Good idea either way I would keep horses away from the contaminated area until it's there isn't even a hint of it ever being there. Maybe run a temporary line around the area?
Pull up a plant and take a picture of it against a contrasting background. It's hard to say from these pictures as so many plants are together it's hard to be sure the leaves etc. are from the correct plant.
Some look like they could be ragwort but a better picture would be a help.
Some is, some isn't. In the first pic, there is some ragwort on the left, but the stuff on the right isn't(looks like birds foot or goldenrod but hard to tell as it isn't in focus). In the lower pictures, it is ragwort.
ETA: It actually looks a bit like oilseed rape in the top photograph.