Lots of weeds huh? What,when and how much you should use depend's primarily on your definition and identification of weeds. Secondary but often extremly important factors are (A) what crop is on or is planed for the field (B) incidental plants you don't want to harm,such as trees on your or adjoining properties (C) rainfall runoff and what plants,aquatic creatures,livestock and wildlife live's in watershed. Those are only a few important considerations,don't be foolish,educate yourself and/or seek professional assistance before squirting chemicals on your property.