If you get a Roundup type weed killer - Glycophosphate based - you can spray once when the weeds are growing strongly (not now) and then give a second spray 2 weeks later. This kills everything!!
It also damages the soil, worms, etc. so don't do it too often. Check the internet for more information on this.
Roundup has its uses but be careful.
Also, the agricultural product is a lot more concentrated than the domestic product. I would test a small area according to the instructions that come with the pack to check the concentration is correct. Do this carefully in order to get the most effective concentration for your land. If you leave it too concentrated, you will need more of the weed killer and may do more damage to the soil. However, if too dilute, it will not kill all the weeds and you will have to re-do the spraying later.
If you have brambles on the land, you may want to use something like SBK rather than Roundup.
NN