A fairly well accepted method of planting into an established pasture is to graze it down fairly low to set back the existing growth, then plant your cover crop with a no-tilll drill into the residue. If you want to just augment your existing pasture plants, hopefully they new plants will become established before the old plants get back on their feet; then they can all live happily ever after. If you want to establish a garden plot or clear some spots to plant trees, your no-till planting might consist of buckwheat. Plant it thickly in the spring, and before it goes to seed, mow or graze it and it should have suppressed a lot of the other vegetation, and you will have a fairly competition-free area in which to dig/plant. If the buckwheat goes to seed, you will have another buckwheat crop, which you may or may not want. There are other "smother" crops but I am most familiar with buckwheat. A "chicken tractor" setup can be used to do the initial heavy grazing before you plant, too, but it can be difficult to get enough cleared that way in time to plant a significant area.