Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org and Frank D’Amico
Some common ragweed in the region (DE, MD, and NJ) have evolved resistance to glyphosate, PPO-herbicides (Group 14), and ALS-herbicides (Group 2). Some populations are resistant only to glyphosate, others are resistant to all three herbicide groups. It is difficult to predict when a population is resistant to one, two or three herbicide groups without testing; and testing common ragweed can take months. So if you are experiencing difficulty with common ragweed control, it is best to manage the field as if it is resistant.
Common ragweed is one of the first summer annual weeds to emerge in the spring, so you need to consider common ragweed when developing burndown herbicide programs. Early emergence also means that delaying crop planting will allow a larger portion of the common ragweed to emerge and be effectively controlled with burndown herbicides. Effective options include paraquat, triazine herbicides, 2,4-D, or dicamba. Liberty can control common ragweed; but my experience is that Liberty performs better with full sun or as a postemergence application.
Common ragweed control in corn can be achieved with atrazine or simazine used at planting. Postemergence applications of atrazine, dicamba-products, or atrazine plus HPPD-herbicide (i.e. Callisto, Laudis, Impact, or Armezon) have provided very good control of common ragweed.
Control of common ragweed in soybeans is more challenging. Using soybean varieties that allow Liberty, dicamba, or 2,4-D is important. Preemergence herbicides options are limited and not highly effective. Linuron (Lorox or Linex), Command, Valor, or metribuzin are the best, but they seldom provide complete control. A postemergence herbicide is often needed and options are limited to Liberty, dicamba, or 2,4-D since uncertainty of resistance to glyphosate, PPO or ALS herbicides. While many of the hard to control populations are resistant to postemergence applications of PPO-herbicides (such as Reflex, Blazer, Cobra), Valor applied as a preemergence herbicide still provides control and is an option for preemergence applications.
In the photo above all the seeds were collected from the same field, grown in the greenhouse and treated postemergence with glyphosate, Reflex (Group 14), or FirstRate (Group 2). The herbicides were applied at 1X, 2X, and 4X rates when the plants were 3-inches tall. The field contains both resistant and susceptible plants. Top left was treated with glyphosate at normal use rate; bottom left was treated with 4X the normal use rate; top right was treated with 4X the normal use rate of Reflex; and bottom right is 4X rate of FirstRate.