2,4D amine concentrate. Weed-B-Gone has 2,4D as an active ingredient, but lacks much power. Follow directions for mix then add just a bit more for a knock-out punch. Add a surfactant like orange oil or old fashioned dish soap so the herbicide will stick to the broadleaf. Apply in the early am when dew is on the grass and the weed's pores are open. It also helps to see your tracks so you get good coverage and not too much overlap. Try to make sure there's no rain in the forecast for a few days. Tank on a wheeler or tractor will save your back for that big patch of grass you have.
I used to do it summers for a large lawn outfit for 4 years. Weeds will be less when your lawn (grass) gets stronger. Aerate and overseed spring and fall. Crabgrass pre-emergent in the spring, slow release nitrogen in summer, weed and feed in the fall. Cut lower spring and fall, higher in heat of the summer; never more than a 1/3 of the grass blade, and water about an 1" a week. That's all there is to it!
