mav,
If dogs eat any large amout of grass, they generally throw up, weed killer or no weed killer. Whatever you use, you want to be sure that you water it in very well. Broad leaf weeds (clover, etc) tend to block sun and water from the ground (this is why the area around the weed patch is dead), so you have to be persistent in watering. Otherwise, the chems will just literally sit on top of the leaves of the weeds. I am also assuming broad leaf weeds. Just keep an eye on them, there's nothing in the chemical that will make that part of the grass/weed more attractive to them to eat.