The 2 big things against Proplan Turkey and Barley are low protein (31%)
and low fat (14%).  Have you tried Hill's Prescription Diet feline d/d or
Eukanuba's response feline lamb and barley?  The d/d canned food has a dry
matter protein of 39% and a fat 28% level.  It is a lamb and rice product.
The Eukanuba can lamb and barley has a dry matter protein level of 41% and
a fat level of 27%.  Both of these are closer to what you want for a ferret
food without having chicken or poultry as the protein source.
 
Food for thought,
Jerry Murray, DVM
[Posted in FML issue 3420]