I have gotten numerous reports of Palmer amaranth becoming established in fields where PRE applied herbicides were not activated by timely rainfall. Indeed I have seen some in my research as well. There were many acres of cotton and soybean planted May 7 to May 14. The rain that was in the forecast for last week was a no-show. The result is many of the PRE applied herbicides sprayed on all those cotton and soybean acres was not activated. This has led to a large flush of Palmer amaranth to emerge with the cotton and soybean in a good number of fields in Tennessee.
If the rain that’s in the forecast for May 16 and 17 occurs then the PRE applied herbicides applied May 13, 14 and 15 should become activated. However, fields where PREs were applied last week will need to be scouted to assess weed control. There is a good chance a “jail break” of Palmer amaranth is up in some if not many of these fields.
If the rain that’s in the forecast for later in the week does not materialize then the timing for the POST application will be critical. All fields will need to be sprayed quickly in order to catch Palmer amaranth before it is 4” tall.
Some have attributed the great Palmer amaranth control in 2017 exclusively to the performance of Engenia or XtendiMax. That was clearly part of it but many have forgotten how well PRE applied herbicides worked last spring.
The PREs last spring received frequent timely rains in May and June and were very successful in providing early season Palmer amaranth control. In many cases they delayed the emergence of Palmer so that Engenia and XtendiMax could be applied timely. Moreover, the PRE applied herbicides decreased the number of Palmer that the dicamba products had to control. Both these results increased the consistency of control. This will not be the case for a significant number of fields planted last week and Engenia or XtendiMax will have to perform very well for us to achieve fields as weed free as what occurred in 2017.