For an overview of organic seed crops please see an article posted on our website via the following link http://www.kingscrops.co.uk/kc/pages/page.jhtml?section_name=500002&page_id=700012 - it is aimed at game cover crops but the principles highlighted still apply.
We tend to work with crops that work reasonably well in low fertlity and have few crop pests such as triticale, fodder radish, gold of pleasure, linseed (can have flea beetle problems but does well on poor soils), white millet, dwarf sorghum and reed millet (the latter two produce no seed but provide good structure supplying staging posts and prevent the seed crops from lodging).
Farm yard manure is essential (if available) and consider a green manure crop such as red clover, phacelia and mustard to trap and create fertility as part of the rotation.
Richard nailed it, but just as an indication of the mix used for Tree sparrow on poor organic land in Yorkshire with good success:
35% Barley
40% White Millet
20% Mustard
5% Phacelia
@ 50kg per ha
at the following rates per ha
17.5kg Barley
20kg White Millet
10kg Mustard
2.5kg Phacelia
= 50kg per ha