josephine said:
They say that changing brands all the time is not good for the dogs...
who are "they"? also, "not good" is a very vague statement.
it is true that some dogs have a little more trouble adjusting to a new food than others, but taking that into consideration you can just make the switch more slowly if you have to. generally 3-5 days is a good time frame, but some dogs need a little longer. some switch "cold turkey" without a problem at all.
as far as nutrients go, if you feed commercial food exclusively, it is much better for the dog to rotate between several high quality brands rather than to feed the dog the same food all his or her life. products are formulated differently and feeding the same thing all the time, without any variety, automatically makes the body accumulate too much of one thing but not enough of another.
the claim that all foods are "complete and balanced" is absolutely ridiculous. requirements differ depending on age, health status, genetics and many other factors, and commercial dog foods are not designed with the individual dog in mind. they cover the "averages", but for many dogs that is not good enough. if it was, we'd all have perfectly healthy dogs that live to 30 years old.
last but not least - you don't eat the exact same thing two or three times daily, every day. why would you expect your dog to do that - a carnivorous animal that has evolved as a scavenger, eating a wide variety of foods.