Historical Paper - Volume 2 2008 - International Seed Testing ...

H.A. JENSEN
performed in 100g. For Trifolium repens, Trifolium hybridum and Phleum pratense, the
examination for Cuscuta should be performed in 50g. If a high number of Cuscuta seeds
were found, the amount examined could be reduced.
Since 1990 it was included in the Rules that the working sample for number count
tests shall be: either a weight estimated to contain 25,000 seed units or not less than the
weight prescribed in table 2A, part I, column 5.
The 25,000 seed Rule allowed the laboratories to make adjustments according to
the considerable differences in thousand seed weight between varieties of several grass
species. At least in the Netherlands and Denmark, countries in which large number of
grass samples is tested, official tables of the seed mass (weights) of small seeded varieties
were prepared. The 1,000 seed weight was used for the calculation of the required weight
of the working sample for determination of other seeds by number.
Tables with seed mass (weights) of different varieties of Lolium perenne and Festuca
ovina were presented at the Budapest Workshop 1997 (Jensen 2000). As an illustration
of this, the mass of 25,000 seeds of the small-seeded Lolium perenne variety Loretta was
only 33g, whereas the standard working sample according to table 2A, column 5 was 60g
in 2005. This shows that for certain varieties considerable time can be saved by using the
25,000 seed rule for determination of other seeds by number.
5.3 Tolerances
The accuracy of purity testing was of concern from the beginning of seed testing within
ISTA, and a formula for calculating the allowed tolerance between two tests was presented
by Munn (1931). In the equations below, p represents the ‘greater part’ and q the ‘lesser
part’ (e. g. % pure seed and the sum of % inert matter, % other crop seeds and % weed
seeds, respectively).
The equation for pure seed was:
Tolerance = 0.6 + 20% of p × q ÷ 100
The equation for other crop seed, weed seed and inert matter was:
Tolerance = 0.2 + 20% of p × q ÷ 100
Munn (1932) published tables for the purity test. Leggatt (1932) criticised that the
equation did not take into account that the larger the sample, numerically, the smaller the
expected variation, and pointed out that it would be more correct to use the equation for
calculation of the standard deviation. In other words, if the size of the working sample for
purity was increased, a smaller variation between tests could be expected.
Leggatt published descriptions of the experimental error in seed analyses, including
the normal -, binomial - and the Poisson distributions, and a theoretical study of a purity
tolerance with special reference to pure seed during the following years (Leggatt 1934,
1935a, 1935b, 1936).
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