Palmer Amaranth Control Issues In Soybean And Management

DR. LARRY STECKEL

JACKSON, TENN.
   Compared to the past couple of springs Palmer amaranth control is not near as good in soybean. There have been quite a few reports of PRE applied herbicides not performing well. In more recent planted soybeans, poor tillage or burndown of large Palmer before planting has allowed a large infestation of Palmer amaranth to become established in soybeans. All the frequent showers have then made it impossible to control the Palmer timely. It is clear there will be a number of fields of soybean that will have to be destroyed and replanted due to Palmer amaranth.
   The weather has been the main issue where the 2 week dry spell in May when a lot of planting was conducted did not activate PRE applied herbicides. In June, the frequent rain events have caused the PRE applied herbicides to play out more quickly and left little opportunity to apply POST herbicides timely. In some cases even where POST applied herbicides were applied timely they were rained off.
   The situation described above coupled with the ideal Palmer amaranth growing conditions have many fields with Palmer ranging from borderline too large to control in RR soybeans (3-4” tall) to over a foot tall. The good soil moisture conditions and 90 degree temps are allowing Palmer amaranth to go from a seed no larger than a period at the end of this sentence to 8” tall in about 12 days. Add in the fact there are many more soybean acres to spray in a short period of time and it is no wonder we are having issues controlling Palmer this year.
   The question is how do we proceed from here? In a few cases where the field is pretty much covered up with 8” to 3’ tall Palmer and the soybeans are Roundup Ready or conventional the answer has been a disk and replant.
   In other cases where the Palmer is still in that 4 to 6” range and not just covered up then a sequential treatment has a chance to salvage RR or conventional soybeans. I define salvage as suppressing the Palmer enough that the crop can be combined. Remember that yield loss has already occurred in these fields. A study published in 2012 conducted at NC State, found that just 1 Palmer in 3 meters of row reduced soybean yield by 21 percent. Those results are similar to a study conducted at the University of Arkansas by Dr. Oliver in 1994 that found 1 Palmer in 3 meters of soybean row reduced yield 17 percent.
   The salvage treatments I describe below will provide at best 60 percent control. So one must weigh the expected yield loss from the Palmer that will escape these treatments to the yield penalty associated with a July soybean planting date if you elected to disk up and replant. We examined a number of sequential PPO treatments where the first application was the maximum rate of FlexStar GT plus 1 percent MSO followed a week later with a sundry of follow up applications. The FlexStar GT application applied to Palmer in that 4 to 6” range provided about 30 percent control if no sequential herbicide was applied.
   The best sequential program was FlexStar GT followed a week later by Ultra Blazer applied at 1.5 pts plus 1 percent MSO. That sequential program got us 67 percent Palmer control. This of course is not great but in a soybean field that could be enough to get a combine across.
   In Liberty Link soybeans the picture is more rosy  as a sequential application of Liberty applied about a week apart even on large Palmer has provided >90 percent control. Control of Palmer amaranth is best when applied between about 9:00 a.m. to 6 p.m. this time of year. However, in our more recent research tankmixing in a PPO herbicide like Ultra Blazer or Cobra can make the application effective even if applied early in the morning or later in the evening. ∆
   DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee