Hi midnight! congrats on your rescue
Our pup is 6 months and 60 pounds. I read the labels for both puppy food and adult dog food, and the only difference I can see is the protein %. I feed some meats and canned food, so I have already switched to adult food.
Now, people will say that there are nutrients in large breed puppy food that they need until 1 year, but I can't find where those are listed on the bag. Glucosamine is for joint growth, but you can give that as a supplement.