I have a 2-year-old Quarter Horse gelding who has been in training for three months. When should I start giving him a joint supplement? Also, when should I begin giving him a vitamin supplement?

Well, to be honest, your question supposes that you need to be giving him such things at all. Horses were made to be able to satisfy all of their own vitamin requirements, assuming that they've got access to good feed. Many of the available supplements actually contain very little nutrients compared to the horse's daily requirements. So as long as you're feeding him appropriately for his age and exercise requirements, vitamin supplements are likely to be unnecessary.

As for joint supplements, that too is a big gray area. In fact, there's little to indicate that such products are effective in horses, although they certainly don't cause any harm. There's certainly no indication that such products prevent joint problems–the best way to do that is with appropriate exercise schedules and good care. Your veterinarian should be able to help you determine if your horse needs either of these products, and if so which ones to buy.