Perennials like nettle and especially bull nettle can be a tough kill because of the large root. I dug up a bull nettle just to see how large the root was. It was 37 inches long and 4 inches in diameter. Had to use the tractor to pull it out of the ground.
We use a spot spray for the bull nettle because we don't have that many in the fields. I mix 1 to 1.5 ounces per gallon in the hand sprayer and spray all leaves thoroughly. Even with that large root, we've had very good success. Probably a 75% or better kill rate. By next year we should have them pretty much eradicated from our fields and the edges of the fields.
For the tank in field spraying, mix rate is 1 qt per acre. Spray rate depends on your rig. Use a good surfactant. DO NOT USE SOAP. The tank doesn't spray them as thoroughly as spot spraying, but it still puts the hurt on them. Using a tank rig, may take a couple of apps during the year and a couple of years to eradicate them, but worth it! They are bad news.
Don't remember the cost on the GrazonNext. It has been worth every penny. Its been very effective. Lots of nettle, berry vines, goat weed, sour (curly) dock, and many other weeds, perennial and annual, are history on our place.
GrazonNext will also take out small trees about 2 ft tall. We have a problem with persimmon sprouts and its effective on them. I've even used it as a spot spray on persimmons that were 3 & 4 ft tall. Some took a second application, but it did take them out.
Spraying is by far the most cost effective weed control, can say that through personal experience.
Be sure to check with your seller. There is a concentrate coming out that the mix rate is lower, but of course price is higher. GrazonNext is a restricted use chemical in Tx and you have to have a license to buy it. Its not hard to get a license and it doesn't cost much either.