Two or three strategies are usually needed. Roundup is very effective on waterhemp. It s not perfect. There are times when waterhemp survives an initial application of Roundup, but a subsequent application should control surviving plants. Another promising choice is Authority from DuPont, which is a pre-emergent herbicide.
Authority can provide full-season control of waterhemp if the infestation is moderate.
If follow-up is needed, consider using Flexstar, Cobra or Blazer. Another possibility is Prowl and Treflan, for a good start, followed by a post-emergent herbicide. If triazine-resistant waterhemp are not present, consider starting with atrazine in corn. This provides good residual control. But, remember that corn does not canopy as well as soybeans so waterhemp can emerge late.