The sense of touch is one of the most developed and important for the horse in terms of human interaction. The nose, lips, mouth, and possibly the ears are the most sensitive areas to touch. Although hooves do not respond to touching, various parts of the hoof are able to feel touch.
Understanding the degree to which horses are sensitive to touch can be valuable to the trainer. Horses can feel the slightest touch with their lips; therefore, it is important to develop a light touch on the reins and make sure the bridle fits correctly on the horse's head and the bit fits correctly in the mouth.
Knowing that a horse can feel a fly that lands on its back makes it even more obvious that the slightest shift of weight in the saddle can affect the way a horse moves. This acute sensitivity is why position is so important when the rider is asking the horse to perform specific maneuvers. Poor position, exaggerated movement, or excessive force are confusing to horses and result in poor performance. The sense of touch is also important in interaction between horses. Foals seek bodily contact with their dams. Mares respond to the touching behavior of their foals in various ways, including milk let-down in response to the nuzzling/suckling stimulus of foals.
From eXtension HorseQuest