I end up asking the vet this every year! I'm always told to just ride lightly, and not ask them to collect too much. Any of the horses in the lesson program who have to work that night (they all get done at once) get some bute for any stiffness, and the instructors take it easy on them that night. If your horse tends to get too stiff and ouchy from his shots ask your vet if you can give 1/2 tablet of bute, depending on size.
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RIP Georgie~~Spotless Appaloosa
RIP Zenobia~~Dutch Warmblood