Research Highlights

Research Highlights
Managing resistant weeds in Kentucky

By Dr. J.D. Green and Dr. James R. Martin

Much of the recent weed science research and educational ef­forts in Kentucky have focused on managing glyphosate resistant Palmer amaranth, waterhemp, and marestail (horseweed). The following is a summary of some of the results of these activities.


A survey of 75 fields in Kentucky involving Palmer amaranth and waterhemp was conducted to determine types of herbicide resistance in these two weed species. A number of fields had resistance to glyphosate as well as to ALS inhibitor herbicides (i.e. Classic or Synchrony STS) for both Palmer amaranth and water­hemp. While glyphosate resistance gets a lot of attention, it is im­portant to recognize that herbicides such as Classic or Synchrony STS may not effectively control these problem pigweeds due to ALS resistance.

PALMER AMARANTH (Fulton Co. 2012)

One of the studies where we were able to get meaningful data on Palmer amaranth showed a strong trend in using a soil-applied herbicide followed by a timely application of a postemergence herbicide. The dry soil conditions in this study seemed to limit the amount of late-season emergence of Palmer amaranth.

WATERHEMP (Hancock Co. 2013)

  • Spray Volume: The use of Liberty post-emergence in Liber­tyLink soybeans provided better waterhemp control than Prefix when applied to larger plants. On the other hand, the benefit of increasing spray volume from 10 GPA to 20 GPA was more notice­able for Prefix than for Liberty. The fact that the waterhemp pop­ulation was somewhat low in this trial may explain why increasing spray volume was not necessary for Liberty.
  • DNA Assay Tests: A multi-state study involving other universi­ties (Illinois, Nebraska, Missouri, and Kansas) showed that sam­ples of waterhemp collected in Hancock County, Kentucky, had a different mechanism of resistance to glyphosate compared with samples from other states. The significance of this finding is that some waterhemp populations, like the one at this location in Ken­tucky, may not show glyphosate resistance based on the current DNA assay that tests for gene amplification.

MARESTAIL (Woodford Co. 2013 and 2014):

  • A standard treatment in Roundup Ready soybean trials includ­ed glyphosate preplant foliar (0.75 pound acid equivalent per acre) followed by glyphosate (0.75 lb ae/A) in-crop. The fact that this treatment resulted in only 40 to 58% marestail control confirmed that a significant portion of the marestail at this site was resistant to glyphosate.
  • Including either 2,4-D Ester (2 pt/A) or Canopy (6 oz/A) with glyphosate as an early preplant application followed by glypho­sate, improved marestail control; but, was not as effective as tank mixing both 2,4-D and Canopy together in this program.
  • Marestail control with Authority First (6.45 oz/A) was similar to that observed with Authority XL (6.5 oz/A) in 2013 but tended to be greater in 2014.
  • Including Sharpen (1 oz/A) with either 2,4-D Ester or Cano­py provided at least 96% marestail control in 2013 and 2014. Whereas, programs involving Valor XLT (3.5 oz/A) or Fierce XLT (3.5 oz/A) were not as effective as those with Canopy pus 2,4-D Ester or Sharpen plus either 2,4-D or Canopy.
  • A number of effective options were identified using the LibertyLink soybean technology. Applying glyphosate 0.75 lb/A (i.e. Roundup PowerMax 22 oz/A) plus 2,4-D Ester (2 pt/A) early pre­-plant followed by Liberty (29 oz/A) in-crop, resulted in 94 to 100% marestail control. Applying Liberty burndown followed by an in-crop application of Liberty resulted in 100% marestail control both years. However, it is interesting to note that when using glyphosate alone in the burndown application followed by Liberty post-emergence to the crop resulted in only 80 to 86% control of marestail.
  • Applying a burndown treatment of Gramoxone SL (3 pt/A) in combination with either Tricor (8 oz/A), Canopy (6 oz/A), Envive (3.5 oz/A, Trivence (8 oz/A), Valor XLT (3.5 oz/A), or Authority XL (6.5 oz/A) provided 95% or more marestail control.

MARESTAIL (UKREC 2013-2015):

Limited testing of newer herbicide traited technologies Enlist Duo (2, 4-D +glyphosate tolerant) and Roundup Ready 2 Xtend (dicamba + glyphosate tolerant soybean) showed promising re­sults for managing marestail. Stewarding these technologies to avoid developing resistance to 2, 4-D or dicamba, and off-target movement will be critical to ensure their sustainability.

Published: Dec 4, 2019

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.