The recent hot, dry spell has finally afforded many Tennessee producers the opportunity to get in the field. Unfortunately, this weather has also been very conducive to pigweed growth. In this weather, the window of opportunity for POST control of Palmer amaranth while it is < 3 inches tall is narrow. These conditions have led to an increase in the number of calls about how to control large Palmer amaranth. In soybeans there are no silver bullets, but there are still some options.
In Roundup Ready or conventional soybeans, PPO (WSSA Group 15) herbicides are the only option currently available. In our research, we have looked at Ultra Blazer (aciflurofen), Cobra (lactofen), and Cobra + Resource (flumiclorac) applied either 7 days before or after a Prefix (fomesafen + S-metolachlor) application on 4 – 12” tall Palmer. We have noticed little difference in control from any of these systems regardless of whether Prefix was applied first or second.
The greatest control was from Prefix and Ultra Blazer at about 75% with Prefix and Cobra being second at about 70%. These systems do cause substantial soybean injury (25-35%), however, injury was transient and the beans recovered in 7-10 days. Palmer control from Prefix and Cobra + Resource was similar to that of Prefix and Cobra however, soybean injury increased to around 40%.
In Liberty Link soybeans, we have the added benefit of being able to use Liberty (glufosinate). We have been examining Liberty tank-mixed with Flexstar (fomesafen), Cobra, and Ultra Blazer on similar-sized Palmer. In these glufosinate-based systems, control was greatest (85%) from Liberty plus Cobra or Ultra Blazer. Liberty and Flexstar provided about 80% control. Injury ranged from roughly 25% with Cobra and Ultra Blazer treatments to about 15% with the Flexstar treatment.
Most notably, we have seen that PPO herbicides help to improve the consistency of Liberty and vice versa. Earlier research has shown that application time of day can affect the efficacy of both Liberty and PPO herbicides. We have looked at sunrise and noon applications of Liberty alone and in combination with Flexstar, Cobra, or Ultra Blazer and found that the time of day effect is largely overcome when Liberty is tankmixed with a PPO herbicide.