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ISTA Position Paper on Seed Lot Size

Introduction
From the outset maximum lot size remains the basic requirement for seed lots for which an ISTA International Seed Lot Certificate is issued. The first issue of the ISTA Rules of 1931 prescribed maximum seed lot sizes i.e. 100 bags for grass seed and 50 bags for other crops. Maximum lot size for each crop species is now defined taking into account the specific properties of seed and conditions for seed trade. Since the conditions for seed trade change over time, ISTA maximum lot size also revised periodically, but remains as part of ISTA sampling system. Once again changing the seed lot size is under intense discussion in international forum.

FIS proposed to increase seed lot size from 25 to 30 tonnes for cereals and from 10 to 20 tonnes for grasses within the context of ISTA/FIS Experiment on Herbage Seed Lot Size. In view of harmonisation of the ISTA Rules and the AOSA Rules it was also proposed whether maximum lot size could be repealed from the ISTA Rules. This paper describes the position of ISTA on maximum lot size approved by the Ordinary Meeting on 21 June 2001.

Scientific Background on Maximum Lot Size
Sampling errors affect the accuracy of seed testing results. Drawing primary samples from the seed lot and sampling procedures in the laboratory are basic part of sampling. The statistical basis of the ISTA Rules for calculating the accuracy of seed testing results is based on the assumption that within seed lots there is a random distribution of seed quality. This is the best possible homogeneity expected and sampling seed lots is not considered as separate variance component in most tolerance tables in the ISTA Rules. For example tolerance tables for comparing two results of other seed counts are the same whether the two samples came from the same or from different submitted samples. In purity testing, however, a certain amount of non-homogeneity of seed lot is taken into account and consequently tolerance tables are different for the two samples submitted from the same seed lot showing differences between these two tests.

The two basic measures justifying this assumption in tolerance tables are the definition of ISTA maximum seed lot size and the ISTA heterogeneity test in Appendix D of the ISTA Rules. Whereas the heterogeneity test provides information in case of any doubt on the homogeneity of a seed lot, the maximum seed lot size is a precautionary measure to avoid heterogeneity in seed lots. The efficiency of this measure is demonstrated in several scientific studies which indicates that with increasing lot size heterogeneity of the lots increases linearly (Figure 2). Due to the design of these studies, experimental data of the heterogeneity of ISTA maximum lot size could not be included in this figure, but the expected value is about 1% heterogeneous seed lots. So the figure shows that doubling the present ISTA maximum lot size, the average percentage of heterogeneous seed lots is higher than the expected value for the present maximum lot size.

Furthermore, results from Germany indicated that in small-scale seed production heterogeneity is a greater problem than in large-scale seed production. However, in large-scale production heterogeneity also increases with lot size as observed in larges scale grass seed production in the United States. Copeland and colleagues said "Excessive heterogeneity often exists in large seed lots, especially from natural production units (e.g. fields). Such seed lots are seldom if ever blended by state-of-the-art equipment, but are simply conditioned, bagged, and marketed. Thus, the inherent heterogeneity commonly occurs among the containers (bags) in seed lots". And the authors concluded "As explained earlier, there are good reasons for the seed size restrictions on seed lots."

Heterogeneity in seed lots is a greater problem in chaffy seeds than in non-chaffy seeds. In the first phase of ISTA/FIS Experiment on Herbage Seed Lot Size, 88 seed lots with twice ISTA maximum seed lot size were tested for heterogeneity. According to ISTA heterogeneity test 75% of grass seed lots were classified as heterogeneous. This shows that increasing or repealing ISTA maximum lot size will increase the heterogeneity of lots.

Consequences on Seed Quality and Trade
The increasing heterogeneity due to increased or repealed ISTA maximum lot size will lead to higher sampling errors when drawing the primary samples from the seed lots. The first strategy is to stick to the heterogeneity test and the assumption that within seed lots only random distribution is acceptable. But in this strategy the statistical basis of the ISTA Rules will not meet the practical situation and is not acceptable. To overcome increase of heterogeneity in seed lots, it is possible to increase the sampling intensity, the number of primary samples drawn from the seed lot, which is shown to be efficient. However, increasing the number of primary samples over the present maximum of 30 samples restricted the efficiency and offers no solution. Consequently, the accuracy of the seed testing results will be impaired. This will lower the reliability of the decisions made on the seed lots during certification or trade. Consequently, in contrast to the present practice, the heterogeneity of the seed lots should take into account the statistical system of the ISTA Rules for all quality tests. The tolerance tables and the heterogeneity test have to be changed to reflect the new situation. Since experimental data for this modification is presently limited, assumptions must be made and agreed among responsible technical committees within ISTA. Therefore, at present there are no exact figures by what factor to broaden the tolerance, but preliminary estimates are between 5 and 20%.

Another issue for discussion is the test results on ISTA International Seed Lot Certificate which represent the average quality of the lot. By drawing a number of primary samples from the seed lot, the heterogeneity in the seed lot affects not unrestricted the accuracy of test results. In practical terms, the seed buyer expects that the results given on the certificate and on the label represent the quality of the seed in some of the containers he purchased. Although this expectation is statistically unjustified, it is a common understanding in seed trade. And here it has to be realised that the reliability of the test results with view to this expectation is not unrestricted but directly impaired by heterogeneity among the containers in the seed lot. So from the consumer viewpoint, who buys some bags from a seed lot, heterogeneity of seed lots is much more important than from the producer viewpoint, whose interest is for the lot to meet quality standards for seed certification.

Conclusion
ISTA concentrates on improving the quality of seed testing results and on assuring the accuracy achieved in its member laboratories. Since the reliability of seed testing results on the ISTA International Seed Lot Certificate will definitively be impaired if the maximum lot size is increased or repealed, ISTA will not actively promote a change of the ISTA Rules. However, if all other partners in seed trade such as seed sellers, buyers and certification agencies agree to bigger lots, ISTA will adapt the Rules and will manage the technical consequences, particularly by setting up new tolerance tables reflecting the lower quality of seed testing results. Source: ISTA News Bulletin No. 122, September 2001.