My thoughts on kibble fillers:
 
Corn is the worst.  It is hardest to digest, and can artificially inflate
the protein levels, especially corn gluten meal.  Gluten is high in
protein, but this protein is not readily accessible to ferrets.  Sukie
also points out that it raises the pee pH too much.
 
Wheat is more likely to cause allergies, at least in humans and some other
animals (as MC mentions).  I don't know if ferrets have this problem, but
it is something to watch out for.
 
Rice is the easiest to digest, and least likely to cause allergic
reactions.  These are the reasons that rice cereal is the first cereal
you give a human baby.  For the same reasons, I think it makes the best
filler for ferret food.
 
Of course the best filler is no filler, which makes recipes like Bob's
chicken gravy so beneficial.  Most people will not go through the hassle
and mess of making chicken gravy, or of feeding mice or other natural
foods, so it pays to pay attention to the fillers in your ferret's food!
 
Linda Iroff
Oberlin OH
[Posted in FML issue 2864]