Oesophageal obstruction or “choke” is caused by food getting stuck in the oesophagus.
Since horses are unable to vomit or belch (soft palate), they may suffocate, and urgent attention from a veterinary surgeon is vital.
The causes are very often due to feeding at irregular times which gives rise to “greedy” behaviour in horses when feeding, with insufficient chewing and, hence, poor salivation.
This is also observed with horses fed outside the stable and in rivalry phenomena between the “dominant and dominated” in which the dominated animal, anxious that it will not be allowed access to its ration, will guzzle its food. These problems also arise in the event of rapid transitions from water-rich rations (pastures) to dry rations (feeds); the best solution in this case is to provide hay before feeds so as to encourage your horse to chew its food and, hence, to salivate properly.
Nervous horses are more prone to these problems.
This phenomenon is exacerbated by a lack of water but also by inadequate functioning of the salivary glands and poor teeth (an annual visit by the horse dentist is essential).
A visit by the veterinary surgeon will enable the latter points to be investigated.

The size and hardness of pellets are not to blame in this situation since horses’ teeth are efficient enough to grind conventional pellets.
It is true that flaked feeds have an advantage over pellets since they allow horses to chew their food for a longer period of time. 
ROYAL HORSE has formulated its feeds avoiding certain raw materials which tend to decrease the problem.