Changing hay on horses

The horses intestinal tract is very sensitive and the bacteria in the intestine (Microbiome) are important for digestion and the horses immune system.  When ever we change hay, for example from grass to alfalfa we change the type of bacteria in the intestinal tract.  Too quick a change can cause problems like colic and an interference of the horses immune status.  Therefore the change needs to be slow over 7 to 10 days to allow the intestinal bacteria to change to the new feed.