Horse nettle is hard to get rid of in one spraying, as it has extremely long and deep roots, and the roots is where it stores it's energy. Spray at the wrong time, or at too high rate as CB alluded to, can be a waste of time and chemical. Use too strong of a concentration or too slow travel speed even with the max recommended mix rate, and you burn the leaves off the plant before it has time to absorb the chemical into the root system and that applies to most weeds and brush. Looks like it's dead, but it will be right back. I didn't have nettle here until after 2011 when I got some hay in here that I didn't know the origin of.
The roots--the plant reproduces by seed pods, but spreads every growing season thru the roots. Sends a deep tap root down that's easily broken off by pulling the plant up, but that won't kill it. Roots also radiate out a foot or so, then send shoots up to form new (additional) berry producing plants, which in turn also send their roots out.
Scroll down to pg 32 in this old article and you can see one plant's extensive root system.
http://www.newss.org/ne42/ne_42_horsenettle.pdf
Spray it just as it begins to bloom. If you wait till berries or seed pods form and set you may knock the above ground plant back for this year, but not kill it's root system or even hurt it.
The 1qt Remedy-One qt 2,4d per 100 gal water CB mentioned will sure kill most everything I have too, including croton, sweetgum, thistle, and tallow.
I have both a 300 gal home built spray trailer and a 35 gal atv elec pump sprayer that I bought from tractor supply. Had a little old 4' wide scratch harrow that I didn't use and stripped it down to the frame and made a mount for the 35gal elec pump sprayer for using behind my small tractor when I didn't want to use the big sprayer. It's ugly as sin, but works. Takes 2 minutes to take the tank off the 4 wheeler and mount it on the 3 pt frame. Boomless nozzles like the link in the other post shows.