I agree with JTK (though perhaps without all the shouting
)
Commercial pet foods are terrible for your pets. The industry is completely without regulation.
Some of the contents of pet food include:
euthanized pets
meat from the supermarket that has passed it's best before date (they don't take the time to remove the styrofoam and cellophane)
horses that have been "put to sleep" (it isn't glue factory that gets them these days)
Farm animals that are not safe enough for the human food supply (i.e. a sick cow -- including all of the medicine that was in their system)
etc.
It really is crap.
I started looking into this years ago when I got my dog. He is a Dane/Lab cross. Dane are known for their short life span -- 9 years on average. The stats show that this is only true in North America. In Europe where there is more of a tradition of feeding table scraps the Danes were living to 15 on average.
Further tests were done and it was discovered that diet had everything to do with it.
My dog eats only table scaps or Organic dry food (HundenFlocken or Fromms are the two we suppliment with they are made from human grade food). The dog eats better than I do.
The result is amazing. I have never had a dog with whiter teeth (the vet frequently comments with surprise that he doesn't need scalling) and better coat.
He is going on eight-years-old and while he getting older he is still quite youthful and vigourous.
As for cat food... most is high in ash. I had to put two cats to sleep before I learned that lesson. High ash = bladder problems.