Grading is done by the gravity separator. The machine is used for separation on the basis of specific gravity. The process will raise the germination by taking out the light seeds and raise the purity by taking out stones, soil particles and other heavy materials.
To reach and follow the demand of high quality, seeds are more often sown by precision drilling today. Sized seeds will also show a more uniform germination. The sizing is done by a screen sizer normally with round holes.