Here every field of spring planted alfalfa I have ever seen is full of pigweed the first year.  We tried it in 24, stand of alfalfa is fair, the stand of pigweed was excellent, first cutting was 90% weeds, second cutting was about 30% pigweed full of seeds and pigweed stickers. We are feeding all of it on the ground.

There is a group 15 pre that can be used to help with pigweed control, I have not used it as I did not know about this chemical until after it was too late. Eptam was recommended by at least one other agtalker.

I would think with grazing you would want to mob graze it for just a few days.