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Patrick J. Tranel Patrick J. Tranel
Associate Professor
Molecular Weed Science
Department of Crop Sciences
(217) 333-1531; tranel@illinois.edu
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William L. Patzoldt
Graduate Research Assistant
Department of Crop Sciences
(217) 333-4723; patzoldt@illinois.edu
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Dean S. Volenberg
Postdoctoral Research Associate
Department of Crop Sciences
(217) 244-6832; dsvolenb@illinois.edu

Waterhemp Response to Glyphosate: Is Waterhemp Resistant, Tolerant, Or is it All in Your Mind?

Waterhemp continues to be a troublesome weed to manage in Illinois corn and soybean production fields. Aside from the biological and ecological attributes which make waterhemp a successful weed is its ability to evolve resistance to a variety of herbicides. At one time, waterhemp could be managed with triazine, acetolactate synthase (ALS), or protoporphyringen oxidase (PPO)-inhibiting herbicides. However, waterhemp has evolved resistance to all these herbicides. The introduction of glyphosate-resistant corn and soybean seemed to be a panacea for growers with waterhemp resistance problems. However, over the past few years reports around the Midwestern grain belt suggested waterhemp varied in its response to glyphosate. Has waterhemp become resistant or more tolerant to glyphosate compared to the past or is it all in your mind?

We conducted greenhouse experiments to evaluate the response of 100 waterhemp populations mostly from Illinois to a reduced rate of glyphosate. The populations consisted of three groups: those collected 1) prior to 1996, 2) between 1996 and 1998, and 3) in 2003. The untreated plants of all three groups did not differ based on shoot biomass. Similarly, the mean response of all three groups of plants treated with glyphosate did not differ based on shoot biomass. All three groups treated with glyphosate had mean dry weights that were approximately 40% of the untreated based on shoot biomass. This suggests that the application of glyphosate over the past years has not selected for glyphosate insensitive waterhemp. Therefore, the perceived evolution of less glyphosate sensitivity in waterhemp may be all in your mind or, at least due to other factors such as improper application (e.g. too low of a rate or applied too late). However, when the data is examined on an individual population basis within the three groups, a very different picture emerges. The shoot dry weight of the prior to 1996 waterhemp populations were all 50% or less compared to the untreated, whereas approximately 40% of the 1996-1998, and 2003 waterhemp populations had dry weights that were greater than 50% of the untreated. Also, five populations of the 2003 group were very insensitive to a low rate of glyphosate.

Results from these experiments suggest that the use of glyphosate may have selected for waterhemp which has decreased sensitivity to glyphosate. However, the overall response of populations of waterhemp has not shifted dramatically towards glyphosate insensitivity. There seems to be a gradual shift from glyphosate sensitive- to glyphosate insensitive-waterhemp. If the shift from glyphosate-sensitive to-insensitive waterhemp continues at the current rate and selection continued with a reduced rate of glyphosate, over 80 generations would need to occur to achieve glyphosate resistance. However, waterhemp populations which are extremely sensitive to glyphosate still exist in production fields. This is good news, considering the forced outcrossing mating system of waterhemp. These glyphosate-sensitive plants may be crossing to glyphosate-insensitive plants and therefore making next seasons waterhemp susceptible to control practices.

Current research is aimed at understanding the heritability of glyphosate responses in waterhemp.  Using five waterhemp populations with reduced susceptibility to glyphosate, we are trying to determine if repeated cycles of selection among these populations will result in a continued reduction in control.  Preliminary results suggest that glyphosate responses in waterhemp involve multiple genes, with a significant genotype by environment interaction.  Nevertheless, the right combination of alleles/genes, in waterhemp may confer substantial levels of glyphosate insensitivity.

 
Department of Crop Sciences
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension
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