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 Main subject - jan/feb 2005
Sampling a decisive tool for seed quality assurance
Dr. Osvaldo Ohlson de Castro
Claspar
lascuritiba@terra.com.br
The sampling theory is a study of the existent relations between a population and the samples extracted from it.
In Statistics, one of the ways that a representative sample can be obtained, is a process denominated random sampling, in agreement that each element of a population has the same probability of being included in the sample.
When we come across with the subject seed quality, immediately the word sampling is used to explain some action, because it is part of the quality control process.
The uniformity in the procedures and detailed information are fundamental in the sampling of a seed production field, of a seed lot, as well as in the laboratory for taking the working.
An incorrect sampling can lead to incorrect decisions, discarding a product of high quality or approving others of inferior quality, resulting in inconveniences for the business.
The sampling is fundamental in all of the stages of the seed quality evaluation, starting from its obtaining, production, reception process, conditioning, analysis and, finally, in the market.
The crescent increase of grain production in Brazil, besides the expansion of the agricultural border, is the result of the productivity increase, consequence of higher quality of the seeds offered to the producer. The breeders, thanks to new cultivars that are being developed, more and more productive, best adapted and resistant to the diseases, contributed a lot to the success of the Brazilian agriculture. However, to get to the farmer, those new materials need to be multiplied in great scale, generating available volume to assist the needs. In that multiplication process, the materials go through constant evaluations of the quality, for identification and preservation of the favorable characteristics.
From the initial stage, when in small volumes, few portions, under the constant control of the breeder, all of the material goes through the evaluations for the quality control. Simplified process, in small populations is almost possible to observe each individual, as for their desirable or undesirable characteristics. Afterwards, in the multiplication process, with the increase of the volumes and of the areas, that individual observation becomes impossible.
In the field, besides the size of the areas, most of the plants from the same species, in the same area, reach the same development phase and need to be inspected in a short period of time and without having each plant observed individually. The technician will need to accomplish a complete inspection that will allow him, with safety, to determine the acceptance or not of the area for seed production. Then, that inspection will involve an estimate for the quality, based on the collection of samples, on which a decision will be made for acceptance or rejection. Those samples will be areas of the farming, randomly taken inside of a course previously established, in which the size will be according to the tolerance limit for the factors that will be observed.
The sampling for field inspection, as any other sampling, is subject to limitations. The inspection procedure should take into consideration the time, the randomness and biased, but above all, to allow an estimate quite close of the field quality. In this case, the sampling should take into consideration the uniformity of the field, to be representative of the area and to make possible a necessary evaluation of the undesirable factors, within the requested precision.
If in a seed lot, all of the individuals were the same, we would have a lot perfectly homogeneous and it would be enough to remove a single sample for its correct evaluation. The sampling process is of extreme importance for the sample to be representative of the lot from where it comes from being taken into consideration that a seed lot is never perfectly homogeneous. In the process of seed conditioning, besides the cleaning, it is expected that the handling of the seed lot promotes the distribution of all the present components randomly.
It is considered as a seed lot a specific quantity, physically identified, in which the portion is, within the allowed tolerances, homogeneous and uniform for the information contained in its identification.
The uniformity is related with the probability of a component to occur in a sample, of a certain size to be constant through the entire lot. In an uniform lot of seeds, there will be a different, but constant probability associated with each component. That leads us to the conclusion that, if the components of a seed lot are randomly distributed in the entire lot, a representative sample removed from any of the points should not be significantly different from other representative sample removed from any other point.
The uniformity of a seed lot is determined by, the proportion of the seed of the specie in subject and the present of undesirable material, the variation that occurs with those constituents and its distribution in the lot. As for the seed of the specie in subject, variations may exist in relation to the moisture degree, size, forms, weight, level of damages, viability and other.
Sampling at the different stages of the seed business
The uniformity can be then considered as a dynamic characteristic, being influenced from harvesting, where generally it is more uniform, passing through transport, conditioning and during storage. In the storage, physiologic and sanitary qualities will be more subject to alterations, as much inside of the containers individually as well as inside of the entire lot.
For planning the sampling techniques, it is necessary to take into consideration all of the variation possibilities, the purpose of the sampling and the stage that it is being accomplished.
The following types of samples are defined:
- Primary sample - it is a small quantity of seed taken from a seed lot in one simple sampling action.
- Composite sample - it the result from the combination of all primary samples taken from a seed lot.
- Submitted sample - it is the sample received by the laboratory for the execution of the tests. It must possess a size specified in the Rules for Seed Analysis.
- Official sample - sample taken by an official, for inspection purposes.
A sample is obtained from a seed lot by the capture of small portions, randomly, in different places of the lot and combining them. From this sample, smaller samples are obtained by one or more divisions.
The purpose of the sampling, in a certain volume of seeds, is to obtain a sample of appropriate size for the tests that intend of being accomplished, in which must be present the same components of the total volume and in similar proportions, and that the probability of any component to be present in the sample must be only determined by the occurrence amount in the lot.
The amount of seeds that compose a sample, on which the individuals will be examined, is extremely small when compared to the entire seed lot. When a result is expressed as a simple number for a seed lot, it can be presumed that any heterogeneity type does not exist, that is to say, there cannot be significant variation in the different parts of that seed lot.
No matter how good the procedures used in the analyses are, the results will only indicate the quality of the seeds contained in the examined sample. Therefore, care should be taken by the seed sampler, in the storage, as well as for the analyst, in the laboratory, so that those samples are representative, respectively, of the seed lot and of the sample received by the laboratory.
If a lot is heterogeneous, above the tolerance, thr samples taken from it will not be representative. The seed sampler should pay attention for possible visual differences among the primary samples, what could characterize the heterogeneity of the lot.
The collection of seed samples, in any of the stages, must be executed only by trained and authorized personnel, that know about the responsibility and importance of that operation, considering:
- type of seed regarding, its forms and if it has chemical treatment or covering;
- type of recipient where the seeds are placed;
- access to the containers that compose the lot;
- access to the complete information on the lot;
- instrument appropriate for the sampling;
- disposal of a sample divider;
- disposal of packing to condition the samples.
page 2 ->> Sampling a decisive tool for seed quality assurance
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