Waterhemp survives after postemerge herbicide
click image to zoomFigure 2. Control of glyphosate-resistant waterhemp with sequential herbicide applications (data collected in 2008). 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.
click image to zoomThis waterhemp plant survived 12 lb glyphosate ae/acre in field research trials. 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.