In the past 10 days, Aaron Hager with the University of Illinois reports he and his fellow researchers have received more calls and inquiries about waterhemp plants that have survived applications of postemergence herbicides. The most common scenario is having a "noticeable" percentage of plants survive after applying glyphosate in soybean (at rates from 0.75 to 1.5 lb ae/acre). Many producers have said that within about 7 to 10 days after glyphosate was applied, it became obvious the waterhemp plants would survive. In some instances it appears that survival could be attributable to an application rate too low for the size of plants, to rainfall soon after the application, or to poor coverage of the target vegetation. In other instances the best explanation appears to be the evolution of a glyphosate-resistant population. Remediation of resistance situations will likely be challenging, and it is altogether possible that herbicidal control of surviving plants will not be achievable.
Options to control surviving plants, whatever their herbicide sensitivity/resistance profile, include interrow cultivation and hand roguing. Some may scoff at the suggestion, but in many areas of the mid-south and southeastern United States, these tactics represent the few remaining viable options to manage emerged populations of herbicide-resistant weeds.
Viable herbicide options for control of surviving waterhemp plants depend on their resistance profile. Keep in mind that more than one type of herbicide resistance may be present in any given field. For example, surveys in 2010 (Figure 1) indicated that about a third of glyphosate-resistant waterhemp populations also demonstrated resistance to PPO inhibitors, and virtually all were also resistant to ALS inhibitors. If plants survived for a reason unrelated to herbicide resistance, retreating them with glyphosate could provide effective control. Be sure to select an application rate appropriate for the size of the target plants (up to 1.5 lb glyphosate ae/acre/application), to include NIS (if recommended on the glyphosate product label) and/or AMS, and to apply at a spray volume sufficient for good coverage of the target vegetation.
If the surviving plants are in fact resistant to glyphosate, retreating is not likely to provide much control. Glyphosate-resistant waterhemp frequently survives treatment with glyphosate at rates far exceeding those allowed by label. Figure 2 presents data collected from previous field research on a glyphosate-resistant waterhemp population. The first two groups of columns report control following sequential applications of glyphosate (0.75 lb ae followed by 1.5 lb ae, or 1.5 lb ae followed by 0.75 lb ae); whether the higher rate was applied first or second, control did not exceed 65% one month after application.
PPO-inhibiting herbicides are the final option for control of glyphosate-resistant waterhemp. Research has demonstrated that products containing fomesafen, lactofen, or acifluorfen can provide good to excellent control of waterhemp, but control is generally greatest before plants exceed 5 inches tall. If you will be using one of these products in an attempt to control glyphosate-resistant waterhemp, be sure to apply at the full recommended rate with the appropriate spray additives and with recommended spray tips and application volume. PPO inhibitors do not translocate extensively once absorbed into the target weeds, so thorough spray coverage is essential. Be aware that these active ingredients may not provide sufficient control of waterhemp that survived the initial postemergence herbicide, either because the plants might simply be too large or because they might also be resistant to PPO inhibitors.
For waterhemp plants resistant to both glyphosate and PPO inhibitors, no viable postemergence herbicide option exists for use in conventional or glyphosate-resistant soybean varieties. Glufosinate, used in conjunction with a glufosinate-resistant soybean variety, can provide good to excellent control of waterhemp resistant to glyphosate and/or PPO inhibitors.
As we have noted in other articles, waterhemp plants resistant to multiple herbicide families will become increasingly common in Illinois. Weed scientists at the University of Illinois will screen waterhemp at no cost if you would like help determining the herbicide resistance profile of waterhemp populations in your fields. Please see "Screening Waterhemp for Glyphosate Resistance" in issue 10 of the Bulletin for details.