How long after new grass grows from seed can I apply weed killer to those spots?

I seeded in the spring and the grass has now grown up with the rest of the lawn (about 90 days) so is it OK to put weed killer on it now?How long after new grass grows from seed can I apply weed killer to those spots?
Read the instructions on the bottle or bag.


e.g.: http://www.hardwarestore.com/media/pdf-m鈥?/a>





FWIW, even though they rarely tell you this on herbicide containers, there's no point in applying herbicides when the plants aren't actively growing. High temperatures (and we're pretty much into the high temp month or so in the Northern Hemisphere) tend to decrease effectiveness of herbicides, as do low temperatures.





You can also maximize the effectiveness of herbicides by choosing the right stage of growth of the weed for application. Likewise, you may need to water before using an herbicide.





http://www.turf.uiuc.edu/teaching/NRES30鈥?/a>


http://scarab.msu.montana.edu/CropWeedSe鈥?/a>





What are you trying to kill? There are often easier ways to do it.