What was she eating before? When the girls came from the breeder, they were eating Pedigree. When I switched them to Wellness, and when I have switched foods on other occasions, I've looked at the amount of calories and tried to provide about the same amount of calories with the new food for starters.
When you first switch, it's best to go slowly (try 3:1 ratio of old food to new for several days, then 1/2 and 1/2, and so on) - to prevent GI issues. However, if she has already switched and is doing fine, disregard that
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