Ohio Fall Weed Survey Follow-Up

By: Harold Watters, Ohio State University Extension Agronomic Systems Field Specialist

So I got some calls after our Extension Fall Weed Survey — if these are the problem weeds, then how do you deal with them?

It is becoming apparent that with the move to herbicide tolerant crops, we aren’t necessarily getting rid of all of our weeds — only 30% of our fields are weed free. Giant ragweed moved back into first place for worst weed, seen in 34% of fields overtaking marestail seen in 30% of fields. And then there is the pigweed problem — waterhemp appeared frequently, so did redroot pigweed and then there are the concerns about Palmer amaranth and its escape across Ohio.

So how do we deal with problem weeds? I look in the back of the Ohio, Indiana and Illinois Weed Control Guide — all of these weeds are there, so it’s not just a problem for you but these appear to be problems across the eastern Corn Belt as well. Mark Loux has the weed control guide posted on his website: https://u.osu.edu/osuweeds/, along with several other resources.

• Herbicide classification chart: If you know what herbicide class is missing the weed, then this will help you choose another direction for your campaign.
• Pigweed (amaranth) ID factsheet: You have to scout to know what’s out there — then you have to identify it. Once upon a time we had a whole industry that spent their time scouting, that’s gone but now you can be that crop scout.
• Palmer amaranth in OH factsheet: Where have they been spotted? How to keep them off your farm? And learn how to remove them. “Go Rogue” is the new phrase from the university weed science team — and to get rid of the seed (a must) then you are likely to have to remove them by hand.
• Marestail control factsheet: For those of us who have had to deal with this for 16 years, we know the drill: use a fall burndown, then a spring burndown with residual, and use multiple modes of action with those pre-emergent herbicides. If you miss it with your pre-emergent herbicide then you better have a LibertyLink variety or Extend beans because glyphosate will not take it down (and neither will other post herbicides in soybeans).
• Herbicide Resistance Screening: We don’t do that much anymore. Contact the University of Illinois for pigweed species screening. We can probably help you with identification, but likely you will need to send the sample you wish to have screened for resistance to Illinois: https://web.extension.illinois.edu/plantclinic/downloads/herbicide.pdf.

What else is in the Weed Control Guide? I tell farmers in meetings that you need to think like a weed to understand how they grow and how to control them. The guide starts with “Weed Control Principles” and for the first 23 pages that’s what it covers. It’s about the longest chapter in the guide, and yet no one reads it. Start there, then go to “Control of Problem Weeds” starting on page 182. After you read those sections, then I’ll allow you see the charts for herbicide selection. Read those last, even though these are the pages that most people start with first.

Some other useful information is to be aware of how much we have overused our (very effective) herbicides. These three images come from Ian Heap at Weedscience.org. He is the guy that tracks resistant weed development across the world. How do we “develop” resistance? Only after exposure to a herbicide do we make selections that will reproduce, and then become the dominant population. Typically, it is a very small number of weeds within a species that have the genes to make it resistant, then with the herbicide pressure we place on the weed, there is a shift in the population until there are a bunch of resistant weeds growing in your field.