Comparing Seed Lots and Cost, Calculating PLS and Seeds per Row Foot Based on PLS Pounds per Acre

Most of the cereal crops a farmer plants are consistent in percent pure seed and germination. Based on these consistencies the industry has adopted a set of Seed Standards for these crops when sold on the market. Consequently, the producer may plant the seed at a regular rate of pounds of material per acre and expect to obtain the stand they desire, providing proper seedbed and planting procedures were used, and the weather conditions are favorable. However, many of the grasses and legumes are so variable in both purity and germination it is impractical to set a commercial seed standard. For this reason a different means is necessary to determine the value of the planting material and to determine the amount of material to plant for a desired stand. The Pure Live Seed method provides a reliable means of determining these factors.

Pure Live Seed (PLS), expressed in percent, is the term used to designate the calculated quantity of viable seed to plant. The percent PLS of a seed lot is obtained by multiplying the percent purity times the percent germination and dividing the product by 100. In the case of legumes, where "hard" seed are counted, the percent of "hard" seed is added to the percent germination before multiplying it times the purity.

The PLS is important in determining the amount of material needed for planting and in determining the quality of the seed to be purchased. It is the best way to determine the actual cost of the seed. As an example, examine the following lots of seed:

The cost of one pound of PLS = (cost per pound of material x 100)/% PLS.

Lot Number 1
Cost per pound PLS = ($2.35 x 100)/83 = $2.84 per pound PLS.

Lot Number 2
Cost per pound PLS = ($2.15 x 100)/56 = $3.84 per pound PLS.

At first glance Lot Number 2 might appear to be the best buy, but it actually costs $1.00 (35%) more per pound of PLS. Differences in some situations are often greater.

Examples

Following are several examples of the use of PLS in determining costs and planting rates (all examples are based on the following hypothetical lot of seed):

• Purity: 90%
• Germination: 80%
• Planting rate: 6 lbs. PLS per acre
• Seed units: 150,000 seeds per pound of pure seed
• Cost of seed: $2.40 per pound.

Pure Live Seed:
% PLS = (% purity x % germination)/100 = (90 x 80)/100 = 72% PLS.

Amount of Planting Material Needed per Acre:
Pounds of material per acre = (planting rate in pounds per acre PLS x 100)/% PLS.
Pounds of material per acre = (6 x 100)/72 = 600/100 = 8.33 pounds material per acre.

Cost of Seed per Acre: = pounds of material x cost per pound = 8.33 x $2.40 = $20 per acre.

Cost per Pound of PLS:
Cost per pound PLS = (cost per pound material x 100)/% PLS = ($2.40 x 100)/72 = $3.33 per pound PLS.

Pounds of Material to Yield 1 Pound of PLS:
Pounds of material to yield 1 pound PLS = 100/% PLS = 100/72 = 1.39.

Planting Rates:
Viable seed (PLS) per square foot = (seed units per pound of pure seed x pounds PLS per acre planted)/square feet per acre.

Viable seed (PLS) per square foot = (150,000 x 6)/43,560 = 900,000/43,560 = 20.6 viable seeds per square foot.

Viable seed (PLS) per row foot = (seeds per pound of pure seed x Planting rate pounds PLS)/row feet in acre at width to be planted.

Example:
In 36-rows there are 14,520 row feet per acre. (43,560 square feet per acre / 3 feet = 14,520).
Viable seeds per row foot = (150,000 x 6)/14,520 = 62 viable seeds per row foot.

Example:
You plan to seed production planting with above lot of seed and want to seed approximately 35 viable seed per row foot in 36-inch rows. It would be computed as follows:

Planting rate pounds PLS = (viable seeds per row foot x row feet per acre)/seed units in pound of pure seed.

Planting rate pounds PLS = [35 x 14,520 (row feet in 36-inch rows)]/150,000 = 3.39 pounds of PLS per acre.

You would then need to buy:
(3.39 x 100)/72 = 4.7 pounds of material for each acre you were going to plant.

Seeding rates of PLS for various row spacings are contained in Montana Plant Materials Technical Note Number 46, Seeding Rates and Recommended Cultivars.

When comparing two lots of seed usually the one with the higher purity and germination will give the highest quality seed and will be cheaper per unit of viable seed. High purity assures the buyer of freedom from contamination with troublesome or noxious weeds. High germination assures vigorous seedlings that will establish good stands in a minimum period of time.

When the germination of a given seed lot is a great deal below the usual germination for seed of the same variety or species of comparable age it should be suspected at any price. Many of the factors which contribute to unusually low germination also damage the seeds which are viable. Seedlings from this seed may be so defective or low in vigor they will fail to produce good stands under the best field conditions.

Formula for calculating seeding rates for row spacing greater than 12 inches:

First Step:
Row spacing desired (inches)/12 inches = adjustment factor.

Second Step:
Pounds per acre at 12-inch row spacing/adjustment factor = actual seeding rate (pounds per acre)

Example for using footnote in Table 3-Ia for calculating pound per acre and seed per foot of row.

Assumption: Seeding alfalfa in 30-inch rows on irrigated land.
Find: Seeds per foot of row and pounds per acre.

Seed per foot of row = 28 or the same as the 12-inch row spacing.

[6 (pounds per acre at 12-inch row spacing)]/adjustment factor of 2.5 (from footnote chart for 30-inch row spacing).

Pounds per acre for 30-inch row spacing = 2.4.

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Last Modified: 07/05/2007