Hay testing can be done at many laboratories in the Midwest. If there are specific test you want done on hay for your miniature donkey, I would suggest you view their websites first. 
Here are some more "local" labs that do forage testing:
Dairyland Labs - Battle Creek, MI https://dairylandlabs.com/locations
A & L Great Lakes Lab - Fort Wayne, IN https://algreatlakes.com/
For equine specific:
Equi-Analytical Laboratory https://equi-analytical.com/

Thank you,