Mancan is a mid-season buckwheat (Fagopyrum esculentum Moench.) cultivar developed at the Agriculture Canada Research Station, Morden, Manitoba. It has a seed size 26% larger than Tokyo and is the first large-seeded buckwheat cultivar licensed in Canada. It is especially suited to southern Manitoba.
Go here FordGuy: http://www.jeffersoninstitute.org/pubs/buckwheat.shtml
50# should be enough for about one acre, and you will need to drill it in. If you want to look for older varieties that are also organic certified, you might try Johnies or Fedco - both can be found on the web.
I will be interested in hearing how you do with this. I may be planting some buckwheat as a cover crop on part of my farm. Good luck and keep us posted!