Managing Waterhemp With A Devious Nature

As you think ahead a few weeks, and make your herbicide plans, you remember that nagging patch of waterhemp that just won’t go away. You have tried to control it (kill it) with a seemingly toxic shower of glyphosate, but the waterhemp just keeps growing. At some point you have to realize there is a need to manage it (live with it as best you can.) So how do you manage waterhemp that is resistant to glyphosate? (Glad you asked!)

The number of acres of corn and soybeans treated with glyphosate increases annually. And that trend is paralleled with the trend of weeds that are resistant to glyphosate, both in number of species, and in number of patches within that specie. That is not rocket science. And neither is the assembly of a plan to manage waterhemp that is glyphosate resistant. It is offered by weed specialist Aaron Hager at the University of Illinois.

Hager’s plan is based on the need to control waterhemp in Roundup Ready soybeans. He says if you know what types of herbicides your pesky waterhemp will resist, then post-emergent herbicide selection and utilization can be made before planting. Hager knows your waterhemp may be resistant to an ALS-inhibiting herbicide. In addition to your glyphosate resistant waterhemp, there is also waterhemp that is resistant to PPO inhibiting herbicides. He anticipates the near-term discovery of waterhemp that is resistant to both glyphosate and PPO inhibitors, which he says is a “worst case scenario.” That means there are no post emergent herbicides that will kill waterhemp. But there is something that could be done to help your problem and that is any practice that will hasten crop development and early canopy development. Hager’s recommendations are:

1) Apply a full rate (according to label guidelines for soil type and organic matter content) of a soil-residual herbicide no sooner than 7 days before planting or no later than 3 days after planting. This may work to curtail germination of waterhemp seeds, and a full rate will last longer to catch some of the late germinating waterhemp seeds. Any time you can delay waterhemp growth and maturity means the less competition for your soybeans. To minimize potential injury to soybeans, make your application no later than 3 days after planting.

2) The initial postemergence application of glyphosate (alone at 0.75 to 1.0 pound acid equivalent per acre) must be made when waterhemp is 3 to 5 inches tall. Since there is limited data on waterhemp control with a glyphosate and PPO tankmix, and few answers regarding additives, the recommendation is for a singular application. Resistant waterhemp will not be controlled at more than 50% efficiency with the labeled in-crop application rate or maximum labeled rate. However, if resistant waterhemp are under 5 inches, they are sensitive to the 0.75 pound a.e. rate. Closely monitor waterhemp that survive that application rate at that size.

3) Fields must be scouted 7 days after the initial glyphosate application to determine treatment effectiveness. Resistant waterhemp will continue to grow following treatment, but susceptible waterhemp will have wilted in that time period. However, at the end of the growing season it is impossible to tell the difference between waterhemp that germinated after the glyphosate was applied and the waterhemp that resisted the glyphosate.

4) If waterhemp control is inadequate and retreatment is necessary, consider applying a PPO-inhibiting herbicide (lactofen, fomesafen, or acifluorfen) at a full labeled rate (with recommended additives) as soon as possible. PPO inhibiting herbicides are the only recourse for glyphosate resistant waterhemp. By this time, the waterhemp may be 8 to 12 inches tall.

5) Rescout the treated field within 10 to 14 days to determine effectiveness of the PPO-inhibiting herbicide treatment. If scouting reveals that plants treated with a second herbicide application might survive, implement whatever tactics are available or feasible to rogue these surviving plants from the field before they reach a reproductive growth stage. The PPO resistance gene in waterhemp is transmitted by pollen, so it is important to physically eliminate plants before flowering. That same mode of transmittal is not known regarding glyphosate resistance. Since female waterhemp plants can produce in excess of one million seeds, running them through a combine is counterproductive, needless to say.

Summary:
The widespread use of glyphosate has allowed an increase in the number of waterhemp plants that are resistant, and because their ability to multiply is of Biblical proportion, it is important to manage them at their most sensitive stage. That stage is the 3-5 inch mark, with first a glyphosate application, then soon after with a PPO-inhibitor, if needed. Survivors should be physically removed from the field. An integrated system of herbicides and scouting is needed for management of glyphosate resistant waterhemp.