I would not recommend using Purina. It's not very high-quality food. Here's my advice for choosing a dog food:

Read the ingredients on the food you buy. Go with a high quality dog food. A grain should not be in the first couple ingredients ingredient (corn and such are mainly fillers, dogs don't digest it well). Avoid foods that have a lot of "by products" listed.

Here is an article about byproducts:
http://www.dogfoodproject.com/index.php?page=ingrd

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Some GOOD foods are :
* Merrick - http://www.merrickpetcare.com/
* Solid Gold - http://www.solidgoldhealth.com/
* Canidae - http://www.canidae.com/
* Timberwolf - http://timberwolforganics.com/
* Orijen - http://www.championpetfoods.com/orijen/orijen/
* Wellness - http://www.omhpet.com/wellness/
* Chicken Soup brand - http://www.chickensoupforthepetloverssoul.com/
* Innova - http://www.naturapet.com/brands/innova.asp
* Innova EVO - http://www.naturapet.com/brands/evo.asp

Or check this website for good foods: http://www.dogfoodanalysis.com/dog_food_reviews/
(I recommend only feeding foods rated 4, 5, or 6 stars. Anything 3 stars or less, I would stay away from.)

NOTE: It may seem like quality foods cost more, but in the end, it evens out. With a high-quality food, you don't have as many fillers. Also, with a high-quality food, your dog will eat less and poop less. So while that bag of higher-quality dog food looks expensive, your monthly feeding bills won't increase much, if at all.

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Beware "premium" foods. "Premium" does not mean good nutritionally, and is not a nutritionally high quality food. It has the same types of ingredients as grocery store foods, just a bit better quality of those not-so-good ingredients. (Premium foods are those like Iams, Eukanuba, Science Diet, etc..)

Another thing to be wary of: A lot of vets will recommend what they sell in their office. They get profit from the brands they keep on their shelves, that's why they push it. Truth is, vet schools don't focus a lot on nutrition. It's not saying that a vet is a bad vet because he recommends those foods, a lot of vets just are told "this is good food", so they pass the message along without proper nutrition knowledge. Also, some dog food brands (like Hills) support vet schools, so vets have heard of it from the time they start college, which makes them think it's good as well.

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When switching foods, do it slowly. I do this over about a two week timespan:
25% food A, 75% food B
50% food A, 50% food B
75% food A, 25% food B
100% food A