Be careful!
http://www.acronymfinder.com/lists about a dozen means of IIRC.
Returning to ragwort. I have a 12 acre field which I use to grow plants and tree seedlings. Part of it is still as it was when I bought it. It had been set-aside land for several years and was covered in rough grass and weeds many of which were thistles.
I had it deep ploughed in order to break up and compressed layer, to bury the weeds and give me a clean sheet to start working on.
The result was an impressive crop of thistles and poppies which I have reduced over 3 years with a topper and a chain harrow.
I know have some weeds growing but much less in the way of thistles. BUT, in the meantime I have attracted a lot of ragwort plants.
In the early Spring (late March - early April), these appear as flat rosettes which grow into small mounds making them very obvious against the partly bare earth. So I go round with a back pack sprayer and a 10% solution of Roundup. I CAREFULLY spray each plant and repeat after 2 weeks. This knocks out about 90% of the ragwort. However, the other weeds and grass are now growing (late April) and the mounds of the ragwort plants are harder to spot. Then they start to spring up as taller weeds preparatory to flowering, so I repeat the spraying.
The remaining ragworts continue to grow and try to flower until late October so the spraying has to continue until then if it is to be effective.
I have found that pulling up a semi-mature plant leaves its roots - or enough of them - to produce 4-5 plants where there was initially one. This also applies to using a plough or disc harrow. Removing the flowers stops them seeding - temporarily. They will grow new flowers or regrow the next year.
I don't like using Roundup but it is effective. I only spray the ragworts and try hard to avoid any other plants, even weeds as the killed plants can poison worms and other wildlife which contribute to a good balanced soil culture.
I have seen this web site:-
http://www.barrier-biotech.com/ragwort.phppromoting something called Barrier H. It might be worth trying if you want a more specific poison, particularly if your fields are used to house horses.
Good luck