There seems to be more weeds, particularly thistles in grass pastures this summer than normal. Excellent conditions for growth over the past six weeks is a likely factor along with the fact that more damage was done this week due to wet weather, so pastures are a bit more open than normal.
The best time to kill a weed is at the seedling stage. So spraying for weeds in newly sown reseeds is a must. Why go through all the work and expense of reseeding and then decide not to spray for weeds post emergence?