This looks to be a young Japanese Knotweed or closely related species, a very invasive weed. As you noted, it will spread from a running, underground root system. While the plant isn't toxic to livestock, it can really get going and become a real problem. At this stage, I would dig and remove every shoot you see as often as you can, every day if possible. The object is to not let it make sugars that turn into more leaves and stems. With no leaves, the plant can't make sugars from sunlight. But be aware, the plant is tricky! It can send root "runners" out many feet from a given plant with new shoots poking up where you least expect them. Email me if you have questions and to let me know how well control is working out.  

How do you think it got there? It isn't started from seed but from root fragments of an existing plant.  Chip

Chip Bubl   OSU Extension/Columbia County  <personal data hidden>