need to be accounted for even with good seedbed conditions.  Poor seedbed conditions may create a need to increase seeding rate more to reach the desired plant population.

Formula:

(Plants per acre / seeds per lb) / germination X (1 + mortality) = seeding rate in lbs per acre

Example (Field Peas):

1.         Desired rate: 300,000 plants/acre

2.         Seeds per pound: 2,190

3.         Germination: 87 % or .87

4.         Seedling mortality: 10% or .10

(Plants per acre / seeds per lb) / germination X (1 + mortality) = seeding rate in lbs per acre

(300,000/2,190) / .87 X (1 + .10) = seeding rate of 173 lbs per acre or 2.9 bushels per acre.

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