Cover Crops and Residual Herbicides Team Up Against Palmer Amaranth

Palmer amaranth is famous for dodging herbicide control options, but it appears that a one-two punch of cover crop residue and residual herbicides may be able to knock this troublesome weed down for a nice long count. 

Vipan Kumar, with an infestation of Palmer amaranth in a sorghum field. (Photo credit: Vipan Kumar)

That’s the conclusion of new research led by Kansas State graduate student Sachin Dhanda and his advisor, weed scientist Dr. Vipan Kumar (now with Cornell University). 

The researchers’ test fields were in a wheat-sorghum-fallow rotation and came well stocked with a seedbank of glyphosate-resistant Palmer amaranth. From fall 2020 to fall 2023, after wheat harvest wrapped up, two of the four test strips received a fall-drilled cover crop stand of winter triticale, winter peas, radish and rapeseed, while the other two strips were left to overwinter in bareground.  

Then each spring, researchers headed to the field with burndown/termination herbicide applications ahead of sorghum planting, as the triticale entered the heading stage. 

The two cover crop sections both received a glyphosate application, but one of the plots also got hit with a residual herbicide premix of acetochlor and atrazine. Of the two bareground plots, one also received the glyphosate + residual combination, while the other remained untreated entirely. 

The cover crop plot treated with both glyphosate and the residual herbicides proved to be a clear weed suppression winner, the researchers discovered.That combination of cover crop residue and long lasting spring herbicides both delayed and reduced Palmer amaranth emergence better than any of the other combinations. On average, over three years, that treatment reduced total Palmer amaranth emergence by about 50% compared to the bareground plots with the same herbicide application, and by 84% compared to the bareground plots with no herbicide application. 

That double punch of residual herbicides and cover crop residue also simply slowed the Palmer amaranth seedlings down, the researchers discovered. Palmer plants facing that treatment needed anywhere from 105 to 1,257 growing degree days to catch up to their neighbors in the bareground plots. 

On the left, a plot with no cover crop and no chemical control of Palmer amaranth. On the right, a plot with both cover crop residue and residual herbicide use. (Photo credit: Sachin Dhanda, KSU)

Growing adequate cover crop biomass remains a crucial step in getting this kind of weed suppression from a cover crop stand, and often the unreliable rainfall patterns of the central Great Plains can make that a challenge, the researchers noted. That, plus the continual evolution of herbicide resistance in Palmer populations means researchers and growers should continue to seek out additional alternative weed control options, such as harvest weed seed control or electric weeders, they concluded. 

You can read the full study here

For more information on using cover crops for weed suppression, see this GROW webpage and these GROW news stories

Article by Emily Unglesbee, GROW; feature and header photo by Vipan Kumar, KSU