Dogs love mud, and they end up soiling their collars, as well as themselves.
Photo by Jody, Stevenson

No matter how clean your dog stays or how fresh his coat, the collar he wears will eventually absorb enough skin oils, dirt, and grime to develop an odor. Dogs who spend a lot of time outdoors rolling in the mud, swimming in lakes and streams, chasing balls, or playing at the dog park are more prone to collar funk than the small lap dog who rarely ventures out and takes a weekly trip to the doggie salon. But eventually, all collars will need to be washed to keep them smelling nice—and to prevent unhygienic bacteria buildup.