My lawn was treated with roundup, core-aerated and re-seeded. I gave the new lawn it's first cut yesterday. A small weed (see attached pictures) has sprouted recently in most areas that are sparse and is very prevalent. Can you identify the weed and suggest a treatment?
The weed looks like chickweed. http://extension.umd.edu/hgic/common-chickweed
If you have very small patches you may be able to handpull. However, do not disturb the root system of the new grass.
If you cannot handpull without disturbing the new grass, then you will have to use an herbicide. However, you cannot do that until your new grass has been mowed at least 3-4 times and that may not be until spring. Follow label directions for the herbicide and See above control.