The first reports of small Palmer amaranth escaping PPO and glyphosate tankmixes started today (5/13/16). Friday the 13th is probably a fitting day to first notice this problem.
At this point we do not know if these Palmer amaranth escapes are due to PPO resistance or just poor PPO herbicide performance. However, looking at how widespread we found PPO-resistant Palmer amaranth last year, coupled with weeds that were obviously sprayed small enough to control last week, the smart money would be on that most of the Palmer amaranth is resistant.
We are in the middle of conducting a more extensive PPO-resistant Palmer amaranth survey in TN this spring and should have a better estimate of how far this issue ranges across the state later this summer.
Once Palmer escapes that first application it is almost impossible to rescue the soybeans with any type of follow-up herbicide application. This is certainly the case if the Palmer is PPO resistant. Even if the Palmer amaranth in question is susceptible to a PPO herbicide, once they escape the first application one can only hope to salvage some yield with a high rate of FlexStar GT + 1% MSO or 12 ozs of Cobra + 1% MSO.
This works best if the herbicide is supplemented with cultivation and/or a chopping crew. I only consider those options if we are late in the soybean planting window.
It is still early so the best recommendation would be to disk down and replant with Liberty Link soybeans and utilize a Liberty based weed management system. This does not mean to just plant LL soybeans and spray Liberty.
It means be sure to have a Pre applied herbicide down. Then in that first post emergence application of Liberty, tankmix in Prefix or some other PPO herbicide. If it gets into late June then substitute Prefix with either Cobra or Ultra Blazer so there will be no plant back problems to corn next year.
The reason we recommend PPO herbicides in with Liberty is that in our research those tankmixes have improved the consistency of Palmer amaranth control. Moreover, it can also help delay the development of Liberty-resistant Palmer amaranth.