Managing fall weeds in your fields

Tony Nye - OSU Extension

This past week, I completed the annual soybean weed survey across Clinton County taking observations from 100 soybean fields (approx 4,250 acres).

Overall, we are doing a satisfactory job with weed control in soybeans as 43.0% of the fields were rated “clean.” Other fields that had not rated as clean were rated based on the level of weed infestation such as: occasional weeds present, large patches of weeds or weeds were widespread throughout the field.

Giant Ragweed has claim to our #1 weed control issue in soybeans with 34.0 percent of the fields surveyed having giant ragweed. The good news is that is down from surveys done in 2018 and 2019 for the county.

The #2 weed observed is marestail with 15 percent of the fields surveyed having it. That too is down from the past couple of years.

The third-most weed observed in field was volunteer corn at 13 percent.

The concern I have is the number of other weeds observed throughout the county that included common ragweed, giant foxtail, redroot pigweed, common lambsquarter, water hemp and pokeweed.

With weeds on my mind, I wanted to share some thoughts, Mark Loux, our Ohio State University weed specialist provided in a recent CORN newsletter regarding fall herbicide tretments.

According to Loux, he has seen a drop off of fall herbicide treatments over the past several years for a couple of reasons, among them the effectiveness of new soybean trait systems for managing marestail, some generally crappy weather in late fall, and efforts to reduce input costs.

We are seeing a resurgence in some weeds, such as dandelion, which respond well to fall herbicides, though. Some growers have also experienced issues with messy fields and late spring burndowns that could have been avoided with fall herbicides.

Here is a history lesson how fall herbicide treatments can be of benefit to managing weed issues in our operation.

According to Loux, in the late 1990s, a few years after the initial introduction of Roundup Ready soybeans, a number of growers were experiencing problems in spring with dense infestations of winter annual weeds such as: chickweed, purple deadnettle, mustards, cressleaf groundsel, and also dandelion.

These weeds were generally interfering with spring tillage and crop establishment, slowing the drying and warming of soils, and harboring insects.

Spring burndown herbicides could be variably effective and, under cool conditions, slow to kill the weeds. One of the reasons for the increase in these weeds was the use of only glyphosate in soybeans, and the oversimplification of herbicide programs.

This included a failure to apply burndown early enough which allowed winter annuals to go to seed, and a failure to include residual herbicides, some of which could possibly persist long enough to shut down some of the late-season winter annual weed emergence.

This approach also allowed dandelion to proliferate and become more difficult to kill, because it had too much time to increase it’s root size and go to seed unimpeded.

Application of herbicides in fall largely solved these issues, providing for a weedfree seedbed well into spring, and reducing dandelion back to manageable levels.

Fast forward to the mid-2000s when glyphosate-resistant marestail became widespread.

Loux continues, while the spring-applied mix of glyphosate and 2,4-D worked for a while on marestail, the increase in the level of glyphosate resistance shifted all of the work to the 2,4-D, which is really only about 70% effective on overwintered marestail. The net result was a failure of many burndown treatments for control of the overwintered plants.

The solution was fall application of 2,4-D mixtures, which controls fall-emerging plants, so that the spring burndown only has to control small spring emerging weeds. Fall herbicide treatments have been a standard component of marestail management programs for many growers since then. More recently, the availability of some alternative spring burndown treatments that can include Sharpen, glufosinate, Gramoxone, and/or dicamba have reduced the need for fall herbicide treatments on marestail.

A consequence of this, along with a move once again to oversimplify herbicide programs, appears to be an increase in dandelion and winter annuals again. The Xtend, Enlist, and LibertyLink soybean programs cannot adequately control some of these weeds if not used in an integrated, multi-application system that includes an occasional fall herbicide treatment.

Bottom line is that fall-applied herbicides, even if used only every other year or so, go a long way toward preventing issues with these weeds and maintaining a more problem-free no-till planting situation. This can be especially true when wet weather in spring delays herbicide application and planting, and the result is a big, dense weed population that herbicides struggle to control.

Fields with a fall herbicide treatment are likely to stay much more manageable into late spring compared to those without.

Keep in mind, this is just a suggestion to think about if you plan on making fall herbicides part of the weed management program again, and especially where the increase in winter annual weeds, dandelion, wild carrot, and curly dock has been noticeable and problematic.

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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.

Tony Nye

OSU Extension