The amount to feed a dog varies with a whole plethora of factors: what food is fed, the body condition score of the animal, the dog's individual metabolism, and others.
As a generalization, you should feed about 90% of the amount the dog food recommends. This should be split into two meals a day. In addition, as a six month old puppy of a large breed, I would strongly recommend feeding a puppy formulation for large breed dogs. The bag will say to feed puppy food till the dog is 18 months old or so. But I would recommend feeding puppy food only till the dog is about 9-10 months old.
It is also wise to keep the dog on the lean side during the growth period (up till about a year old). In large dogs that are overweight, they have a much higher rate of becoming clinical with hip dysplasia. So keep her lean.
I am a believer in premium foods as opposed to OTC foods. Three brands stand out as having very good quality control, consistency of ingredients, quality of ingredients, digestibility, and research behind them, etc. There are certainly other premium brands, but these have the longest track record and best reputations for research and development. Those are IAMS, Eukanuba, and Science Diet.
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