Seed Info 33, July 2007

S E E D I N F O

Official Newsletter of the WANA Seed Network

No. 33, July 2007

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HOW TO___________________________________________________________________

This section provides technical/practical information for technical staff involved in seed production and quality control.

How to No. 34: Simplifying Physical Purity and Number Count Test
In seed testing, raw (un-cleaned) and cleaned seed may need to be analyzed for physical purity and number count test:

•	To ensure that seed price paid to contract growers reflects the quality of seed produced and delivered
•	To identify proper machinery for cleaning and grading seed lots
•	To determine if seed lots meet quality standards for certification

The type and amount of impurities in raw seed lots varies depending on several factors. For example, the impurity of small seeded grasses such as Dactylis glomerata is naturally high and difficult to distinguish from the pure seed because the empty and full florets look rather similar. For most field crops, however, the difference between pure seed and impurities can be easily distinguished, but the rate may considerably differ from one seed lot to another depending on growing conditions and harvesting methods. The rate of impurities in hand harvested, threshed and winnowed seed lots are usually low compared to combine harvested large seed lots. In the former, the rate of breakage, inert matter and other seeds are usually low whereas in the later it is much higher because of mechanical operations such as drum speed and concave clearance which may lead to breakages particularly under conditions of low seed moisture content or excessive drought. Seed samples from such seed lots are very tedious to analyze for physical purity and number count test. In order to simplify the task of seed analysts handling large number of samples in seed testing laboratories, two methods are employed:

Seed blowers
Blowers are commonly used for testing small seeded grasses. They consist of an aspiration system, which generates the air stream, and a vertical glass tube through which the air stream travels during operations. The glass tube is closed at the bottom with a piece of fine mesh on which the purity working sample is placed and at the top with a two overlapping glass lids which can be rotated to adjust the speed of the air current generated by the aspirator. When operated, the air goes through the working sample, lifts up light contaminants such as empty florets, chaff, immature and shriveled seeds, and drops them into the cavities located under the top rotating lid for collecting light impurities. If the blower is calibrated well using standard samples (e.g. ISTA calibration samples) the lifted portion represents the lighter impurities whereas the portion, which is not lifted by air current, represents the pure seed fraction, other seeds and heavy impurities, if any, and can be separated mechanically.

Seed blower showing heavy(left) and light (right) fraction

Hand screens
The machine consists of a shaking deck with set of screens with different sizes and perforations. When the sample is placed on small screens with the right perforation shriveled, immature, broken, weed seeds and small soil particles pass through whereas the pure seed and weed seeds remain on top and can be removed from the pure seed fraction. The screen system is used mostly for combine harvested seed lots of small to large seeded field crops for which light contaminants are removed effectively during harvesting. The hand screens can also be used to select the right screens for cleaning seed lots. Using this simple method, the physical purity and number count tests can be simplified considerably and efficiency and effectiveness increased significantly.
Abdoul Aziz Niane, Seed Unit, ICARDA, P.O. Box 5466, Aleppo, Syria; E-mail: a.niane@cgiar.org