Invasive weed farmers don’t want


Tony Nye - OSU Extension



What a great feeling it was this week to be in a meeting that was face-to-face with farmers from around Clinton County!

It’s been since March 2020 that I’ve been able to do this due to COVID-19 restrictions. It was great to interact in-person with farmers about soil fertility, effective herbicide use, weed ID and control as well other related items.

One of the discussions we dealt with specifically during this meeting was waterhemp, including weed identification, effective herbicide options and control. This, along with Palmer amaranth, are two of our most feared weeds in crop production.

According to Mark Loux, Ohio State University weed specialist, 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:

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 (give) 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.

Weed specialists aren’t sure what the correct recommendation is for stewardship of herbicides once a single mutation can confer resistance to multiple sites of action. That’ss 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 the study by OSU Extension Educators or clientele.

For the first several years scientists focused on the possible resistance to glyphosate and group 14 herbicides. Essentially all waterhemp populations are resistant to group 2 herbicides (ALS inhibitors) so there isn’t any point in looking for it – it’s assumed.

Starting with 2019 populations, OSU scientists 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 no point in looking for it.

Overall, the data show that Ohio waterhemp populations vary in their sensitivity to these herbicides.

For all 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 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.

The reality of this weed is if you have not experienced it in your farming operation you probably will. So, based on this information I encourage producers to take some time now to learn what waterhemp looks like, learn what herbicides are showing success and providing effective control and plan ahead for effective control strategies.

During the growing season, utilize residual herbicides in corn and soybeans, and scout fields from mid-July into early September.

The ultimate goal is to not have waterhemp establish itself in your fields but if found, eliminate plants from the field to prevent seed and above all do not run it through the combine spreading seed to other fields.

A great place to start learning more about this invasive weed and many others is going to the OSU Weed Science Website at http://u.osu.edu/osuweeds here you will find many resources including bulletins, pictures for ID and videos to help with the management of invasive weeds such as waterhemp.

Another great resource to have is the 2021 Weed Control Guide for OH/IN, OSU Extension Bulletin 789, this is available through the Clinton County Extension office by calling (937) 382-0901.

Tony Nye is the state coordinator for the Ohio State University Extension Small Farm Program and has been an OSU Extension Educator for agriculture and natural resources for over 30 years, currently serving Clinton County and the Miami Valley EERA.

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Tony Nye

OSU Extension