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Seed Coating

A Tool for Stand Establishment; a Stimulus to Seed Quality

Summary: Seeds are coated for ease of handling, singulation, precision placement and the incorporation of beneficial chemicals or microbials. Coated seeds are accepted widely as a standard product for many crops.

Quality demands for seed suitable for coating, have improved knowledge of physiological seed quality. Higher, better defined quality standards in the seed and coating industry, combined with additional quality demand for enhanced seed, will continue to improve stand establishment potential for growers.

Coatings first were developed for cereal seeds in the 1930’s by Germains, a British seed company. Large scale commercial use of coating began in the 1960’s with precision sowing for the European greenhouse transplant industry. When California outlawed the short handled hoe in the mid 1970’s, the use of coated seed for precision field seeders increased.

Precision sowing greatly reduced the number of skips and doubles in soil blocks or cell trays for transplants. Field precision sowing spaced seeds and thus, individual seedlings sufficiently to permit accurate thinning with a long handled hoe, while reducing damage to the root systems of remaining plants.

The combination of U.S. field precision seeding and greenhouse transplant production created a demand for high quality coatings to achieve accurate sowing, satisfactory seedling emergence and stand establishment. University and commercial research programs responded to this demand, producing coatings now widely used for seeds of vegetables, flowers and some field crops. (Kurosawa, 1976; Mayberry and Robinson, 1982; Markey,1990; Robinson et a, 1983; Valdes and Bradford 1987; Bradford et al, 1985)

In the United States, the major high-volume vegetable crop being using seed coatings is lettuce. Roughly 95,000 Ha (235,000 acres) are sown with coated seed. Brassicas, carrot, celery, endive, escarole, onion, pepper and tomato also are coated to a significant extent, varying with growing season, individual grower preference, the use of direct sowing or transplants, the economics of seed and coating costs etc.

Begonia is the flower crop most frequently sown in coated form. Impatiens, marigolds and petunias are also coated commercially and the market for coating these and other species is growing strongly. Alfalfa and tobacco are two agronomic crops that are coated.

Why is Seed Coated?

Seed is coated when growers need a precision-sown crop and the non-coated (“raw”) seed is too small, light or variable in size or shape to be sown accurately with existing equipment.

Precision sowing is desirable when growers need singulation, e.g., for cell-tray plant production in a greenhouse or strict control of spacing or depth of placement (e.g. onion spacing is critical to achieve desired bulb size at harvest) Singulation and controlled spacing are also vital for crops that are direct sown and then thinned back to the desired population. The field thinning operation is faster, cheaper and more accurate when coated seeds are used.

One Florida grower who used raw seed before 1984 informed me in 1985 that reduced thinning costs paid nearly all of the additional cost of coated seed. He stated that the stands were superior, and that this improvement was essentially “free” Superior stands meant that the incidence of skips or doubles was reduced and the plants were spaced closer to the ideal of 11.5 to 12 inches on center in the row.

The objective of coating is to deliver the seed in a form that is larger, rounder, smoother, heavier and more uniform than the original seed. The coated seed can then be sown with a belt, plate, cup, vacuum or other type of seed. The coated seed or “pills” can be placed individually, with improved spacing and depth control. The pills also flow better through the seeding mechanism, because their surface is smoother than that of non coated seed.

Coating can be a carrier. Fungicides and beneficial microbials that protect the seed and emerging seedling are carried in the coating. For example, alfalfa seed coating with incorporated rhizobacteria is used to inoculate the field with beneficial microbial.

How is Seed Coated?

Seed coating relies on technology developed by the pharmaceutical industry to make medicinal pills. Commercial seed coating operations put seed in a rotating pa, mist with water or other liquid and gradually add a fine inert powder, e.g., Diatomaceous earth, to the coating pan. Each misted seed becomes the center of an agglomeration of powder that gradually increases in size. The pills are rounded and smoothed by the tumbling action in the pan, similar to pebbles on the beach. The coating powder is compacted compression from the weight of material in the pan.

Binders often are incorporated near the end of the coating process to harden the outer layer of the pill. Binders can also reduce the amount of dust produced by the finished product in handling, shipping and sowing. Care must be taken with binders to avoid delaying or reducing the germination percentage.

Specific details of the materials used as binders are closely held as proprietary information by the coating companies. We are unaware of any public information on the classes of materials used as binders.

Blanks and doubles are eliminated by intensive screening and other techniques. Uniform size and uniform rate of increase in size are evaluated throughout the process with frequent hand screening. At intervals during coating, and at the end, all of the pills are removed and mechanically sized on a set of vibrating screens. Smaller pills are returned to the pan and built up to the size of the remainder of the lot. After drying, usually with a forced air system at controlled, moderate temperatures, the pills are screened a final time before packaging. Undersized pills may be built up or discarded. The recovery rate (number of pills divided by the original number of seeds) has been 97% +/-2% for commercial seed lots at one commercial company for the past 10 years.

Size uniformity after coating is a major criterion of coating quality. The usual tolerance for coated seed is +/-1/64th inch (0.4mm). This is the US seed trade standard for sizing, established long before coatings were introduced. For example, coated lettuce seed is sown most frequently with a belt planter through a 13/64 inch diameter round holes in the belt. This hole size requires that the lettuce pills be sized over a 7.5/64 inch screen and through an 8.5/64 inch screen. These tolerances result in levels of singulation well above 95% in the field with placement in the row controlled within < 1/2 inch.

Accuracy of seed placement can vary with weight of the pill as well as the size tolerances. Sowing accuracy also depends on the skill of the equipment operators, the adjustment and the wear of the seeder, and the speed of the tractor through the field. The same constraints are true for greenhouse seeding: experience, attention to details and appropriate equipment are necessary to obtain the full benefit of coated seed.

Types of Coatings

Two basic types of pill produced with inert coating powders are dissolving or “melt” coats and “split” coats.

The melt coats dissolve when wet and gradually wash away from around the seed. Split coats initially retain their shape when wet and , by capillary action, pass moisture through the pill to be imbibed by the seed. The seed swells and cracks the pill by internal turgor pressure. The split coat often permit germination with less water, as they split, allow uniform, rapid oxygen access to the surface of the seed. The melt coats often require more water to wash the coating material away from the seed, and more time for the oxygen to reach the seed through the saturated coating material. Melt coats may offer advantages when soils are saturated , but oxygen availability always influences the speed, uniformity and total percentage of germination.

Powder coatings, both split and melt, multiply raw seed rate and depending on the coating, the number of seeds per pound may decrease dramatically (Table 1)

Species Seeds / lb (raw) Pills / lb (coated) Weight increase (%)
Begonia 70,000,000 250,000 28,000
Lettuce 430,000 12,500 – 40,000 3,400 – 1,100
Onion 120,000 15,000 – 30,000 800 – 400
Tobacco 5,000,000 165,000 3,000

Generally, the heavier pills are easier to because they bounce an roll less than the lighter pills. However, the sowing equipment must be able to handle the pill weight. This can be critical, especially for vacuum type seeders.

In addition too the types of coating products described above, there is recent and increasing use of “film-coating”. A thin polymer film smoothes the surface of the seed for better flow ability. The polymer also influences water uptake and the adherence of chemical fungicide treatments. Film coating only increases the raw weight of the seed 1% to 5%, far less than the powder coatings.

Seed coating aims to influence the external physical properties of the seed, affecting the sowing characteristics only. By itself an ideal coating would be neutral in its influence on the speed, uniformity and percentage of germination when compared to the original raw seed lot. The ideal seed coating would perform in the same manner as the raw seed under a wide range of environmental conditions: light, moisture, temperature, pH, soil type etc. Also the stress of the coating process should not influence the germination pattern or longevity (shelf life) of the seed lot adversely, nor induce secondary dormancy – i.e. affect seed quality.

Seed Quality

No single test is sufficient to distinguish all the attributes that contribute to the physiological quality of a given seed lot. From a growers view point, a high quality lot is defined by results. A high quality lot is one that gives fast, uniform, high percentage emergence, resulting in healthy, near 100% stand under the conditions in the growers field or greenhouse.

Unfortunately, defining quality by historical results is not enough. a predictive test of field performance is needed to provide growers a reasonable assurance of obtaining given good cultural practices and typical (or at least not greatly abnormal) weather, insect or disease conditions. The stand potential needs to be assessed both before and after coating. The goal for a commercial seed or coating company is to develop a fast, reliable series of tests that will predict accurately the stand establishment of a seed lot under a range of possible seed conditions, including stress.

The Federal Seed Act and various state laws stipulate that seed germination be labeled according to standardized tests established by the Association of Official Seed Analysts (Copeland 1981). For example, lettuce must be labeled with the percentage of germinated seeds counted after 7 days at a constant 20C in the light. Celery is tested under the same conditions but the percentage is counted after 21 days.

The official germination figure is extremely useful, but additional tests are needed to determine if the seed lot is suitable for coating. This single laboratory test does not provide sufficient information to determine if the seed lot will retain its germination pattern (percentage, speed, uniformity, reaction to high or low temperatures and shelf life) after coating or ho it may perform under actual conditions in growers fields or greenhouses. Additional tests can include variations on the standard lab test, thermogradient table tests and greenhouse tests.

Laboratory Tests

Variations on the standard germination test increase the stress level, usually using temperature and/or darkness. Lettuce might be tested at 15C or 25C or alternating at 20/30C, perhaps in the absence of light.

The lettuce variety Etna was rejected for coating based on such a test, while Domingos 43 was accepted (Table 2). Etna failed to meet the coating companies standard of > 93% germination with a < 3% abnormal seedlings at 20C and was not coated.

Time Percentage
Raw Seed at 20C
Raw Seed @ 20/30C Coated Seed Coated Seed at 20C Abnormal Seeds
Etna lot 1292
1 day 89 90 * % abnormal seeds
7 days 92 + 7* 92 + 2* * % abnormal seeds
Domingos 43 (lot 5980)
1 day 100 68 99 * % abnormal seeds
7 days 99 + 1* 99 98 + 1* * % abnormal seeds

This standard is empirical, not theoretical, and was chosen based on years of experience with lots that give satisfactory or unsatisfactory field results. The standard is conservative and if no other lot is available,, the seed might still be coated provided that the grower knows, and explicitly accepts the limitations of the seed and the risk of a weak stand.

In another test, the onion variety Sweet Perfection tested as an excellent raw seed lot (Table 3). Germination patterns for onions differ from lettuce; onion seed lots with total potential 80% to 85% frequently are encountered and accepted by growers. Lettuce seed lot germination < 90% are strongly resisted by commercial growers, who prefer seed with a germination of > 95%.

Germination % of raw and coated “Sweet Perfection” onion seed. lot 57412, after two time periods in the lab at 20C and four time periods in the greenhouse with no temperature control.
Time Percentage
Raw Seed at 20C Coated Seed Abnormal Seeds
Laboratory at 20C
3 days 74 83 * % abnormal seeds
7 days 94 + 2* 95 + 2* * % abnormal seeds
Greenhouse, no temperature control
6 days 18 * % abnormal seeds
8 days 82 * % abnormal seeds
10 days 95 * % abnormal seeds
14 days 95 + 2* * % abnormal seeds

Thermogradient Table Tests

A Thermogradient table is a uniform sheet of metal with a cold water bath along one side and a hot water bath along the other. This creates a continuous temperature gradient, typically with a range of 20C. e.g. 12C to 32C or 16C to 36C. ten individual tests can be run on blotters across the width of the table, at 2C increments. This type of test determines the maximum and minimum temperatures at which an individual lot will germinate, as well as its ideal, or preferred temperature range for fastest germination and growth (Figure 1 and Figure 2) The different tests are used in combination. For example the lettuce variety Domingos 43 showed heat dormancy at 20/30C, with reduced germination after24h (Table 2). The severity of the heat dormancy was tested on the thermogradient table. The test indicated a potential germination of >87% in 24h and 96% in 48 h in temperatures up to 23C. This is acceptable for this variety at the usual soil temperatures when it is sown in Southern California and Arizona.

Greenhouse Tests

Greenhouse tests give a stand establishment estimate under real conditions. These tests also can be prolonged to give a subjective estimate of seedling quality. The greenhouse test can, for example, identify seedlings with necrotic tissue on the cotyledons that might not be apparent in other tests. The percentage of seedlings with significant necrosis is deducted from the stand establishment potential of the lot.

Seed quality testing is an integral part of seed coating. Without it the performance of coated seed would be erratic and unreliable for commercial growers. Precision sowing without dependable, precise emergence wastes the advantages of coated seed. For example, the Etna probably would have a field or greenhouse stand well below 90% (Table 2). Depending on circumstances, this might be acceptable, but it would fail to meet the expectations of many growers.

No single test is adequate to accept a raw seed lot for coating, but several in combination can identify acceptable and unacceptable lots with a high level of accuracy. Prudent coating companies repeat all or part of their battery of tests on the final coated product to ensure that the commercial product meets the expectations of the pre-coating tests.

Testing methods, which differ among companies, have been developed to evaluate coatability of a raw seed lot and to check the coated lot’s quality. This intensified testing for subtle quality distinctions has improved understanding of quality and how to measure it. This helps both seed and coating companies to improve quality with techniques employed in seed growing, harvesting, conditioning storing and coating. When it is possible to measure the quality differences it is also possible to develop and choose superior techniques.

The result has been a gradual increase in quality standards throughout the seed industry. Thus, “The Stimulus to Seed Quality” in the title of this article. For example 15 years ago, lettuce could be sold at 85% germination and 90% was considered good. Today, 90% is considered a weak stand and growers seek 95% or better. Commercial growers have higher expectations of physiological performance , mirroring their higher expectations of genetic performance by new varieties of each crop species.

This developing ability to measure and deliver higher seed quality reinforced by the markets higher expectations, is the major long term benefit of seed coating. The particular economic benefits of precise placement, reduced thinning costs, ease of handling etc., motivate growers to change from raw to coated seed and pay the associated research and production costs. Meanwhile, the on going pressure to maintain and exceed existing quality standards benefits all participants.





Seed Enhancement

There is obvious value to improving performance beyond that of raw seed Techniques to evaluate seed quality have been refined further to evaluate methods of enhancing performance.

Enhancement techniques can include seed production and conditioning methods, addition of chemical and biological agents, and seed priming. The purpose of these techniques include increase total germination, faster germination, better uniformity and temperature range of germination, to break post harvest dormancy, to reduce light sensitivity and to prevent disease.

Many of these techniques can be combined with coatings. The “Royal Green” lettuce , like nearly all green leaf lettuce, can be a slow irregular germinator and/or the absence of light (Table 4).

Time Percentage
Raw Seed at 20C, dark Raw Seed @20/30C, light Primed and Coated @ 20/30C, light Abnormal Seeds
1 day 98 98 99 * % abnormal seeds
7 days 98 93 + 1 98 + 2* * % abnormal seeds
Primed and coated seed, no temperature control, light
4 days 85 * % abnormal seeds
6 days 99 * % abnormal seeds

* Raw seed is not greenhouse tested for this product because only the primed, coated seed will be sown commercially. The tests on raw seed evaluate coating suitability; the primed , coated product receives this final; test to ensure that the final quality has been achieved.

Low light conditions make such varieties especially prone to heat dormancy at relatively cool temperatures. Priming the seed can reduce the need for light and simultaneously improve the range of temperatures where germination is rapid and uniform. The thermogradient table test, in the dark, is indispensable for quality evaluation of primed and coated product illustrated (Fig 1 and fig 2).

Fungicides are another example of an enhancement applied to seed to protect vulnerable seedlings from various fungal diseases. Dust or slurry dithiocarbamate treatments are used widely and generally are successful. However, dosage is variable from seed to seed, and there is a degree of “dust off” when the seed is handled.

Fungicide treatments can be combined with film coatings and thereby applied at even more dosage rates, simultaneously eliminating dust off for a cleaner safer product. Powder coatings achieve similar results by blending precise amounts of fungicides in the coating powder for nearly identical dosage on each seed.

Then a final layer of coating powder without fungicide can be applied at the end of the coating process, eliminating the chemical from the pill surface (Canerday, 1990; Dzlezak, 1988; Jackson et al., 1989).

The preceding paragraphs are meant only to hint at the exciting potential of existing rapidly developing new products for enhancing seed performance. Secure production of excellent stands, even under stress conditions, is a worth while goal for researchers, the commercial seed industry and growers.

Real achievements have been made and more will follow, both in seed coating and seed enhancement technology. The achievements are reinforced by the use of coatings and enhancements on on high value hybrid or open pollinated seed. Coatings, enhancements, and new varieties combine advances in physiological and genetic quality, making each more successful and cost effective for the grower.





Cited literature

Canaday, R 1900. Coating creates nutrient environment. Seed World June p48 -49

Copeland, LA (ed) Rules for testing seeds. AOSA, J. Seed Technol. 6:1-125

Dzlezak, JD. 1988 micro encapsulation and encapsulation ingredients. Food Technol. Apr p136-139

Jackson, IM, S. Roberts, P.Timmins and H.Sen.1989 comparison of laboratory scale processing techniques in the production of coated pellets. Pharm Technol. Intl Nov/Dec, p29-32

Kurosawa, T 1976. Effect of seed coating with Calcium peroxide on seedling stand in the mechanized direct sowing rice culture on the paddy field. Rpt Tohuku Br. Crop Sci Soc Jpn17;42-43

Mayberry KS and FE Robinson.1982 Lettuce seed coatings Amer. Veg Grower38(7):32

Markey AE 1990 Growers benefit from Seed Technology. Amer. Veg. Grower 38(13)14-16

Mayberry KS and FE Robinson and Dj Scherer 1983 Lettuce stand establishment with improved seed pellets. Trans Amer. Soc.Agr.Eng26:79-80

Valdes, VM and KJ Bradford.1987 Effects of seed coating and osmotic priming on the germination of lettuce seed. J. Amer. Soc. Hort.112:153-156

Valdes CS and RJ Mc Govern 1990. Seed treatments, target soil borne diseases. Amer. Veg Grower38 (13):63-64


I thank Cher Brant and Chip Sundstrom, Sales manager and Research and Development Manager, respectively of Incotec Salinas, CA for review of this report. Their help is deeply appreciated. Any errors of fact or interpretation are solely mine.
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