When a dog advances to a new stage of her life, or her nutritional needs change, you probably need to rotate her food. Otherwise, changing her menu just for the sake of introducing variety isn't necessarily a good idea. When you do opt to switch her from one brand or one type to another, you also have to be sure you do it the proper way. Otherwise, the digestive problems that may occur during a switch are more likely to become an issue, compromising both your dog's nutrition and her overall health.
Puppies to Dogs
All puppies have to transition to dog food sooner or later. Puppy food is loaded with extra nutrients and calories that a rapidly growing pup needs, but once she physically matures, eating that same old puppy food can lead to obesity and other health problems. Because different breeds mature at different ages, you'll have to defer to your vet's judgement regarding the proper transition time. For example, a dog that maxes out at around 20 pounds may be physically mature and transitioning to dog food at 9 months of age, while a dog that will weigh over 50 pounds may be on puppy food for as long as two years.
Taking Professional Advice
Dogs may need to rotate their food based on changing health needs. For example, if your veterinarian determines your dog is overweight, he may recommend switching to a lower-calorie food formulated for indoor dogs. Senior dogs may also have to switch food as they age, as chewing hard, crunchy kibbles can prove uncomfortable. Switching to wet food, then, makes mealtime less of a chore. If you suspect your dog's health would benefit from changing her food, consult your veterinarian.
No Good Reason
You don't need to rotate your dog's food on a regular basis -- in fact, you shouldn't. While you would get bored eating the same thing every day, dogs benefit from the consistency, and upsetting her eating habits could also upset her constitution. Her body is used to receiving and breaking down the same product every day, and if you switch her to something else, it could cause some intestinal distress and diarrhea. When it comes to her diet, if it isn't broke, don't fix it.
How to Rotate
If and when you do rotate your dog's food, you should do so gradually. An overnight switch is too much change, too fast, and it increases the likelihood of upsetting your dog's stomach. Instead, introduce the new food gradually by mixing it in with the old food. Over the course of a few days, feed your dog a mix of the old and new food. Every day, increase the amount of new food and decrease the amount of old food -- after a week or so, she should be eating just the new food.