Will Atrazine on weed filled lawn harm st. augustine gr?

Yesterday, I mixed up 2 tablespoons of atrazine to 1 gallon of water and I sprayed it on the whole front lawn of my aunt's house because I had planted st. augustine gr sod there a year ago and it is trying to grow but the weeds are combatting it. 90% of the yard is weeds. So, I now put atrazine on the front yard. There is also bahia gr growing there too. I want to know whether or not the atrazine will harm any preexisting gr.