Well here is what I am hoping: that you have yet to draw up the lease and that you address the matter in the lease from the get go.  Make the issue part of the rider to the lease agreement so that it is clear cut from the beginning.  I think that clearly the neighbor would be responsible for damages that result to your property from the cows but I can see the argument from the other end that the situation could have been anticipated.  So if you have not already signed the lease put in these provisions.  If you have then seek help from an attoreny to interpret the lease agreement that you already have in effect.  Good luck.