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The percentages of purity and of pure live seed are two items that are easily overlooked when making purchasing or planting decisions, but they are very important.
There are two types of purity of concern to seedsmen: varietal purity and mechanical purity. Varietal purity is trueness to variety. It can only be determined by complex and time-consuming techniques, like field grow-outs, greenhouse trials, chromosome counts, and other techniques.
The purity that concerns us here is mechanical purity or the composition of the seed lot. Purity analysis simply tells what percentage of the seed lot is pure seed and what is not.
Size of the seed sample depends on the size of the seed and is specified in the rules for seed testing. For example, some crops require the sample to have 2,500 seeds for a proper purity analysis. The sample is usually sorted by hand, but sieves or mechanical blowers may be used for certain types of seeds.
Each component is weighed to obtain a percentage of the total seed sample. The seed analysis report will list percentages of:
Pure seed is the percentage of the sample that represents the crop species for which the sample is being tested. In seed mixtures, it includes the percentage of crop species present in levels of 5% or more. Pieces of crop seed larger than one-half the original size are classified as pure seed. Shriveled, immature, or frosted seeds are also classified as pure seed.
Other crop seed is the crop species being 5¾. the percentage of crop seeds, other than tested, present in concentrations of less than
Inert material is the percentage of seed sample that is not seed. This includes chaff, stems, small stones, dirt, and other material. It also includes pieces of crop and weed seeds less than one-half original size. The rules for seed testing define specifically what is and is not considered inert material.
Weed seed is the percentage of seeds from plants considered as weeds. Weeds are usually defined by state law. Sometimes the seed analyst must make an arbitrary determination because some plants are considered a weed in one state, but not in others. Weed seeds counted may or may not be noxious weeds.
Purity tells us what percentage percentage of a seed lot is something other than pure seed. The other components can be very important even if only found in very small amounts.
An example is a “bin-run” soybean seed sample that had been "cleaned” by a portable air screen cleaner. Results of purity analysis were:
¶ Table 1. |
|
| Pure seed | 87.75% |
| Other crop seed | 0.04% (10 oat seeds per lb.) |
| Inert matter | 12.18% |
| Weed seed | 0.03% |
The tiny fraction of weed seed – 0.03% - does not seem important, except that the weeds seeds were black nightshade berries. The number of berries represented by the sample were enough seeds to produce one black nightshade plant every 2 feet when planted in 30 inch rows.
The results of a Kansas State wheat drillbox survey provide another example. One wheat sample had 0.18% weed seeds by weight (1.7 ounces in 60 pounds of seed). This weed seed component consisted of the following species:
¶ Table 2. |
|
| Curly dock | 12 per lb. |
| Cheat grass | 15 per lb. |
| Goat grass | 14 per lb. |
| Wild buckwheat | 18 per lb |
| Total seeds | 59 per lb |
Assuming a 60 pound per acre seeding rate, that's 5310 weed seeds per acre or 1 weed seed every 10 feet of row in 10-inch row spacings.
All planting rates and seed purchases should be based on pure live seed. During purity analysis, no attempt is made to distinguish between seeds that will germinate and seeds that will not. Purity only tells how much of the seed lot is pure seed. Germination tells how much of the seed is alive.
The fraction of pure live seed is obtained by multiplying percent purity by percent germination and dividing by 100. If PLS is 95%, that tells us that 5% of the seed lot is something other than good quality seed.
When buying seed, always compare cost per pound of PLS. This is obtained by dividing total seed cost per pound by %PLS then multiplying by 100.
The data in Table 3 provides an example of how differences in PLS can affect production costs. The example compares two lots or sources of seed, “A” and “B”.
If we simply compare cost per bag, lot "B" looks like the better buy because it is $15 per bag less than lot "A". If we compare cost per pound of PLS, then seed lot "A" is a better buy. Lot "A" has about 10% more pure live seeds than lot "B". To get the same number of live plants per acre using lot "B", would require planting more seed.
Assume 200,000 seeds per pound and a target rate of 75 live seeds per square foot. This would require 16.4 pounds of PLS per acre. Lot B with the lower PLS percentage would require 2.1 more pounds of bulk seed and would cost $3.30 per acre more.
¶ Table 3. PLS Example, Cost Comparison |
||
| Seed Lot | Lot A | Lot B |
| Germination % | 97.5% | 90.1% |
| Purity % | 98.7% | 95.3% |
| Cost for 50-lb. bag | $215 | $200 |
| Bulk seed cost, $/lb | $4.30 | $4.00 |
| PLS % | 96.2% | 85.9% |
| PLS cost, $/lb. | $4.47 | $4.66 |
|
75 PLS/ft2 ÷ 200,000 seeds/lb. = 0.000375 lb. PLS/ft2 0.000375 lb PLS/ft2 x 43,560 ft2/ac. = 16.4 lb. PLS/ac. |
||
| PLS required, lb./ac. | 16.4 | 16.4 |
| PLS % | ÷ 96.2% | ÷ 85.9% |
| Bulk seed required, lb./ac. | = 17.0 | = 19.1 |
| Bulk seed cost, $/lb. | x $4.30 | x $4.00 |
| Seed cost, $/ac. | = $73.10 | = $76.40 |