Can you spray too early?

Every week has a maximum height where it wont be effectively killed, even with glyphosate. Once any weed sets "seed heads" it becomes relatively more difficult to kill as it's not in the growing phase but reproductive phase. Some weeds need to be sprayed before they get to be 6-8" or they might only be suppressed or stunted.

The problem with waiting too long to spray weeds as it helps build up chemical resistance. You want to have effective weed kill and not suppression.

To get more information, here is a great reference
Label Database