Your horse’s manure can give you clues as to his overall condition. Droppings should be in firm, well-formed balls. Very loose droppings or low amounts of droppings are a cause for immediate concern and should prompt a call to your vet.
Horses should pass droppings every 1-2 hours so if your horse suddenly starts to pass droppings at a lower frequency than this, it is worth contacting your vet to discuss the issue. If your horse seems otherwise well it may not be a cause for concern, but always discuss with your vet before adopting a ‘wait-and-see’ approach.
It is true that very loose droppings can also be caused by stress or a change in diet, particularly access to richer grass or a change to wetter food or hay, but it is still worth discussing the issue with your vet. If your horse is otherwise well they may just suggest that you monitor the situation, but it is always better to call them initially to get their take on what is going on.