[Stereo]

Don't touch them until July. Then whack them or pull them. You want a flower but not a seed head. If you top them earlier they will come back stronger. If you pull them earlier you will leave roots in the ground. The reason for topping in July is that the plant has put all it's energy into the flower and if you lop that off, it will probably die. It may take a good few years to clear a field but it is possible. Spraying is just going to kill your soil, whatever Monsanto tells you.

[cotswoldfarmer]

You will always have a seed burden in the soil. Disturbing the soil will make the problem worse. I understand that people are trying to do things the eco way which I understand. Sometime you do need to help things along. You can get sprays such as Thistle X and Dockstar which do exactly what there name say. So do not kill any pasture or wildflower and such like. Spraying will not kill your soil unless you are applying at the wrong rate.

[Bramblecot]

We cut a rough area in early July  and baled it for bedding.  The thistles have re-grown but many of them are pale, almost yellow, at the tops.  It is quite noticeable even when viewed from a distance.  We have not sprayed with any chemicals.  Any idea what is happening to them?  The nettles and docks do not seem to be affected.