Where do you live?
What kinds of seed did you put down?

Weed seeds just love being watered 2 or 3 times per day. You can expect rapid germination of those seeds. When you seed in the spring, crabgrass will be one of your intruders.

I'm not sure why you would use round up on weeds, but if all you have is a nuclear bomb, I guess that's what you have to use. Yes, you can use round up but it will kill any grass seedlings it hits. I guess you knew that. Broad leaf weeds are usually treated with Weed-B-Gone (2,4-d) as opposed to Round up (glyphosphate). Hand pulling seedlings is pretty easy especially when there is no other grass to entangle them.

For anyone else reading this and getting ideas, don't use manure. Use compost if you have to use something. Manure often has fresh urine in it and can burn germinated seed sprouts before they take root. However, simply rolling the seed into the surface of the soil would have been fine.

i am in denver area, i put down heritage blend from earth carpet. basically kbg with small percentage of rye grasses.

i put down manure couple days before i seeded. if there is urine present how long does it stay active?

i started watering month or so before i seeded to have the weeds germinate, and than using roundup to kill it. only place i have germination of weeds is this edge of the whole area. i am talking roundup as there are also grasses germinating along with broad leaves. i thing i have killed all the grasses but other stuff keeps germinating. so if i use wbg am i going to hurt seeds? i dont mind hand pulling, its just it takes much longer.
thannks again

Actually, when the grass is immature, the broadleaf weed killers will kill the grass, also. Your choices basically boil down to ignoring the weeds until the grass has been cut 3x or hand pulling.

all right, now my soil was tested low on nitrogen. when do i go ahead and add that and in what form? also do i fertilize on memorial weekend? what do i use? thanks

If you are worried about crabgrass germinating you could apply some Tupersan (siduron) to your newly planted seed. It is a selective preemergent that will prevent crabgrass from growing but not affect your turf seed.

i am not sure crabgrass is what i have germinating. i will take pictures and try to post them up

>"Actually, when the grass is immature, the broadleaf weed killers will kill the grass, also. Your choices basically boil down to ignoring the weeds until the grass has been cut 3x or hand pulling."

ummmm, actually 28 days after germination, Drive 75 (Quinclorac) can be appllied for broad leaf weeds and crab grass. See link below from Purdue Unv.

Drive 75 can be obtained from Ebay.

Here is a link that might be useful: Herbicide Delays after Seeding

The Tupersan will prevent crabgrass from germinating, it's a preemergent, it's not used to kill crabgrass that has already sprouted. Crabgrass usually needs soil temps over 60 degrees for a few consecutive days to germinate, so if you are in a cooler part of the country it probably hasn't germinated yet.