David:
     When I was doing research at Wisconsin, the rule of thumb was 1/col per
acre but 2 was better.  As I recall, put bees out just at blossom color, since
the largest seeds (ie. outer head) bloom first.  The florets open sequentially,
giving pollen the first day and then the stigma opens the second day.
Pollen/nectar secretion is bimodal, starting about 6AM with another peak at 12
noon. Bees in our study flew down rows, so if you want to cross parent lines,
put them within a row.
 
Hope all this helps.  Jerry Miller at ND State in Fargo can probably give you
more help too.
 
DIana Sammataro