When transitioning your horse to new feedstuff – either to a new batch of hay, new pasture or new concentrates – you need to do so slowly to give bacteria in the horse’s digestive tract a chance to adapt to the new feed.Bacterial populations change according to what the horse is eating, and time is required for different bacterial species to colonize the horse’s digestive tract in this new environment. If there is not enough time for the population to adapt, it can cause digestive disturbances, which can lead to colic.