Introducing QuadStack™ Waterhemp
Department of Crop Sciences
Michael Bell, Graduate Research Assistant
Waterhemp (Amaranthus tuberculatus, syn. rudis) presents numerous management challenges to Illinois corn and soybean producers, not least of which is its demonstrated ability to evolve herbicide resistance. As shown in the figure, waterhemp has continued to “stack” different herbicide-resistance genes. The first case of multiple resistance in waterhemp appeared in 1996, with multiple resistance to photosystem II inhibitors (e.g., atrazine) and acetolactate synthase inhibitors (e.g., the IMIs and SUs). More recently, a waterhemp biotype from western Illinois was confirmed to contain resistance genes for four different herbicides/herbicide groups: glyphosate, PS II inhibitors, ALS inhibitors, and protoporphyrinogen oxidase inhibitors (e.g., diphenylethers).
Although there remain several preemergence herbicides that are effective on waterhemp in either corn or soybean, waterhemp’s continuing evolution of herbicide resistances is seriously limiting effective postemergence options, particular for soybean production. Currently, the only POST option in soybean to which waterhemp has not yet evolved resistance is glufosinate (which of course requires the use of glufosinate-resistant soybean varieties).
Farmers in Illinois continue to rely on glyphosate for POST control of waterhemp and other weeds in soybean. As glyphosate-resistant waterhemp becomes more prevalent in the state, we expect an increasing number of waterhemp-control failures with this approach. Thus, we have conducted research to determine the likelihood that other currently available POST soybean herbicides will be effective on glyphosate-resistant waterhemp populations.
We found that all glyphosate-resistant waterhemp populations also contain resistance to the ALS inhibitors. Most farmers now accept the fact that resistance to ALS inhibitors is widespread in waterhemp, so this did not come as a surprise. However, what was surprising – and somewhat disconcerting – was that 40% of the glyphosate-resistant populations also contained resistance to the PPO inhibitors.
Also of interest is determining whether the multiple herbicide resistances present in these populations exist only at the population level, or also at the individual plant level. In other words, if a population contains multiple resistance to glyphosate and PPO inhibitors, does it possess some plants that are resistant just to glyphosate and others that are resistant just to PPO inhibitors, or does it contain individual plants that are resistant to both of these herbicides? The latter is a more problematic situation: a tank mix or sequential applications of glyphosate and a PPO inhibitor has the potential to control a multiple-resistant “population”, but not a multiple resistant individual. Of 55 glyphosate-resistant waterhemp plants from 10 populations, 64% were resistant to ALS inhibitors and 13% were resistant to PPO inhibitors. Nine percent of these glyphosate-resistant plants were resistant to both ALS and PPO inhibitors. Thus, the multiple resistance in these populations clearly exists at the individual plant level.
Extrapolating the results of our multiple-resistance survey, if you encounter a glyphosate-resistant waterhemp population, you should not expect a POST application of either an ALS inhibitor or a PPO inhibitor to provide greater than 90% control. This is one of the reasons we continue to urge producers to utilize PRE herbicides in their weed management programs.
Unfortunately, the story of multiple herbicide resistance in waterhemp does not end here. We fully expect that the frequencies of multiple resistance we found in 2009 populations are already higher in 2010, and will continue to increase. Furthermore, we expect that waterhemp will continue to evolve resistance to other herbicides, and stack these new resistance genes with those already present. In fact, a report of a new case of herbicide resistance in waterhemp may be just down the road… literally.