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| Seed production / Quality assurance |
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The purity of any commercial product propagated by seed begins with the genetic purity of the seed planted.
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| Introduction |
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 Production of high-quality seed is fundamental to modern agriculture. Most annual crops are established each season from seeds, and seed quality can have a major impact on potential crop yield. Seeds can serve as the delivery system not only for improved genetics but also for new planting and production methods and crop protection strategies that improve the overall efficiency of agriculture and reduce its environmental impact.
The purity of any commercial product propagated by seed begins with the genetic purity of the seed planted.
Genetic purity standards have been established by state seed laws and seed certification agencies to assure growers that the seed they buy is accurately labeled with the correct crop and variety.
Seed purity standards also specify the percentage of contamination by seeds or genetic material of other varieties or species.
The physical purity of seed refers to the presence and identity of weed seeds, and the percentage of other materials such as dirt or plant residues. In addition, the germination capacity of the seed in a standard test must be shown on the label. In some cases, seeds must also be tested for the presence of seed-borne diseases, and hybridity tests are conducted to confirm parentage in hybrid seed.
Production of high quality seed is an exacting task. Seed producers take many steps to protect genetic integrity, including ensuring the integrity of their planting seed, properly identifying and labeling plants and fields, planting seeds on clean land which has not been used to grow the same crop in the recent past, removing rogue plants, or plants which are not true to the variety’s characteristics, and employing physical isolation – via mesh cages, distance isolation, time isolation or hand pollination – to ensure that pollination only occurs among plants of the desired variety.
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| Genetic Purity |
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Genetic purity refers to the percentage of contamination by seeds or genetic material of other varieties or species.
The genetic purity of any commercial agricultural product propagated by seed begins with the purity of the seed planted.
In general, the genetic purity of the seed planted must equal or exceed the final product purity standard required, as purity generally decreases with each subsequent generation of propagation.
It is virtually impossible to assure that no off-type plants or pollen is present in the seed production field and that all handling and conveyance equipment and storage facilities are completely free of contamination.
As a result, commercial planting seed is seldom 100% pure.
In practice, practical seed genetic purity standards have been established by state seed laws and by seed certification agencies to ensure that the purchaser receives seed that is within certain purity tolerances.
These tolerances are established based on the biology of the species (i.e., self- or cross-pollinated), the type of variety (i.e., open-pollinated, hybrid, synthetic), and market-driven standards for final product quality. Earlier generations of seed (e.g., foundation or registered seed) have stricter standards in order to be able to meet the certified seed purity criteria.
The main sources of contamination of a seed crop are the prior crop grown in a field, transfer of pollen from a nearby field, and mixtures during harvesting and handling.
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| Coexistence |
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Co-existence for crop agriculture can be defined as the sustainable production of seed, food and fiber from diverse plant varieties, crop types and production practices.
Co-existence principles have been the key to successful diversification of plant varieties and production systems for food and seed as practiced by growers and shepherded by national and international seed associations from 70 countries over the last 100 years (AOSCA, 2008; ISF, 2008).
The foundation of co-existence is good communication among growers, handlers, shippers and marketers and respect for each others’ practices and requirements.
There is general agreement in agriculture that a zero tolerance or 100% purity standard is not practical in field production systems, but tolerances and thresholds for the presence of low levels of undesired materials allow efficient marketing while meeting end use quality and safety criteria (FDA, 1998).
It is customary that the primary responsibility for meeting specific market standards is on the entity economically benefiting from it, usually the producer who is compensated for higher quality products (CropLife, 2006; Fernandez and Polansky, 2006; SCIMAC, 2006).
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| Certification programs |
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Seed certification programs have been in existence for over 100 years. They have effectively defined and monitored standards to guarantee specific purity standards of the final product or seed. The standards developed reflect the genetic purity and quality of the final product (including seed) based on the final market requirements.
Seed classes: breeder, foundation, registered, certified, commercial, variety undeclared
U.S. National Organic Program (NOP)
The National Organic Program (NOP) in the United States (USDA, 2005) and several other countries define production practices that must be adhered to in order to market products and seeds bearing an “Organic” label. These programs are processed-based rather than being based on the final quality of the product. For example, although only certain approved compounds with pesticidal or fertilizer properties may used in producing organic seed or products, minimum thresholds are established for the inadvertent presence of non-approved compounds. To produce organic products in the US, growers must first begin with organically produced seed. If it is not available, they can use seed that has not been treated with unapproved (usually synthetic) compounds. Although organic programs have chosen to exclude genetically engineered (GE) varieties from the program, there are no established thresholds for the presence of GE materials in organic products. In fact, as long as growers follow an NOP-approved production plan, the USDA has assured that they will not lose organic certification if GE materials are inadvertently found in their seed or products.
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| Hybridity and varietal purity tests |
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Hybrid seed is seed produced by cross-pollinating plants in a controlled environment. Hybrids are bred to improve the characteristics of the resulting plants, such as better yield, greater uniformity, improved vigor, color, disease resistance, and so forth.
Today, hybrid seed is predominant in agriculture and home gardening, and is one of the main contributing factors to the dramatic rise in agricultural output during the last half of the 20th century.
In the U.S., the commercial market was launched in the 1920s, with the first hybrid maize.
Hybrid seed cannot be saved for replanting without losing the benefits of the original variety as these traits randomly segregate among the saved seed, not reliably producing true copies of the original variety.
New seed must therefore be produced for each planting.
As genetic purity is a function of seed production, each hybrid seed lot must be tested for parentage and purity.
To achieve this, hundreds of seeds from each seed lot are planted and observed for uniformity in field tests.
Protein and DNA molecular marker analyses are also widely used for hybrid purity testing.
Protein analysis is often prefered because it is less expensive, but DNA tests are becoming increasingly affordable.
Similar tests are applied to open-pollinated and synthetic varieties to assure varietal purity.
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| Isolation |
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To ensure that pollination occurs only among plants of the desired variety, fields must be isolated either by distance or flowering time from potentially contaminating pollen sources.
The isolation required depends on flower characteristics, sexual compatibility with neighboring crops, pollen quantity and viability, mode of pollen dissemination and purity standard required for specific class of seed.
Self-pollinating crops such as rice or wheat require relatively small isolation distances of 0 to 30 feet because their flower characteristics limit movement of pollen among plants. Isolation is primarily to prevent mechanical mixtures during harvesting.
Cross-pollinating crops, on the other hand, can require distances greater than 1 mile (1.6 km) of isolation from sexually compatible plants to prevent out-crossing, depending upon the flower structure, the mode of pollen transfer, the duration of pollen viability, and the type of seed being produced (e.g., foundation versus certified or hybrid versus open-pollinated).
Recommended isolation distances may need to be increased depending upon the economic impact of contamination. For example, hybrid seed production or production of seeds in which contamination would be readily apparent (e.g., contamination of seeds for white onions with seeds of red onions) often requires greater isolation to achieve purity standards expected in the marketplace. Similarly, greater isolation is required reduce the likelihood that pollen from outside the field will introduce undesired traits intro the crop, such as biotech traits into an organic field.
Isolation can also be achieved by planting crops at different times such that their flowering periods do not overlap. Border rows around the field that are not harvested with the seed crop can also be employed to intercept stray pollen and reduce contamination of the remainder of the field. Related weeds and volunteer crop plants or home gardens can also be sources of pollen contamination. Certifying agencies inspect fields and the surrounding areas to ensure that isolation standards are met.
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