Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
Soybean fields treated before the rains that have not been planted yet are probably experiencing weed breaks. Even the least water-soluble herbicides have lost most of their herbicidal activity. Be sure to scout these fields and if there are emerged weeds at planting, be sure to control them. These fields will need a non-selective herbicide such as glyphosate or paraquat. The challenging scenario is if Palmer amaranth or horseweed are present along with grasses. Paraquat plus some metribuzin will control newly emerged Palmer amaranth or horseweed plants; and burn off the grasses but they will probably regrow if they are over 3 inches. While glyphosate will control the grasses, it will not kill the horseweed or Palmer amaranth. My advice is to focus on the biggest challenge, Palmer amaranth and horseweed and you can attack the grasses after the soybeans have emerged with glyphosate or Select or Poast or something similar.
Soil-applied herbicides generally do not control emerged weeds. However, we have had some success on 1 to 2 inch tall weeds with 3-4 oz of metribuzin plus 2 oz of Valor plus nitrogen plus crop oil. Be aware weeds over 2 inches may not be controlled, and this will not control emerged grasses or emerged horseweed. I am not ready to endorse this approach, but it is the only combination of residual herbicides that have provided acceptable burndown activity on some weed species.
The combination of metribuzin plus Valor with paraquat has looked very good in our trials for treating early burndown fields that needed retreating before planting. However, this combination may not be appropriate if you used metribuzin and/or Valor early preplant.