Seed and Seed Quality
Seeds are the foundation of agriculture. Technology has modernized much of farming's day-to-day operations, but without a steady supply of high-quality seed, yields and crop quality would be greatly decreased.
Seed quality plays an important role in the production of agronomic and horticultural crops. Characteristics such as trueness to variety, germination percentage, purity, vigor, and appearance are important to farmers planting crops and to homeowners establishing lawns and gardens. Achieving and maintaining high seed quality is the goal of every professional seed producer.
This publication presents basic facts about seed, seed quality, and seed laws. This information can help seed producers, farmers, and homeowners understand the important role that seed plays in producing superior crops, landscape plants, and lawns.
Seed Development and Structure
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Seed Development
The process of seed development begins within the flower, the plant's reproductive structure. The flower is a modified leaf structure and can be both male and female. The female part is the pistil, and the male part is the stamen. One flower may contain the pistil and stamen, as in beans, or they may occur in different flowers, as in corn.
A typical flower and its parts are illustrated in Figure 1. The pistil has a top portion (stigma), a middle portion (style), and a lower portion (ovary). The ovary may contain one or more ovules. The surface of the stigma produces a sweet, sticky solution as it becomes receptive to pollen fertilization.
Pollen is produced in the anthers at the ends of the stamen. Pollination occurs when pollen grains come into contact with the stigma. Wind and insects are largely responsible for the transfer of pollen from the anthers to the stigmas. Methods of transfer differ from one species to another.
If conditions are favorable, pollen grains begin to grow on the stigma surface and form pollen tubes. The pollen tube grows down the style and into the ovary, where it comes into contact with the ovule. Male gametes are transferred through the pollen tube into the ovule. Fertilization occurs when the male gametes unite with the female egg in the ovule. After pollination and fertilization, ovules develop into seeds.
In self-pollinated plants, pollen produced within each flower pollinates the stigma of the same flower. Cotton, barley, wheat, oats, tobacco, soybeans, okra, peanuts, and peppers are examples of self-pollinated crops. In cross-pollinated plants, the pollen grains pollinate flowers other than the one from which they originated. Examples include corn, rye, tall fescue, alfalfa, carrots, cucumbers, squash, and onions.
Fertilization occurs shortly after pollination and begins the process of seed development. Early stages in the development of strong plants depend on favorable growing conditions. Seed quality, on the other hand, is largely dependent upon environmental conditions and the promptness with which the seed is harvested after it matures.
Effects of Pollination Type on Varietal Purity
Varietal purity is heavily influenced by the type of pollination. Self-pollinated plants need be only a few feet apart to prevent the pollen of one plant from pollinating nearby plants of another variety. Cross-pollinated species, on the other hand, may need to be separated by several hundred feet to isolate them from plants of different varieties.
In self-pollinated crops, stray pollen from an off-type plant can pollinate flowers on several other plants, thus reducing varietal purity. Off-type plants should be removed before pollination occurs. In cross pollination, pollen is moved long distances. Thus, a single off-type plant in a seed field can pollinate flowers on hundreds of other plants.
Varietal purity is not easily achieved. It requires timely and careful roguing (removal) of off-type plants. Seed certification agencies and seed breeding companies use field control programs to maintain varietal purity.
Seed Structures
Knowledge of seed structure can help in understanding how seeds respond during harvesting, conditioning, germination, and seedling emergence.
Seed can be divided into two major classifications, monocots (monocotyledons) and dicots (dicotyledons), based on the number of cotyledons (seed leaves) in a seed. Monocots contain one cotyledon, whereas dicots have two.
Examples of plant species having monocot seeds are grasses–such as small grains, corn, or turfgrasses–and other crops such as onions. Plants with dicot seeds include legumes–such as peas, peanuts, soybeans, and clover–and other crops such as cotton and tobacco.
Seeds are composed of three basic structures: (1) the seed covering (seed coat or testa); (2) the embryonic axis (embryonic root or radicle and shoot or plumule); and (3) supporting tissues (the cotyledons and endosperm). The structures for monocots and dicots are illustrated in Figure 2.
If unbroken, the seed coat regulates water uptake by mature seeds. Variations in seed covering characteristics, especially in dicots, often affect the quality of seed when exposed to adverse weather. Some seeds, such as peanuts, have an extremely soft and delicate seed covering. This covering can easily break or slip and expose the embryo, making it susceptible to injury, deterioration, and pathogen attack.
Other seeds have extremely hard coverings that protect the seed from almost everything. Common weeds such as puncturevine, dock, knotweed, and pigweed are examples of these tough seeds. It is no wonder that most common weeds survive so long scattered across the land or buried in the soil. Lotus seeds have survived for many hundreds of years because of their tough, hard covering.
The embryonic axis normally includes the miniature plant, consisting of the root and shoot. Cotyledons and endosperm are usually considered supporting tissues. They are useful to the developing plant as a reserve food source through the course of germination and emergence until the plant can make its own food through photosynthesis. Supporting tissues in monocots are composed mostly of nonliving, starchy materials, whereas such tissues in dicots are composed of mostly fats and oils.
The location of seed structures plays an important role in determining the seeds’ susceptibility to mechanical injuries and weather damage. The embryonic axis is often just below the seed covering. Impacts to the embryonic axis can cause severe damage, resulting in abnormal seedlings or death of the seed. Peanut and soybean seeds can be damaged easily.
Figure 1.
Figure 1. A typical flower. It may contain both the pistil (female) and stamen (male). Many flowers contain only one pistil but usually several stamens.
Figure 2.
Figure 2. The major botanical structures of monocotyledonous (for example, corn) and dicotyledonous (for example, soybeans) types of seed.
Sizes and Shapes of Seeds
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Seeds come in many sizes and shapes. They range in size from the micro-miniature orchid seed, as small as a particle of dust, to the gargantuan double coconut seed, over 1 foot long and weighing many pounds. Shapes vary from the simple, round tobacco seed to the complex, aerodynamic, winged maple seed. Some seeds are hairy, such as the cotton or thistle. Others, like grass, have long spikes. Some have thorns.
Dispersal Mechanisms
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The dispersal mechanisms of seeds range from the simple dropping of the seed from the parent plant onto the ground to the more exotic ways such as scattering by wind or “shooting” from the plant. The tumbleweed scatters its seeds in the wind as it rolls along the ground. Light, puffy seeds like those of the thistle and dandelion and winged seeds like those of the maple and pine can ride the wind for great distances. Some seeds, such as the cocklebur, hitch rides on passing animals. Among the most exotic dispersal mechanisms are the “shooting” seeds of plants such as mistletoe or touch-me-not that are “spring loaded” and flung from the parent plant into the air.
For seeds used in farming and landscaping, the primary dispersal mechanism is the planter. This mechanism can be very simple such as the primitive dibble stick used to scratch open a furrow or poke a seed hole into the soil. At the opposite extreme is the modem mechanized planter that plants four or more neatly aligned rows at one time.
Chemical Composition of Seed
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Like all living organisms, seeds are composed of many different types of chemicals, but seeds are unique in that they are a storehouse of chemicals that are used as food reserves for the next generation plant. These chemical foods also serve as a significant part of our food supply.
Seeds store three major classes of chemical compounds: carbohydrates (sugars), lipids (fats and oils), and proteins. The quantities of these compounds stored in seeds vary with the type of seed, as shown in Table 1.
Seed chemicals can be very useful. Certain seed oils are particularly well suited to cooking. A prime example is peanut oil, highly prized as one of the best cooking oils. Others are particularly well suited to lubrication. Seeds of the jojoba (a little known but very useful desert plant from the American Southwest) contain an oil that has lubricating properties as good as those of the finest sperm whale oil. Seed oils are used to make soap, paint, printing inks, and other industrial supplies.
In the case of proteins, seeds may not have the ideal composition in terms of human nutritional needs. Proteins are made up of long chains of amino acids. Some seeds do not have the optimum quantities of amino acids for human nutrition. For example, corn proteins are generally low in the amino acid lysine but relatively high in the amino acid methionine. In contrast, soybean proteins are relatively high in lysine but somewhat low in methionine. When corn and soybean seeds are used together, a nutritionally satisfactory balance can be obtained.
Most seeds are not high in protein but are high in carbohydrates or lipids. Soybean (high in protein and relatively high in lipids) is the exception rather than the rule. The seeds of most plants store their food reserves mainly in the form of carbohydrates or lipids.
Through careful breeding and selection, the levels of seed storage reserves can be modified. In this age of biotechnology, it may be possible to breed a “designer” seed plant with the correct levels of carbohydrate, lipids, and protein to meet human nutritional and industrial needs.
Table 1. Approximate Chemical Composition of Various Kinds of Seed.
Kind of Seed % Carbohydrate % Lipid % Protein
Barley 76 2 9
Bean 56 1 23
Corn 62 5 10
Oat 66 5 12
Pea 40 2 20
Peanut 23 45 25
Soybean 17 20 40
Wheat 67 2 13
Germination and Field Emergence
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The following definitions will help in understanding seed germination and seedling emergence:
Germination is the emergence from the seed and development of those essential structures that under favorable conditions produce a normal plant. Germination is more than just the protrusion of the root or shoot from the seed covering. It is important that all of the seedling structures necessary for continuation of the next generation be present and healthy.
Field Emergence is the elongation of the seedling axis resulting in protrusion of the seedling shoot from the soil.
Viability is the potential to germinate. A nonviable seed will not germinate under any conditions. Viable and a nonviable seeds may look exactly the same.
Dormancy is the state of nongermination in viable seed. During this period, germination is blocked by conditions within the seed. Dormant seeds are often thought of as being in a resting state. Dormancy is broken when the seed is exposed to specific conditions. The condition may simply be the passage of time, or it may be the removal or breaking of the seed coverings, a cold overwintering, or the effects of light or hormones supplied to the seed. Minor and subtle changes in the physical or chemical properties of the seed are usually required to break dormancy. A seed may take up water and look fully able to germinate but, because other necessary conditions are not present, may fail to germinate.
Various seed characteristics result in different germination and seedling emergence patterns.
Germination
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Requirements for initiation of germination include:
a favorable moisture level in the seed;
a favorable temperature in the environment around the seed;
a favorable oxygen supply to the seed.
Note that favorable conditions must be present. Some seeds may require specific light conditions. Also, the seed's dormancy must be broken for germination to proceed.
Germination occurs in several steps. The first is the absorption of water. Water begins certain biochemical processes within seed that accelerate cell activities. The minimum moisture level at which germination begins is known as the critical moisture level.
Critical moisture levels vary among crop seed. Most starchy seeds (monocots) will begin germination when they have a moisture content of approximately 30 percent. Most oily seeds (dicots), however, will not begin germination until they have a moisture content of at least 50 percent.
Germination will not occur above or below the critical temperatures. Each species has a minimum, an optimum, and a maximum temperature for seed germination. Most species have a minimum germination temperature of approximately 55°F (13°C) and a maximum germination temperature of approximately 110°F (43°C). The optimum temperature is usually from 75 to 85°F (24 to 30°C).
Seeds of plants such as beets, corn, cotton, okra, peppers, soybeans, and turnips sometimes germinate better with alternating temperatures than at a constant temperature. Favorable alternating temperatures for the seed of a given crop tend to follow the average day-to-night temperatures during the planting season.
Lack of oxygen is not usually a limiting factor for germination. However, wet or soggy soils may not contain enough oxygen for germination to begin. Seeds planted in such soils will absorb water quickly and have a tendency to decay.
Most agricultural seeds germinate equally well with or without light. However, many seeds such as those of tobacco, lettuce, and many types of grass, will germinate best in light. In contrast, onion seeds germinate best in darkness.
The internal conditions of seed, such as soundness and vigor, as well as the environment, affect the rate of germination. Seed vigor can be affected by maturity, age, mechanical injuries, disease infection, preharvest weather, and storage environment.
Adverse weather conditions after planting regularly influence the germination processes by affecting moisture, temperature, and oxygen levels. Weather patterns sometimes make early spring or late fall planting extremely risky.
Once cell activity is initiated, the root is the first structure to emerge from the seed coat. Root growth is a result of both cell division and elongation. Hypocotyl growth is mainly a result of cell elongation. Shortly after root growth begins, the shoot meristem emerges from the seed coat and continues development by cell division and elongation.
Field Emergence
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Different species exhibit different seedling emergence patterns. The emergence patterns of soybean, pea, and corn seedlings are shown in Figure 3, Figure 4, and Figure 5. The illustrations represent three basic emergence patterns for agricultural seeds.
In soybeans and other crops such as cotton, clovers, squash, and radishes, root growth occurs by cell division and enlargement of the root (Figure 3a). As growth continues, the hypocotyl elongates by cell enlargement, and the midsection emerges from the soil (Figure 3b). The seed coat usually remains below ground. As the bent hypocotyl elongates, it gradually pulls the cotyledons above ground (Figure 3c). During the critical stage when the hypocotyl is still arched above ground and is pulling the cotyledons upward, damage to the hypocotyl can easily prevent seedling emergence.
Figure 4 shows the emergence pattern for peas. Elongation of the root and plumule occurs almost simultaneously (Figure 4a). The cotyledons and the seed coat remain below ground at planting depth, and the epicotyl advances upward through the soil (Figure 4b). When seedling emergence follows this pattern, a weakness in the epicotyl may cause emergence failure. Other crops with a similar emergence pattern include vetches and peanuts. Peanut cotyledons, however, tend to advance toward the soil surface but usually remain slightly below it.
The emergence pattern of corn is illustrated in Figure 5. Most monocots have this kind of emergence pattern. The elongation of the root and the coleoptile (penetrating organ) occurs almost simultaneously (Figures 5a and 5b). The seed remains underground at planting depth, and once the coleoptile (which is pointed like a pin) emerges above ground, it stops growing. The plumule continues to develop and grow upward through the coleoptile (Figure 5c). In this emergence pattern, a weakness in the coleoptile can cause emergence failure.
Figure 3.
Figure 3. Field emergence pattern of soybean seedlings.
Figure 4. Field emergence pattern of pea seedlings.
Figure 4. Field emergence pattern of pea seedlings.
Figure 5.
Figure 5. Field emergence pattern of corn seedlings.
Seed Germination Tests
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One of the official statutory test required for labeling purposes is the standard germination test. The rules for conducting this test are established by the Association of Official Seed Analysts (AOSA) and set forth in a handbook entitled Rules for Testing Seed. Most seed testing laboratories follow these rules. The AOSA has also set standards and procedures for the use of the tetrazolium test as an estimate of viability and germination.
The standard germination test measures the number of normal seedlings produced by a sample of seed under optimal conditions. Germination is reported as the percentage of seed producing normal seedlings. Normal seedlings are those that produce a vigorous set of primary and secondary roots; have a healthy hypocotyl, epicotyl, and cotyledon; and produce a healthy shoot meristem. Abnormal seedlings are considered nongerminative in the Standard Germination Test, and would not be counted in the total percent germination for that sample. See Figure 6.
Some seed producers use tetrazolium, cold test, growth rate, or some other testing technique to estimate seed vigor. These tests are designed to evaluate the seed’s ability to germinate and grow under less than favorable conditions.
SEED QUALITY MANAGEMENT SYSTEM
Remember growing beans in cotton and water when you are younger? This is often how it starts: planting a small seed in a pot and waiting to see a tiny green shoot out. On a much larger scale, it is the farmer who sows his fields with the aim of a "good" harvest. Seeds are the reproductive organ for plants. They come from the flower, and contain a miniature version of everything needed to form a new plant: root, stem, leaf and bud. The seed can also contain reserves and other components to help the new plant emerge and grow. All these things will decide whether a plant is cultivated for food, animal feed, or other uses. To choose the right seed, farmers need to ensure the seed’s variety, its ability to form good, healthy plants, and a reasonable probability of a high-quality harvest.
Seed: plant potential and a technological product
To produce high-quality seed, it is necessary to:
Control field production and ensure varietal identity (tested by molecular biology tools)
Sort the seeds produced to obtain a lot of clean seeds (with only the seeds of the species and the desired variety)
Test the seed lot to check the effectiveness of sorting, ensure purity, germination and in some cases seed health. Biochemistry tools can also provide information on the technological quality of seed.
Field seed production is based first and foremost on the work of the seed multiplier. A specific device keeps track of upstream and downstream production. Field control is carried out upstream by precise mapping of seed planting to avoid unwanted crossing. This takes neighbouring cultures into account. In vegetation, the field is controlled several times (control on foot). After the harvest, seeds are sorted and seed batches are approved. When the sorting operations are completed, the seeds can be primed either with a simple coating or with a film coating of crop protection or biostimulant treatment.
Throughout these stages of production, the seeds are controlled in the laboratory. At the end of these operations and before marketing, the seed lots undergo laboratory product control testing. Controls are carried out within the regulatory framework, and seeds and certified in accordance with regulatory standards.
Seed quality criteria
In the laboratory, the first quality criterion is purity. The objective: to have the purest seeds possible of the species in the lot. This criterion is essentially based on a visual analysis. It consists of separating the sample into three fractions: pure seeds, seeds of other plants and inert materials. The result of the analysis is a percentage by weight for each of these fractions. This first analysis is often coupled with a second analysis: enumeration. Within the "seeds of other plants" fraction, the aim is to identify the seeds as precisely as possible, at the finest taxonomic level. The result is given in the form of a list of identified species and their proportion.
After purity analysis, the sample is ready to undergo further tests to examine its physiological qualities. Germination is the key criterion: under optimal conditions (defined by ISTA international rules), what is the highest level of germination that the sample is able to achieve?
Depending on the species, a certain number of seeds are put to germinate. At the end of a fixed period, a count of the seedlings meeting normality criteria is carried out. Abnormal or ungerminated seedlings are counted specifically.
Seed health is also essential for species whose seeds may be disease vectors for crops. Specific analyses are used depending on the pathogen in question. Mycology (study of search), bacteriology (study of bacteria), virology (study of viruses) or nematology (study of nematodes) analysis can be required.
Seed quality standards
Test results for each species-specific quality criterion are compared against the standards defined in national and international regulations.
A purity result for wheat must be at least 98% (by weight) for the lot to be certified. It should also be noted that the presence of a single seed of prohibited species can lead to the refusal of a lot. In a batch of wheat, for example, no Avena fatua seed is allowed.
In many countries the seed and planting material available to farmers are often of insufficient quality, thus undermining the potential yield and performance of crop production. FAO supports countries in raising the quality of the seed produced locally and used by small-scale farmers and in strengthening their capacity to develop and implement quality assurance systems for the production of seed and planting material.
Seed Quality Testing and Certification: Resources Useful in Organic Seed Production
This is an Organic Seed Resource Guide article.
Introduction
The importance of using high quality seeds in organic agricultural systems
Every grower has at some point observed the effects of poor seed quality: slow germination, damping-off, poor stands, weak seedlings, and mixed or genetically contaminated lots. Because organic growers depend heavily on preventative/cultural approaches to promote crop health, vigorous seed can be viewed as the first line of defense against the challenges of cold soil, soilborne pathogens, and other unfavorable conditions. Selecting appropriate varieties adapted to the area of production with disease and insect resistance, along with other desirable characteristics, is also fundamental to satisfactory crop performance and yield. In the past, many organic growers have been reluctant to use organic seed due to issues of availability, pricing, and lower quality. However, as the demand for organic seed has grown, and the industry has matured, the capacity to produce high quality seed has increased. Seed quality is assessed using the methods described below. For other seed quality tests or more information on the following tests, contact your seed laboratory.
Seed quality assessments
Genetic Purity
Genetic purity means trueness-to-type of the seed lot. It is important to assure the genetic identity which makes cultivars distinct. Genetic purity is best evaluated through a field trial in which the percentage of off-types in a seed lot is determined. Seed companies typically conduct variety trials each season to evaluate the genetic quality of contract lots; ideally, the seed lot is evaluated in comparison to the parent stock seed lot and competitors’ lots of the same variety. The results of these variety trials are made available to the grower; this information is used as a tool to guide on-farm selection of the plants in the seed crop so the seed produced from that crop is true-to-type.
Genetic purity evaluation can also include screening for transgene (GMO) contamination. Corn and beets, for example, are increasingly tested for the presence of transgenes. Seed companies typically request and pay for the testing, which is conducted at independent labs. The current National Organic Program (NOP) regulations do not specifically require testing of organic seed for GMO contamination but an increasing number of certifiers are requesting testing. Avoidance is the best approach. Seed farmers should avoid planting seed crops in regions where cross contamination is likely, observe isolation distances for cross-pollinating crops, and when available, participate in regional pinning networks.
Physical Purity
Physical purity evaluation consists of a purity exam and a noxious weed exam. The purity exam determines the percentages by weight of pure seed, other crop seed, weed seed, and inert matter in a sample. The contracting seed company typically defines the purity standard for a particular seed crop and communicates this standard to the grower. The noxious weed exam determines the rate of occurrence and identity of noxious weeds as specified by the Federal Seed Law. This is called the “All States Noxious Weed Exam.” In addition, there is a noxious weed list specified by each state seed law; these weed seeds are prohibited if seeds are to be marketed as "certified seed". These tests are performed by a registered seed technician at a certified lab (see the list of some seed testing laboratories below). Using seed free from weed or other crop seeds, along with planting seed in a clean seedbed, reduces the cost of weed control program.
Other Seed Quality Attributes
Seed viability, vigor, and the presence/absence of seedborne pathogens or other microorganisms are among the attributes needed to make a reasonable prediction of seed performance in the field.
Viability testing: Viability testing determines the percentage of live seeds in a sample that have the potential to produce normal seedlings under favorable germination conditions. The USDA mandates that all seed sold commercially be tested by a certified lab within six months of sale and must meet minimum germination standards that are set for each major crop group. Individual states have their own seed laws governing viability, so seed companies typically set their own internal minimum germination standards to meet or exceed the strictest of the state laws. One problem with germination testing is that it is ineffective when seeds are dormant (as they won't germinate even when viable). The tetrazolium (TZ) test is a quick biochemical test that evaluates seed viability based on seed respiration. This test is useful as it measures the percent live seeds in a sample regardless of the seeds’ dormancy status. The test can be performed in 24 to 48 hours.
Vigor testing: Vigor testing moves beyond a simple assessment of germination by evaluating how quickly seed germinates and whether the germinating seeds and developing seedlings are “normal” and robust in the early stages of growth. Vigor tests measure the potential for rapid, uniform emergence of seeds under a wide range of field conditions. Examples of vigor tests are the cold test, the accelerated aging test, and the conductivity test. The cold test and accelerated aging tests subject seeds to stress conditions to assess their vigor, while the conductivity test measures the level of exudates secreted by the seeds, or the “leakiness,” which correlates to low vigor.
Seedborne diseases and saprophytic (non-pathogenic) fungi: Seedborne disease testing indicates whether your seed carries diseases that will have a significant impact on the health and productivity of the crop. For more information, see Disease Management in Organic Seed Crops. In addition to seedborne pathogens, many other non-pathogenic fungi and bacteria can grow on seed surfaces, and high populations can reduce seed viability and vigor. Proper harvest, processing, and storage methods are key to avoiding storage mold in seed lots.
Certified Seed
One of the ways in which seed quality is regulated is through adherence to a system of seed certification that is overseen by state seed certification agencies. This system has not typically been used by many smaller, independent specialty seed companies as these companies often have their own internal quality control systems. However, seed certification is an important mechanism for ensuring seed quality and should be understood by growers, buyers, and users of seed.
Seed classes indicate how many generations a given seed lot is removed from the plant breeder or institution that was the source of the variety; these classes provide the distributer or buyer information about the history and quality of a seed lot.
Breeder seed is seed produced by the originating plant breeder or institution/private company.
Foundation seed is seed produced from breeder seed and is controlled by license from that source.
Registered seed is produced from foundation seed and is the typical parent seed of certified seed.
Certified seed is produced from either breeder seed or foundation seed, and is at most two generations from foundation seed.
Certified seed crops must pass both field control and laboratory analysis. The field must be planted from the proper class of seed, have appropriate isolation, and be free of problem weeds and diseases. After harvest, a sample of the seed crop must be sent to an official seed certification laboratory for germination and purity analyses. The seed must meet the standards set by the seed certifying agency.
Seed that has passed the field control and the laboratory analysis can be tagged as Certified seed. In addition to the Certified tag there must also be an analysis tag with information on kind (e.g. corn), variety, and purity (percent pure seed, other seed, inert matter, weed seeds, germination).