Canary Grass

I believe that most of the grass in the field is Reed Canary Grass. I will quote from google: "Reed canary grass is a large, coarse grass that reaches 2 to 9 feet in height. Reed canary grass is difficult to eradicate; no single control method is universally applicable. In natural communities, mechanical control practices are recommended. Small, discrete patches may be covered by black plastic for at least one growing season; the bare spot can then be reseeded with native species. This method is not always effective and must be monitored because rhizomes can spread beyond the edge of the plastic."

It spreads by seeding and by rhizomes (roots). It is great for wetlands (loves wet feet) and withstands heavy grazing. In other words, it is ideal for the use the pasture has been put to in the past.