Waterhemp populations across the Midwest continue to develop more complex variations of herbicide resistance. Multiple resistance to an increasing number of herbicide sites of action is the norm in many populations in states west of Ohio. Waterhemp has on the whole developed resistance to seven sites of action, including the following:
Group 2 – ALS inhibitors – chlorimuron, imazethapyr, etc
Group 4 – Synthetic auxins – 2,4-D, dicamba, etc
Group 5 – Photosystem II inhibitors – atrazine, metribuzin, etc
Group 9 – EPSP synthase inhibitor – glyphosate
Group 14 – PPO inhibitors – fomesafen, flumioxazin, sulfentrazone, etc
Group 15 – long chain fatty acid inhibitors – metolachlor, pyroxasulfone, etc
Group 27 – HPPD inhibitors – mesotrione, isoxaflutole, topramezone, etc
Individual populations with resistance to three or more sites of action are common. Mutations are occurring that confer resistance to several of these sites of action simultaneously, through a resistance mechanism that enhances the metabolism and inactivation of the herbicides by the plant. For example, there appears to be a linkage in the resistance to mesotrione and atrazine, where resistance to one means it’s likely that resistance to the other occurs also. Weed scientists have concluded that this weed is capable of developing resistance to any herbicide site of action used against it. We aren’t actually sure what the correct recommendation is for stewardship of herbicides once a single mutation can confer resistance to multiple sites of action. Which is the reason we stress the need to take steps in mid to late season to prevent seed from plants that survive management strategies.
Since 2016, OSU weed scientists have been taking steps to maintain a rough assessment of the herbicide resistance characteristics of Ohio waterhemp populations. Some of these populations were randomly collected during our surveys and some provided to us by OSU Extension Educators or clientele. For the first several years we focused on the possible resistance to glyphosate and group 14 herbicides. Essentially all waterhemp populations are resistant to group 2 herbicides so there isn’t any point in looking for it – it’s assumed.
2016 – 18 populations
Glyphosate – 100% resistant, group 14 (fomesafen) – 28% resistant
Glyphosate + group 14 – 28% resistant
Some populations resistant to low rate of atrazine (but not metribuzin)
2017 – 13 populations collected in fields from plants surviving group 14 herbicides
77% of the populations were resistant to group 14 herbicides (did not test for glyphosate)
2018 – 8 populations
Glyphosate – 87% resistant, group 14 (fomesafen) – 25 to 50% (rate-dependent)
Glyphosate + group 14 – 25 to 50% resistant (rate-dependent for group 14)
Starting with 2019 populations, we have expanded to assess response of waterhemp populations to foliar applications of 2,4-D, and groups 5 (atrazine), 14 (fomesafen), and 27 (mesotrione). Our assumption at this point is that most waterhemp populations are glyphosate-resistant so there’s point in looking for it. We assessed response of 19 populations collected in 2019, and the results below show assessment of % mortality from each herbicide/rate combination. (1X and 4X indicate a use rate and four times that rate).
We are also conducting soil-applied screens for group 15 herbicides (s-metolachlor). In the table below, assessment at 14 and 28 days after preemergence treatment was based on % control compared with a nontreated for each population.
Some observations on these data:
- Herbicides do not necessarily work the same in the field versus the greenhouse, so results can vary between them for a given rate. It’s evident here that the 1X rate of the 2,4-D and S-metolachlor were possibly not truly a use rate in the greenhouse. This doesn’t change the fact that there was variable response among populations.
- Overall, the data show that Ohio waterhemp populations vary in their sensitivity to these herbicides. For all of the herbicides, at least some populations were resistant to the 1X rate and partially resistant to the 4X rate. We assume this is an evolved lack of response that is developing over time in some fields in response to the use of these herbicides, and also movement of seed from field to field. We expect this to happen, based on the history of resistance in areas west of us with a longer history of waterhemp resistance problems.
- There could not really have been much selection by 2,4-D in these fields prior to 2019, based on the Enlist soybean adoption timeline. So the lack of response of some populations to this herbicide may be due to a mechanism that confers resistance to multiple sites of action. A population from Illinois was identified several years ago where on mutation conferred resistance to atrazine, mesotrione and 2,4-D.
- Some populations were completely sensitive to all of these herbicides, and other populations had a reduced response to all. In one Darke County population, mortality from foliar applied herbicides did not exceed 60% at the 1X rate, and ranged from 77 to 96% at a 4X rate. Control from S-metolachlor did not exceed 60%. Darke County is one of the counties with the longest history of waterhemp issues, so selection for resistance has occurred for a while. But – another Darke County population was still sensitive to all herbicides.
- The populations tested are a composite sample from several plants at a field site, so we assume that where some degree of resistance occurs, there are individual plants that may be mostly resistant and others that are still susceptible.
We are in the process of screening populations collected in 2020. We are also part of a regional project that screens waterhemp populations from various states for resistance to glufosinate and dicamba. So far, that project has not identified resistance to either herbicide in the populations sampled from any state, although dicamba resistance has independently been confirmed in other non-Ohio waterhemp populations.
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