2. Sifting
The working sample is passed over two sieves – one with round holes and one with slots.
When the sample is sifted over the round-hole sieve, the canola drops through the holes and the large dockage material stays on top. When the sample is sifted over the slotted sieve, the canola stays on top and the smaller dockage drops through to the pan.
Choosing the correct sieve sizes is a key part of the dockage assessment process. When selecting sieves, the goal is to remove enough conspicuous admixture so that the grade tolerance can be met, while also minimizing the loss of reasonably sound canola. Conspicuous admixture is material that stays in the sample after cleaning and is easily distinguished from canola such as flaxseed, mustard, weed seeds… etc. Choosing sieve sizes that will produce this result requires care and attention by the person doing the assessment.
Important Update: The Official Grain Grading Guide was revised on August 1, 2023, to provide important clarification of the dockage process. The Guide indicates the smallest slotted sieve (0.028) should be used first, and the larger sizes only to clean for grade improvement. Now, the 1.0% tolerance for conspicuous admixture must be exceeded to justify moving to a larger sieve. The change clarifies the dockage assessment process, eliminates a degree of subjectivity, and allows for clearer understanding by farmers.
Pay close attention to what is considered dockage. A range of sieve sizes can be used and operators use their discretion in choosing the most appropriate. If you observe many sound, smaller seeds being included as dockage, request a smaller sieve be used to minimize the loss of sound canola.