If you're not familiar with it.....do some research into the concept of "rotational grazing" also called "mob grazing".....and look at how they use some of the same principles. Also check out the specific traits of pigweed and the soil conditions that it favors. See if there is any correlation between those conditions and what you have in your field. I'm sure the problem did come in with the bay but maybe it will help you get rid of the problem long term instead of only having a temporary fix