I've looked into this for fruit trees, but don't have any practical
experience so far. Three breeds that seem suitable for the job are:
1. Shetland,
2. Soay, and
3. Olde English Babydoll Southdown
The Babydoll sheep are quite expensive right now since they have a cute name and are being bought up to keep as pets. The Soay sheep are supposed to be ridiculously hardy as they come from an island of feral sheep, but they are somewhat uncommon since they were only brought to the states a couple decades ago.
Shetlands are easier to get and relatively inexpensive, so my plan is to try growing a pair in the orchard for a season as a learning experience.