Answer: Well, now that you know that what you used not the right product, let's concentrate on trying to fix the problem. Obviously, grass killer is going to kill the grass, roots and all. And, it will have some residual effect until the chemicals biodegrade (degrade in the presence of sunlight and fresh air). I don't know how long it will take to degrade - that will depend upon how much was applied. So, the quickest fix would be to dig out the dead grass in the affected areas, along with some of the soil. Dig down 5-6 inches and remove all the debris. You can pile this soil up somewhere else in the yard and use it again, after all the weed killer has weathered out. Then take some soil from another part of your yard and spread it into the holes in your lawn. Then reseed. The new grass should sprout in 7-10 days as long as you water often enough to keep it from drying out.
In general, turfgrass needs about one-inch of water per week. To figure out how much water your sprinklers are putting out, set several empty cat food or tuna cans on the lawn and water as usual. Then measure the amount of water in the cans. If it's about one-inch, you're applying the right amount. If it's less, you'll need to water longer to achieve the one-inch goal. If water runs off the lawn before you collect one inch in the cans, you can adjust your watering - water for 15 minutes, turn the water off for 15-30 minutes, turn it on again, then off, repeating this process as often as necessary to supply one inch of water to your lawn.
Obviously, the safest thing to use on your lawn to keep weeds at bay is a weed and feed product. Weed and feed fertilizer products contain 2,4-D which is a broadleaf weed killer. It will not harm grasses.
Best wishes with your lawn!
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