The following is from Larry Steckel, Tennessee Extension Weed Specialist, who this week addressed the fact that wet weather in the upper Delta has caused a huge - read that, expensive - problem with Roundup-resistant Palmer pigweed. He offers options, none of which seem all that promising, considering the way this weed operates in its Roundup-resistant form - Owen Taylor
Roundup-resistant Palmer pigweed is geting away from us fast.
Due to all this wet weather, the pre applied herbicides are giving out quickly. Moreover, it has often been too wet to spray post emergence as timely as we need to.
This is shaping up into a perfect storm.
I received the first calls of the year on Palmer pigweed issues in soybeans and cotton. One consultant called and said that Prefix applied last week had not controlled 4-inch-tall Palmer.
This is not surprising. A 4-inch-tall gramoxone-resistant (GR) Palmer pigweed (amaranth) is getting too large for any PPO herbicide to control. Ignite at higher rates (32 to 40 oz/A) can control Palmer up to about 6 inches, though this can be inconsistent at times.
The unfortunate truth is that if Roundup/glyphosate is unable to control tall Palmer amaranth due to resistance there is no herbicide that will.This coupled with the fact that a 2-inch Palmer amaranth can quickly become an 8-inch Palmer amaranth even with 44 ozs/A of Roundup PowerMax on it (as seen in last year’s picture at right) leaves us very few days to get these post herbicides applied.
Other reports have come in on RR Flex Cotton where the management plan was to use overlapping residual herbicides. The problem being that it was too wet to get the second overlapping residual on and now there is a heavy flush of Palmer pigweed in a good cotton stand.
There are not any real good options for fields in this shape with resistant Palmer pigweed. It is still early enough to replant soybeans and if we can get in next week cotton, as well. Depending upon the density of the GR Palmer this would probably be the best option.
The number one question of the week is from folks wanting to spray Ignite on cotyledon WideStrike cotton. The first Palmer pigweed flush is right with cotton emergence in a number of fields. In other cotton fields we are seeing some late regrowth of horseweed.
The main question is how safe is it to spray cotyledon WideStrike cotton with Ignite to control these weeds?
The answer from research we have conducted here as well as some from Georgia is that cotyledon WideStrike cotton tolerates Ignite a little better than 5-leaf cotton. My observations from this work have been that the waxy cotyledons do not seem to show effects from Ignite like a true cotton leaf will. In general we will see about 10% injury on cotyledon WideStrike cotton and typically 15 to 20% injury on more mature cotton.
What about tank-mixing Dual Magnum with my Ignite on WideStrike cotton?
My answer is in areas where GR Palmer amaranth is already established the benefits outweigh the risk. We have conducted several trials with this tankmix (I would like to run several more) and seen anywhere from 0 to 5% more injury compared to Ignite alone. If the Dual Magnum gets activated this may save one or even two over the top Ignite applications to the cotton.
What about tank-mixing Prowl H2O with Ignite on WideStrike cotton?
I have not looked at this tankmix on WideStrike cotton. I have seen it on one trial several years back in Liberty Link cotton. I did not like the results. We saw antagonism on the Palmer amaranth control compared to Ignite alone or Ignite tankmixed with Dual Magnum.
Moreover, the injury to the LL cotton from the Ignite tankmixed with Prowl H2O appeared to be more than I saw with the Dual Magnum tankmix. This is only one trial, but it suggested to me that we need more research on it before we recommend Ignite tank mixtures with Prowl H2O on LL cotton as well as WideStrike cotton.
