There may be simple refusal to pull. This often comes from overloading, and especially when the animal has been sick or idle, and comes back to work with soft flabby muscles unequal to any violent exertion. After one or two ineffective efforts he sets himself back in the harness refusing to try again and the vice is started. Ordinary loads on bad roads full of holes from which it is impossible to drag the wheels have a similar effect. The danger is greater if the animal is naturally of a nervous or impatient disposition, and if he makes a desperate plunge forward and fails at once to move the load. Such a horse hitched with a slow steady mate is liable to have expended his effort before the latter has had time to join him in the pull, and it becomes impossible to move the load because the two cannot be started simultaneously. The conditions are aggravated if the driver is irritable and by voice and acts further excites the already too excitable animal.
Lesions of various kinds, such as shoulder bruises, abscesses, abrasions and callouses, saddle bruises, callouses, abscesses or fistulæ cause acute pain whenever the effort is made, and render the animal more impatient and indisposed to try again.
Too small a collar or one that fits badly (too narrow, uneven) has often a similar effect.
Among other causes may be named a hard bit harshly used, a sharp edge of the lower jaw bone where the bit rests in the interdental space, sores of the buccal mucous membrane in this situation, and caries or necrosis of the superficial layer of the bone. Also chaps, ulcers, or cancroid of the angle of the mouth.
Young horses, that are as yet imperfectly trained, are more readily driven to balk than old trained animals.
Mares are more subject to the vice than geldings, by reason apparently of a more nervous disposition, but much more because of the excitement to which they are subjected, under the periodic returns of heat.
Pench speaks of rare hereditary cases in which the habit is uncontrollable and the animal incurable.
Friedberger and Fröhner accuse chestnut and sorrel horses as being especially liable to balk.
However started the continued exercise of the act fixes it as an incurable habit a virtual psychosis. Yet the inclination of the animal, his likes and dislikes to a certain extent control its manifestations, thus a horse rarely balks in going home, and shows it mostly in going in the opposite direction, and above all on a new or unknown road.
The Symptoms vary greatly in different cases. One animal stands stock still propping his legs outward and absolutely refusing to budge. This may occur even in the stall when it is attempted to take the animal out. When on the road he is usually willing to turn and go back, but no persuasion by voice or whip can force him forward.