Fortunately, as a junior in high school, you don’t have to decide right now. Most universities that have great business programs will offer a range of focuses. And if you go in as a business major, you will probably have to take a wide variety of classes during your first 2 years. After you’ve taken those courses, you can decide how you want to finish your degree.
Unless you really fall in love with an ag or other specialized program, I highly recommend going to a university that has the best program you can get into, that you can afford without killing yourself with debt (which will nearly permanently exclude you from a career in horses), and that has nearby access to horses and training programs what you can lesson from or work with during the year or during your first two summers.
Then graduate and take the traditional career path. Don’t do horses. Spend 2-4 in a the professional world. Then, and only then, if you decide horses are the only thing for you, join the horse industry. The 2-4 years won’t make much of a difference, and the horse world could use more people coming from professional backgrounds. It’ll help you learn your worth, gain skills, and truly know what your other options are if/when you decide to leave the horse industry. Or, you may find that you love getting a paycheck so you can actually afford to ride and compete.