Feeding a treat or two to your horse may seem like a great way to show your horse you are a friendly and loving owner. However, before you hand over the peppermint flavored alfalfa cube, think about whether the horse's behavior warrants the positive reinforcement.
A good education program for any animal (including humans) follows a similar pattern: realistic goals, consistent approach in asking for the achievement of those goals, and positive reinforcement when the goals have been met. Whenever you are working with a horse, you are communicating with them what you consider acceptable social behavior. Giving positive reinforcement in the form of treats at inconsistent moments while working and being with them may cause the horse confusion, and possible problems in the future, especially when unintentionally rewarding bad behavior (ex: pushing on you resulting in getting fed a treat). Be aware what you are communicating to your horse, and what their response was to that cue before you decide to feed a treat or not.
Treats should also be kept in their proper niche in the training scheme: as a positive REINFORCEMENT to a previously given cue that the horse responded in the desired manner towards. Therefore, the treat should not negate the need for an initial cue. Holding out an apple tauntingly in front of a horse so that they may approach a trailer destroys the training process. There is a notion of a goal, but it is not been divided into realistic steps (if the horse walks towards the trailer following the apple - great! Let's see if he will walk INTO the trailer as well before he gets the apple). There is no initial cue besides holding out a treat to follow, and therefore no connection with affirmation to desired behavior if the horse responds correctly. Instead, when approaching a task, think about the end goal, and the steps the task might be divided into. Decide on a consistent cue that you will be giving the horse to achieve those steps, and be prepared to give positive reinforcement when those steps are completed in a desirable fashion. To use the trailering example again: you decide the end goal is to get all 4 feet into the trailer. You decide approaching the trailer is the first step, each foot in the trailer is a separate step, with four feet in the trailer as the final step, and have decided to unload and re-approach the trailer between each step. You give a treat as a positive reinforcement after each successfully completed step, and keep your cues consistent throughout the task. With this approach, the horse correlates a treat with responding in a certain way to your cues, rather than a treat with something he may or may not get by walking forward after it.
Libby Keenan, co-author of TacknTalk Blog sees feeding treats as positive reinforcement to cues as able to "help build a conscience [in the horse]" as ultimately they will gain an understanding of what is desirable behavior and what is undesirable behavior. Ie: the horse will start to "know the rules." "They will work very hard for a treat...but some days they cannot resist the naughty behavior and do not even come looking for a treat as they know what the deal was." Additionally, riders who have used Clicker Training for their horses which works on the premise of a *click* and a treat for positive reinforcement for desirable behavior note good manners around food, and a general understanding of why treats would be given.
On a final note, I do not believe that treats are the only positive reinforcement that can be used in horse training: a pet, a verbal affirmation, or a release of the asking cue, could all be used effectively. The important thing to remember is a positive reinforcement is a necessary step in the training process, and leads to trust and enthusiasm for work. What form that positive reinforcement takes is up to you - and may vary from horse to horse. Looking at various training methodologies and see how the goal --> cue --> horse's response --> affirmation process is managed, and see if it works for you.
Have fun and happy horse training!!