In this article from Kentucky Equine Research they look at studies that show using food rewards are helpful in training horses.
Rewarding horses with food may keep horses more interested in the trainer and the lesson that is being taught, according to research conducted at the University of Rennes in France. Anticipating the positive reinforcement of a treat seems to motivate the horses to pay attention while the training is being conducted. Because attention is a key element in learning, anything that keeps a horse focused on the task can be expected to increase training performance.
Previous studies have indicated that positive reinforcement, particularly in the form of a food reward, leads to faster learning by horses as well as training results that last longer when compared to negative reinforcement (rewarding by taking away pressure). Scratching the horse’s withers as a reward was also found to be less of a motivation than giving a food reward.
In a study using 30 yearling and two-year-old horses that had only minimal previous training or contact with humans, the researchers divided the equines into three training groups on the basis of reward (food, wither-scratching, and no reward). The horses were taught to stand still in response to a vocal command for up to 60 seconds. During this time, the trainers walked away from or around the horse.
Members of the team monitored signs of attention while the horses were being trained, such as where they were looking, where they were pointing their ears, and how they curved their necks. They also watched the way the horses interacted with the trainers by smelling them and touching them with their muzzles.
Horses in the food-reward group spent the most time smelling, nuzzling, looking at the trainers, keeping one or both ears turned toward the trainers, and curving their necks toward the trainers. Horses that were rewarded with wither scratching or were not rewarded were more distracted by their environment, paying more attention to other sights and sounds away from the trainer. Horses in these two groups showed the same level of distraction, indicating they did not consider wither scratching to be a significant reward.
The researchers commented that the horses learned they would get the food reward if they completed the task of standing rather than if they searched for food held by the trainer. This was seen as constituting significant restraint, a behavior that represented solid learning.