Weed Management in Ohio, Update 2019

By: Harold Watters, OSU Extension Ag Crops Field Specialist

Our OSU Extension AgNR educators observed soybean fields across the state again this fall to see what was out there for our annual fall soybean weed survey. I was supposed to share this early enough so you could at least get a fall application on to get a head start on controlling marestail, but it seems we have more problems than that to deal with.

Statewide our most frequently observed weed problem was again marestail. It was present in 36% of the fields. The second most likely observation was weed-free — at 29% of the fields. That’s a big jump over several years ago, and likely due to LibertyLink, Enlist, and Extend soybeans. Third, fourth and fifth places in a three-way tie were giant ragweed, volunteer corn and then giant foxtail (or just generic grass) — all in about 19% of the fields. Next, and getting ever more widespread, is waterhemp at 15% of the fields across the state.

I also split the state into regions to see if maybe some were worse off than others. This is in Table 1.

Table 1. The table below shows the number of fields observed in each region, the percent of fields without weeds and weeds observed ranked by appearance.

No area is without some resistant weed. All areas of the state except the southwest had lower numbers of weed-free fields than they had last year. Waterhemp and the pigweed species (Amaranthus) generally are looking a little scary in western Ohio. We know these guys are resistant to numerous herbicides already, and will likely become resistant to more. Mark Loux, OSU Weed Scientist, says mix it up and to rotate herbicide programs across all the packages that work. Don’t rely on just one or you will be the one who Mark comes to visit to see just how you blew it.

Who conducts the Fall Soybean Weed Survey by driving 80 miles on the road in each county? In Table 2, is a listing of the counties in the survey, the Extension educator, the number of fields and the acres they checked on.

Table 2. County, educator, acres and field number by county in the 2019 Fall Soybean Weed Survey.

That is about 2,000 fields and 100,000 acres sampled to make these observations. You can see in the list we had a significant number of counties and fields surveyed — enough that we have a good idea of what is happening in each region of the state. The OSU folks will report on local results and answers as we have our winter programs.

What works?

As I toured Ohio soybean growing areas over the summer, I heard remarks from growers on what worked well for them. They reported the efforts they have gone to that reduced their weed problems in soybeans. Many found good yielding Liberty varieties and are happy they went all LibertyLink. Some also tried the Enlist technology and more also moved to Extend beans this year.

This is the list that works — and sounds an awful lot like the recommendations of Mark Loux our Ohio State University Weeds Specialist.

1. Apply a fall burndown that includes 2,4-D.. plus dicamba, plus glyphosate, or whatever – just don’t spend the money now on a residual, especially for marestail control. It’s likely too late in January.
2. Increase the use of metribuzin. Always a residual in the spring, even on worked ground.
3. Consider a switch to LibertyLink varieties, and use due diligence on the other herbicide technologies.
4. Use of full rate of pre-emergent herbicide at planting in the spring. Even on the worked ground.
5. I added number 5 in 2018 and updated it again for 2020. Consider dicamba or 2,4-D resistant soybean varieties.

• But a couple of items have come upon this option — the formulations labeled for soybeans are restricted use herbicides because we had some herbicide movement in all of the past three years.
• This means you need to have a pesticide applicator license, and take continuing education classes with the manufacturer on managing drift, volatility and the environment.
• And your likely target weeds — marestail, giant ragweed, and waterhemp — have all shown a great genetic capacity for evading control.

To learn more about managing weeds in Ohio, attend your local county Ohio State University plant health recertification program. It was the Pesticide Recertification program but now includes fertilizer as part of the updates so I am calling it to plant health recertification. Also, recertification is now a four-hour program, up from three hours we had in the past.

Winter programs?

Both Pesticide Applicator license holders and Fertilizer Applicators will attend recertification programs at the same time and place, check the PestEd website https://pested.osu.edu/privaterecertification for a program near you.

And as we get closer to the winter meeting season we will post those regional and area agronomy and update meetings. Our Agronomic Crops Team calendar is pretty thin now but will be full by the first of the year. Check our website: http://agcrops.osu.edu/events/calendar, for the events and their locations.