The soybeans in the picture without any pigweed were managed the following way:
1) Started out clean.
2) Used 2 residual herbicides (1 PRE + 1 POST). For RR soybeans, look to Boundary or Dual Magnum or Warrant – PRE followed by Roundup + Reflex or Prefix (Reflex + Dual Magnum) – POST. In 2016 , a new formulation of Warrant + Reflex (Warrant Ultra) will be available for those growers who prefer Warrant over Dual (FYI, UGA Extension aka Dr. Prostko does not have a preference).
3) POST applications were timely (24 DAP).
4) This particular soybean field was grown under a center-pivot. It is very unlikely that dryland soybean growers will be able to achieve optimum weed control on a consistent basis when relying on rainfall for the activation of residual herbicides. That is one of the biggest perils of dryland farming.