About three weeks ago I overseeded a lawn. Now the weeds have enjoyed the starter fert and are growing big. I did not kill the weeds, since there were not that many at that time. First mistake. Some additional seed needs to be added to the yard, but what can I do with the weeds. Hand pulling is not an option. What can be used to kill the weeds and not affect the seed or new grass? Thank you for your advise.