I would put my energy into making a secure area for my dog when I can't be right there to supervise.
It does not need to be 3/4 of an acre. It can be 10'x20' inside your fenced yard. It has to be impossible for climbing out of or jumping out of and impossible to dig out of. Then your dog will be safe from a lot of things, including the cow.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.