glyphosate

It's absorbed through the leaves, so if you cut them all off,  the weeds won't absorb any!

Definately do not cut it down.

Follow the instructions on the bottle and you will get good results. Especially important is the 6 hours of dry after application.

A good way to apply glyphosate is with a watering can and dribble bar. These are available at your local garden centre and avoid drifting fine droplets that you get from a sprayer on to other crops.

After about 10 days the glyphosate will have penetrated right into the roots and you can cultivate whether the plants are showing yellowing symptoms or not.

If more green leaves appear after you have sprayed, you have either missed a patch, or there were weeds that had no green leaves at the time of spraying. They will need a second application.