You need a bigger spray rig with a pump. At work we modified a Gator with a 25 gallon tank and 50' hose with a trigger sprayer. It takes about 8 hours to fill, spray and refill 3 times.
If you can get out there in the Spring when the seedlings are just getting started you can use diquat (Reward) or gluphosinate (Lifeline). They're great for fast top-kill and the gluphosinate has a weak systemic action as well.
