It is very dangerous to feed a rabbit new food suddenly. A responsible breeder will provide you with transitional food upon sale.
If you wish to transition the feed, the first night you have your rabbit feed them 100% of their old feed. Mix the rest with the new feed at a 50/50 ratio until all used up, then fully transition to the new feed.
Feed a high quality pellet from a feed store like Tractor Supply Co. Don’t use feed from a pet store as they often contain unhealthy fillers like corn.
Purina Show is what I use with all of my rabbits.
Up until 6 months: NO TREATS. I know it’s hard because they are so cute, but don’t do it. Their digestive systems are not used to it and they can die.
Up until 1 year: free feed pellets and hay.
After 1 year: 10oz of pellets per day.
Throughout their life: unlimited Timothy hay from a feed store like Tractor Supply Co. A good guideline is to put the same amount of hay in with the rabbit as compared to how big they are.
Unlimited water in a ceramic (not plastic) crock or Lixit hanging water bottle. Putting apple cider vinegar in the water aids digestion and can be administered as 1 tablespoon per gallon.