This is now a critical time for Palmer amaranth control in soybean. Despite most of Tennessee being very dry, Palmer amaranth is growing very quickly with the warm temps.
On May 16th we planted soybeans in several tests at the station here at Jackson without a pre applied herbicide. We started clean at planting with a Gramoxone application. On May 29th just 13 days later, the soybeans were 3” tall and just opening their first trifoliate leaves. The Palmer on the other hand was ranging in size from 2 to 8” tall (picture left). The growth rate of this weed species in late-May and June always amazes me. Just 13 days after soybean planting and the Palmer amaranth was way too large for a PPO herbicide (Flexstar, Cobra, etc.) to control. Right after looking at our soybeans, the first calls of the year came in on how to deal with fields of Roundup Ready soybeans planted at the same time as ours with Palmer at this large size. Unfortunately, I had to make the first recommendation of 2012 to destroy the soybeans and replant.
In a number of cases the consultants and producers are reporting that pre applied herbicides like Prefix and Valor were not activated and the Palmer grew through them unchecked. In better situations, pre applied herbicides were at least partially activated and have reduced the number of pigweed in this first weed flush. In these situations the next few days will be critical for getting post applied herbicide applied timely. In many cases whether or not a field can be sprayed in a 3 day window will dictate if the crop will need to be destroyed and replanted or not.
Despite the overall dry conditions, the scattered showers in many areas did activate pre applied herbicides. This is a great first step to successful weed control. However, be prepared to be timely with a follow-up herbicide application in these fields as well. An overlapping residual herbicide if no Palmer has emerged is a good second step. In cases where Palmer has broken through the Pre applied herbicide, a follow-up PPO herbicide or Liberty application on newly emerged Palmer would be needed quickly. I know some producers are utilizing Liberty Link soybeans on their worst pigweed fields. Liberty will control larger Palmer than Flexstar and Cobra. However, in fields with overwhelming pressure, spraying small (<3”) Palmer as well as good coverage will be critical to obtaining good control with Liberty. In short be prepared to run the sprayers hard over the next week or two regardless of the herbicide trait technology.
