Here are some sites so you can check out different kibbles.
This one has a large food chart:
http://www.ohioferret.org/info/nutri_prn.asp
This one has lots of info:
http://members.tripod.com/ferret_luv/
click on food chart for the diagram
click on ferret nutrition for the explanation of ferret needs
(also check out their humor page for a laugh!)
 
The following is some information I've read on ferret kibble needs.
What ferrets require from their food:
Protein level of 32-38%  midrange being best
fat content of 20-22%
(as the ferret gets older, move to the lower ends of those above scales)
Ash content no more than 7%, the lower the better
Fiber content 3% or lower, the lower the better
 
Ingredients are listed in order of quantity, so the first three
ingredients are the most important.  The first ingredient should always
be a meat other than fish (ideally all first three ing. are meats).
Corn is hard for ferrets to digest and should not be one of the first
three ing., ideally it won't be in the food at all.
 
Risa
[Posted in FML issue 3945]