Winter annuals, i.e. mustard, are the fourth type of weed you may be concerned about. They sprout in the fall and flower early in the spring. Good pasture health will also take care of them. These two grasses spread by underground roots in the fall. Spring applied nitrogen tends to promote top growth at the expense of root growth.
Some people like to do a fall interseeding.
Always a risk, of course, but you might be better to wait for an early spring attempt to thicken the stand. We'll talk more about that next spring. In the meantime, you might be reading journals or visiting with neighbors and extension personnel to get a handle on the species that might be most useful to you. The same goes for a magic bullet to improve pastures but it just seems to boil down to creating and maintaining a proper environment for the plants and they'll respond to their best ability. Acreage Living is published monthly.