One proven method to get rid of the sumac trees (ailanthus altissima-tree of heven) is to do a Low Volume Basal Spray.
You will have to buy Garlon 4 or Crossbow(herbicides labled for this use) And mix it with an oil carrier (diesel fuel) according to label rates.
It will be applied to the entire lower 12-18 inches of each plant you want to treat... wet the bark surface only....Not to the point of runoff. It will be absorbed by the plant and kill them off slowly.
Now is a good time to treat and get any new sprouts next spring.
I am not clear on how tall the sumac is... and how extensive is the problem. Gopher is right, sumac spreads by underground rhizomes and runners.
Myself, I suggest knock it down with a bobcat or cut it all off with loppers and a saw. Then plan to spray it frequently in spring when the new sprouts come up up from the roots. If possible, get the area smooth enough to mow it weekly. Try to spray it with glyphosate, a three-way, a brush killer with triclopyr or chlorpyarlid. when it sprouts from the roots in spring. Try to never let it get taller than 6 inches. It takes a few treatments to exhaust the ability of the new roots to produce new sprouts from storage reserves,
This is a problem...IF...part of the patch of sumac is on the next property across the fence. It will keep spreading from across the fence into your yard. Not much of a problem if it is mowed regularly, or can be sprayed, but if it is a situation where the sumac is invading a shrubbery bed--it may have to be removed by hand--OFTEN.
Here again is where lopping off the top at ground level and dabbing some roundup(full strength) on the cut trunk with a paint brush should kill the roots... A concentrated and clean solution that doesn't involve spraying anything...
Crossbow or Tordon RTU does well when painted on cut tree stumps or suckers. Watch out for root grafting onto desirable trees. Scratch the Tordon if you are in sight of any desirable trees. It is some bad stuff. I remember when the DOA used toothpicks dipped in Tordon to kill banana trees vectoring virus diseases. Desirable trees and shrubs near the banana were killed. RoundUp will transfer via root grafting as well, but it does not transfer as well and most trees are not as sensitive.