Sheep are not twig browsers like goats but ...... I've always been told its a bad idea to mix any grazing animal in an orchid. Why take a chance I'd say.
Mike

The folks from whom we got our sheep had them in an orchard of dwarf apples. They had covered the trunks of the trees with hardware cloth but the low hanging branches were trimmed up to sheep-head height. Overall the whole system looked good, and we plan to replicate it once our trees get a little taller. I doubt they'd decimate the trees unless they ran out of grass. My advice would be to get a small (short) breed of sheep (Shetlands? Jacobs?) and add chickens to the mix to keep down the flies, sheep parasites, and tree fruit borers!

Enjoy!

Also, you might try asking this question over on the farm life forum. There are quite a few shepherds over there. There will probably be at least one grumpy person who runs you over because it sounds like you care more about the orchard than the sheep, and you shouldn't run out and buy an animal without long consideration, blah blah blah, but if you can ignore them you'll get some good advice from the rest who know what you're talking about.

Katey

And you might find out what you could use to improve the pasturage.

Sue

Have you considered weeder geese?? They do a really good job in orchards, are lower maintainance than sheep and for sure won't eat your trees.
We have sheep and I adore them! If you do get sheep or geese for that matter, why don't you consider getting a heritage breed that also could use a little protecting? The American Livestock Breeds Conservancy has a list of livestock animals and poultry/ waterfowl, etc that are heritage breeds that are in low numbers. The heritage breeds tend to be more sturdy and need less intensive parenting (from you). We have 3 types of heritage chickens, turkeys to follow soon and CVM Romeldale sheep. You would also be helping to maintain biodiversity in our livestock populations. Just a thought.

Here is a link that might be useful: American Livestock Breeds Conservancy

I second geese. You would be AMAZED at how efficient they are to keep a pasture down. I once read 40 per acre- I would say 10-15 would be more than enough.
They're cheap, hilarious, and so much easier than sheep.