Legal Question in Real Estate Law in California

I think the question, as asked, is a trifle short on background facts for us to give a reliable answer. As a cautious lawyer, I'd want to ask about the understanding of the parties, the nature of the pasture-sharing agreement, the wording of the rental agreement, the availability of other access routes, and whether there are other complicating issues, before telling you to break the lock and feed your horses.

The problem is that your sharing of the pasture and your need to enter to care for your horses is an issue that should have been discussed and decided at the time the rental agreement was negotiated. A landlord certainly can reserve access rights for various purposes when renting out property, but on the other hand, a tenant has a right to lock up his rented property.

If there is an emergency, I'd first attempt to contact the tenant, and if that fails, break the lock, but there is no guarantee this won't result in some further issues with the tenant.