How do I take possession of a neighbors cow?

You need to contact the local law enforcement first. It is, more or less, found property. They will take possession of it, or ask you to keep it where it is, and then they will contact the owner, if possible. If the owner doesn't respond or cannot be found, there probably is a method for you to claim the cow. It just takes time. Don't turn it into steaks yet. If you do, you may be responsible for the value of the cow and if it is a breeder, any offspring it may have had over the period of it's life. If you just want him to come get it, send him a bill for pasturage. That will get his attention.