Common ragwort (Senecio jacobaea), has a very poor image listed as an injurious weed it is poisonous and although it is not illegal to have it growing on your land it is illegal to allow it to spread to agricultural land, particularly grazing areas or land which is used to produce fodder.
Unfortunately a lot of people see ragwort and pull it up wherever it is even when there is no chance if it reaching farmers’ fields this is a shame as ragwort has significant conservation benefits.
When a ragwort plant is growing its poisons are there to give it a vile taste so it is protected from being eaten. So if growing in a field animals will leave it well alone unless they are starving and there is nothing else to eat. The problem is if it is cut and wilts it loses its bad taste and animals can eat it and get ill or die very dangerous if it is cut and incorporated into hay or silage.
So when you drive down the motor way and see workmen pulling ragwort they are doing it to comply with the law and stop it spreading onto adjacent farm land.
Over the millennia very special relationships have developed between Ragwort and a number of insects. The best known of these is the Cinnabar moth whose caterpillars have evolved a special way of storing the plants poison in its body as they eat it. This makes the caterpillar taste vile and protects it from being eaten; another two insects have done the same. There are even two species of parasitic wasp that have evolved to lay their eggs only in these insects, so effectively 5 species of insect are totally dependent on ragwort.
In a more general way ragwort is a great nectar plant and if you watch the flowers they will be covered in all sorts of insects feeding. Its vivid yellow is also a great addition to the colour pallet of the countryside.
So if you spot some beastly ragwort growing somewhere before you pull it up consider its beauty and its value in the ecosystem and if there are no fields nearby maybe just leave it to be and let the bugs enjoy it.