TB horse changed from hay/nuts to gr yesterday, now has diarrhoea?

try and limit his intake of gr. allow him to graze only a few times a day and gradually increase his pasture time. call and see if you can get the name of the hay he was eating at his previous stable and buy some so you can wean him down. horses have very sensitive stomachs and any change in feed is likely to cause upset. also I would do lots of reading up on caring for ex race horses. a lot of things with them are different than with most pleasure riding. Pulling back on the reins which usually means stop to my horses will increase speed on a race horse. Do lots of research on rehabilitating ex race horses. also remember that most racehorses are given tons of steroids and performance drugs and will take a while to detox. I would recommend getting a vet out there to do an initial health check. good luck and read up;)