University of Tennessee Extension Weed Scientist Larry Steckel has confirmed the discovery of dicamba-resistant waterhemp across multiple counties in Tennessee. Field tests and greenhouse experiments conducted by both UT and Purdue University found the populations resistant to dicamba appliactions up to four times the labeled rate. And, according to a survey of fall weed escapes, waterhemp is spreading quickly.
“In several counties in middle Tennessee, waterhemp is replacing Palmer amaranth as the top weed in crop fields,” Steckel said. “This finding is troubling and not good news for the soybean and cotton growers of our state.”
Finding resistant populations
Steckel said the troubling waterhemp populations were first called to his attention in the fall of 2019. On his farm visits he found soybean fields exclusively overrun with waterhemp. The level of infestation in these ravaged fields was surprising.
“Until 2019, finding any more than a stray waterhemp plant in Tennessee was rare. When we were told that the waterhemp had survived multiple dicamba applications it really got our attention,” Steckel said.
In the spring of 2020, the waterhemp populations had passed the first test of dicamba resistance. In greenhouse screenings, numerous waterhemp weeds survived an initial dicamba application. That fall, Steckel teamed with Bill Johnson, Extension weed scientist with Purdue University to perform additional tests in the greenhouse and field.
“The waterhemp seed screened in the greenhouse at Purdue was collected in the fall of 2020 from the same areas of fields where the waterhemp had escaped dicamba in 2019,” Steckel said. “The waterhemp in 2020 had once again escaped multiple dicamba applications applied to soybean. The greenhouse results showed that the waterhemp in these suspect fields needed 4.7 times more dicamba to provide similar control to the normal, or susceptible, waterhemp population.”
Two other greenhouse screens found similar results where the waterhemp from suspect fields were 3.5x and another 5.2x resistant to dicamba compared with the susceptible population.
Steckel said the field research conducted this summer found that the 1x rate of dicamba (22 ozs/A XtendiMax) provided less than 40% waterhemp control. Doubling the rate to 1 lb of dicamba gave 68% control. The rate of dicamba needed to provide control that would be considered adequate was 2 lb dicamba (88 ozs XtendiMax).
“In other words, about a 4x rate which would be consistent with the greenhouse results,” he said.
“One surprising find in our field research was that, though dicamba did not control the waterhemp, 2,4-D did,” Steckel added. “This was very different than what we have seen with dicamba-resistant Palmer amaranth in Tennessee. To date, dicamba-resistant Palmer amarant has always been resistant to 2,4-D, as well.”
The annual weed survey conducted by UT in fields across the state found a surprising amount of waterhemp in soybean fields. Survey results show the primary Amaranthus species in fields in middle Tennessee counties visited in 2021 was waterhemp. The counties where waterhemp was very apparent in Xtend soybean fields were Bedford, Cannon, Montgomery and Warren counties. Steckel said even in western Tennessee counties like Carroll, Henry and Weakley he discovered waterhemp in soybean fields along with Palmer amaranth.
“From a weed ecology standpoint, it will be interesting to observe in these fields where both Palmer amaranth and waterhemp are present which one comes out on top,” he said. “Regardless which Amaranth species wins, one can safely say the cotton and soybean in these fields will be the losers.”
What to do?
Steckel said with the discovery of dicamba-resistant waterhemp growers will need to adopt a “pre-Xtend mentality.”
“Overlaying residual herbicides will be key,” he said. “There are some cultural practices we can adopt. Rotating with corn has helped a good bit. Narrow row-width in soybeans is another option. But overlaying residuals is crucial. We have to try to keep these resistant weeds from ever coming up.”
Steckel added that he still has confidence in glufosinate to control the waterhemp populations in his state.
“Growers using the XtendFlex system should be ok, but there are concerns about products like Liberty being in short supply,” he said.
More information on Steckel’s weed research can be found at the UT Crops News Blog.