Someone started a thread about Hemlock in 2007. I'll bump it up so you can read it.

If you don't get a response here, then I would be inclined to call the County extension or a livestock veterinarian in your area and ask them this question.

A link that might be useful:

Wood Choice for Horse Stalls
Is there any kind of wood that horses won't chew on?
July 18, 2008

www.woodweb.com/knowledge_base/
Wood_Choice_for_Horse_Stalls.html

I have 3" hemlock stall floors and wall dividers. It's very strong, not toxic and although most horses do chew it will take them a long time to chew through the dividers. The definitely won't chew on the floor! The rest of my barn is pine.

In the winter here I go out into the woods to pick hemlock boughs for the horses, donkeys and goats to eat. It gives them a nice little treat of green in the dead of winter. When I ride they try to nibble on the hemlock and other leaves of course! It will not hurt them.