Palmer amaranth is a monster. It’s ability to quickly grow and steal space and resources from crop plants is legendary. As a C4 plant (like corn), it grows fast in warm sunny weather, can grow to heights of 6 to 8 feet, and produce a million seeds on one plant. And if that doesn't scare you, it is, after all, in the pigweed family, having the genetic disposition to develop resistant pathways to many of the herbicides we would like to use to control it.
While this weed has not been universally identified throughout the cornbelt, it has been found in Iowa, Michigan, Illinois, Indiana, and Ohio, and in nearly every region to the South. In Georgia, Palmer amaranth was identified as glyphosate-resistant in 2004. Because of weed seed that has moved with livestock, machinery, and grain transport out of the South to the North, it is suspected that even new infestations of Palmer amaranth in regions to the north of Georgia may have the glyphosate-resistant trait. To make matters worse, Palmer amaranth has developed biotypes that are resistant to the standby products for post-emergent control; PPO herbicides.
What does this all mean? It means that scouting for Palmer amaranth becomes an essential task for growers and that a combination of herbicides modes/sites of action will be required to keep this weed at bay. If you see populations of this weed that are left uncontrolled after a glyphosate or PPO herbicide application, corrective action might need to be taken as soon as possible (think hoeing?). As yet, Liberty herbicide remains a viable option for herbicide resistant Palmer amaranth. However the Liberty-Link gene must be in the crop to permit this application. One final note: researchers from Tennessee have indicated that, as with most PPO herbicide applications, applying them during the time of the day when sunlight is at its peak (noon), improves weed control.