Jim,  I pulled up the label for Butyrac 200 (Butyrac 200... 2,4-D,B ... same thing), and it does not have wild parsnip listed as a weed that is controlled.  I agree - you will be safe to use Butyrac, but I am not sure if you will get results.  If you try it, I would use the highest labeled rate...... and maybe try only a small area at first.  It should only take a couple of days to see if it will work. You should know very quickly whether you want to do the whole pasture with it.... or look for some other method.

Just my thoughts.