This does appear to be hedge mustard, which is an annual plant. Your first step needs to prevent this plant from going to seed, and I recommend mowing it right away. Hand-pulling is an option but it’s difficult and takes a lot of time. Earlier in the season, when the plant was in a younger growth stage, applying a broad-leaf herbicide would also be an option. “Management to Reduce Weed Problems” is a good resource to learn more about herbicides labeled for pastures.

In the long term you will want to re-establish perennial forage cover to this area to create competition. If the soil is bare, something will grow. If the soil is compacted or heavily overgrazed, unfortunately, it is often weeds that take root.

Introduction to Pasture and Grazing Management is a free online resource that you might find helpful.

There are references to potential medicinal uses in humans for this plant, but it is not listed in my resources as a toxic plant to livestock. I found one reference that says sheep and goats may browse this plant but aren't necessarily an effective control.