My lawn used to very weedy. I used alternate treatements of
Weed-B-Gone and Spectricide, sparated by a two week period. I used a
pressurized garden sprayer and just sprayed the weeds, not the entire
lawn. What one product did not get, the other DID. It took about 20
minutes or so to "hunt" down all the weeds over a half acre. Use the
weed killer on a bright sunny day with no rain in the forcast for 24
hours. You'll use very little weed killer but this method is very
effective, especially in the spring. I would never put weed killer in
a sprinkler system.