When I had a kennel of 12+ gundogs I used the 6 day rule! I fed raw green bulls belly tripe, when I could get it, (Then mad cow disease played its hand and I went off sale, and I don`t mean the wife!) I`d keep the bellies in a barrel of water and slice it off as I fed it. On the 7th day it was rib bones only. It chalked up the poo, which in turn emptied the annul ducts.
A lot of dogs these days have kidney problems due to the gut becoming lazy, after all the canine is a scavenger and its intestine is geared to extract all of what it consumes. Or so my rural vet informed me.
The dogs coats shone like a clear wine glass and the stamina for a days work was a given.
Spaniel