Welcome aboard and thank you for adopting your 5 year old pup~ Pics are necessary. Taste of the Wild is good food but not all foods will benefit every pup. The reset is great to get her system back on track. Add 1 tablespoon of pumpkin (not the pie filling) to the boiled white rice and boiled meat. Usually 1 cup of boiled rice with 1 cup of meat + 1 tablespoon of pumpkin for 4-5 days will reset the system so you start the new food that you choose to feed your pup. If you do not go the reset way, then do a slow transition to the new food (see below).
Check out Acana Regionals, Wellness Core, Earthborn Holistics, Fromm*s, Innova Natures Table or Taste of the Wild grain free. Chewy.com is a great place to order dog food and they offer free shipping.
Slow transition to the new food is as follows to prevent diarrhea. If at any time during the transition, the pup has diarrhea return to previous amounts of food per feeding. If you are switching flavors made by the same manufacturer, you should not have to do a slow transition.
Amount per feeding:
Day 1-4 ¾ cup of old food and ¼ cup of new food.
Day 5-9 ½ cup of old food and ½ cup of new food.
Day 10-14 ¾ cup of new food and ¼ cup of old food
Day 15 Start 100% of new food
Generally, mastiffs are allergic to grain and chicken found in kibble. You can check
www.dogfoodadvisor.com for dog food ratings and customer feedback. Mastiff puppies should eat Large Breed adult food because puppy food has too much calcium which causes fast growth. Slow and steady growth for a healthy mastiff. Protein in the food is not an issue unless the pup has kidney issues.
http://www.peteducation.com/article.cfm?c=2+1659&aid=652
Dog Food FAQs: Protein
You may want to check out the raw diet for your pup. Raw diet can be fed to young pups. Check out the sub forum on raw diet that has a wealth of information. It is not recommended to feed the pup kibble (one with grains) and raw food. Do your research re: raw diet and form your own opinion.