Mo. Rev. Stat. § 270.060: Lawful fence not necessary.

It shall not be necessary for any person to fence against any of the species of domestic animals enumerated in this chapter, and it shall be no defense to any action or proceeding brought or had, that the party taking up such stock did not have his lands enclosed with a lawful fence, but nothing herein contained shall be construed to lessen or interfere with the obligations of the several railroads in this state to fence the right-of-way of such railroads, as is now provided by law.