Question
you say us roundup
there are many roundup herbicides
witch one will work the best on the locus trees

Answer
Roundup is a trade name. Monsanto chemical co. had the patent on it until a few years ago when the patent ran out an dother companies started make it under the name  glyphosate. Find the highest % content of glyphosate (check the label on the can/bottle) you can and use it. The off brands are just as effective and less expensive. The higher the % the less that you need to use.