Problem: Waterhemp is becoming resistant to everything
- Waterhemp is a dioecious species, with separate male and female plants. Cross pollination results in tremendous variability within populations, which is evident from differences in plant color and other characteristics.
- This genetic variability results in a relatively high frequency of mechanisms that can impart resistance to herbicides, enabling waterhemp to develop resistance more rapidly than most weeds.
- Waterhemp can produce over 1,000,000 seeds per plant.
- The abundant seed from a few resistant plants can rapidly shift the response of a population to an herbicide, increasing the risk of failure the next time that herbicide is used.
|Figure 1. Seedheads illustrating inherent genetic variability within a waterhemp population.||Figure 2. Illinois multiple-resistant population surviving 2,4-D.|
Which herbicides is waterhemp resistant to in Ohio?
- Site 2 (ALS), all populations
- Site 9 (glyphosate), most populations, some in eastern Ohio still sensitive
- Site 14 (PPO), 30 to 50% of populations in areas with longest waterhemp history, lower elsewhere
- Multiple resistance, populations resistant to site 14 are resistant to sites 2 and 9 also
- Only two POST options for multiple-resistant populations: dicamba (Xtend) or glufosinate (LL)
Which herbicides is waterhemp resistant to in the Midwest?
- In areas with long history of waterhemp, populations have developed multiple resistance to herbicide from up to six sites of action.
- In 2018, a Missouri population was determined to have six-way resistance:
- Site 2 (ALS), Site 4 (auxin inhibitor—2,4-D), Site 5 (PSII—atrazine), Site 9 (glyphosate), Site 14 (PPO), Site 27 (HPPD)
- Populations with five- or six-way resistance have several resistance mechanisms occurring concurrently:
- Target site mutation: three separate mutations that impart resistance to sites 2, 9, and 14
- Enhanced herbicide metabolism: mutations that impart resistance to sites 4, 5 and 27
Ohio waterhemp populations will continue to develop multiple resistance with these same characteristics without appropriate management to slow this phenomenon.
Steps to reduce selection for resistance and preserve herbicide utility
- Know whether population is already resistant to glyphosate and site 14 herbicides.
- Have population tested for presence of resistance
- University of Illinois, $50 per field
- Use a combination of PRE and POST herbicide applications.
- Use PRE herbicides rated 8 or 9 in the “Weed Control Guide for Ohio, Indiana, and Illinois”
- Combinations or premix of several PRE herbicides can extend the duration of residual control
- Apply early POST when weeds are small and add residual herbicide.
- Reduces or prevents the need for a second POST application
- Residual products containing one of the following:
- Acetochlor—Warrant, Warrant Ultra
- Metolachlor—Dual II Magnum, Prefix, others
- Pyroxasulfone—Zidua, Anthem Maxx
- Diversity traits and herbicides.
- Use different herbicide sites of action between corn and soybeans
- Avoid repeated use of the same POST herbicide(s) throughout the rotation
- POST applications—use two sites of action that are still effective on waterhemp. Possible examples:
- glufosinate + fomesafen (LL soybeans)
- mesotrione + atrazine (corn)
- glufosinate + atrazine (LL corn)
- Integrate cover crops to reduce the population and selection for resistance.
- Up to 50% reduction in waterhemp population
- Use cereal rye, wheat, or barley as a base
- Terminate cover close to time of planting or after, to extend effect into the season
- Scout after the final POST herbicide and into late season. Remove waterhemp plants.
- It is essential to prevent seed from plants that may have survived herbicide treatment
- Herbicide-resistant plants cannot be allowed to go to seed
- It is not possible to prevent resistance by management of herbicides alone