Herbicide Residue Considerations for Fall Cover Crop Establishment

Source: Alyssa Essman, Mark Loux, OSU

Herbicides with residual that are used in corn and soybeans can affect the establishment of fall-planted cover crops, and should be taken into account when planning cover crop practices and selecting species. Soil characteristics and weather also play a role in the persistence of residual herbicides, which can vary by field and year. More information is needed on rotational intervals for many cover crop species, and this information is often not included on herbicide labels. University weed scientists have studied the effect of residual herbicides on some of the most popular cover crop species in order to provide this information to growers. In general, residual herbicides that control grass weeds can hinder establishment of grass cover crop species. Broadleaf cover crop species are most impacted by group 2 (ALS inhibitors), 5 (PSII inhibitors), 14 (PPO inhibitors), and 27 (HPPD inhibitors) herbicides (Purdue University).

A multi-state study found that the general order of sensitivity of cover crops to herbicide carryover, from greatest to least sensitive, is:

  • Tillage radish > Austrian winter pea > crimson clover = annual ryegrass > winter wheat = winter oats > hairy vetch = cereal rye.

Soybean herbicides that tended to be most injurious were:

  • Fomesafen, pyroxasulfone, imazethapyr, acetochlor, and sulfentrazone.

Corn herbicide treatments that were most injurious to cover crops were:

  • Topramezone, mesotrione, clopyralid, isoxaflutole, pyroxasulfone, and nicosulfuron

(University of Missouri).

Below is a table of commonly used corn and soybean herbicides, the fall cover crops that are safe to plant in rotation, and cover crop species that may be injured following these herbicides (Adapted from Lingenfelter D. and Curran W., Penn State University).



Cover crops provide a multitude of benefits and their use is becoming an increasingly popular practice in Ohio. Including cover crops in rotation with agronomic crops to realize these benefits costs time and money. It is important to evaluate the potential risk of herbicide residue on the establishment of cover crops in order to ensure success. Residual herbicides applied at the time of planting typically interfere with cover crop establishment less than those applied POST. Weather can affect the persistence of herbicides also, especially rainfall in summer.  The risk of residual herbicides affecting cover establishment will be higher in areas that have been dry since herbicide application.  Risk will be lower where the herbicide application was followed by some wet weather to get herbicide degradation started, compared with an application during prolonged dry weather.  One of the least problematic cover crop species is cereal rye, which can be successfully established following a late corn or soybean harvest, and is tolerant to a most of the most commonly used corn and soybean herbicides. Weed control should continue to be the priority in selecting herbicides, and cover crop species selection should be based on potential injury and goals for the use of cover crops.  The introductory section of the “Weed Control Guide for Ohio, Indiana, and Illinois” has some of the same information presented here, and OSU weed scientists also summarize this in a video: https://www.youtube.com/watch?v=ylr0zGnXMfs

The following resources contain information on residual herbicides and cove crops also:

https://extension.psu.edu/corn-herbicides-and-rotation-to-cover-crops https://extension.psu.edu/soybean-herbicides-and-rotation-to-cover-crops



Noxious Weeds in Cover Crop Seed and Seed Germination

Source: Alexander Lindsey, Laura Lindsey, Mark Loux, Anne Dorrance, Stan Smith, John Armstrong, OSU Extension

Seed quality is key to establishing a good crop (or cover crop). Some of the critical components of seed quality are percent germination, mechanical analysis for purity (% other crops, % inert, and % weeds), and a listing of noxious weeds identified by scientific/common name and quantity found. As producers are looking for seed sources to provide living cover on acreage this year that was previously earmarked for corn or soybeans, it is important to pay attention to the quality. These tests may also be required on seed lots for use in some relief programs as well. Commercial or certified seed used for cover crops should have a seed tag that shows variety and the seed quality measurements above. However, if the seed is sourced from out of state, the noxious weeds listed (or NOT listed) on the tag by name may differ from those had the seed been sourced from Ohio.

Only the noxious weeds for the state where the seed was originally going to be sold are required to be listed on the tag by name and quantity (Federal Seed Act, part 201.16). Each state determines which species are included on this list, and can differ from state to state. If seed is outside of Ohio for use on-farm, producers may want to have the seed tested for an “all state noxious-weed exam” prior to planting if this was not done previously on the seed lot. Only 1.1-1.2 lbs of seed is needed for the test, but it is critical the sample is representative of the lot to ensure quality test results. This test would screen the seed sample supplied for the weed contained in this list: https://www.ams.usda.gov/sites/default/files/media/StateNoxiousWeedsSeedList.pdf, and may serve as a more comprehensive exam than was conducted at the time of initial seed lot labeling. One service provider that can conduct this exam is Central Ohio Seed Testing (a subsidiary of the Ohio Seed Improvement Association; https://ohseed1.org/about-our-lab/). Samples can also be sent to ODA for an Ohio noxious weed exam (https://agri.ohio.gov/wps/portal/gov/oda/divisions/plant-health/grain-warehouse-feed-and-seed/). Depending on the source of seed and the planned use, a seed lot may be eligible to be tested for free through ODA between June and December (up to three per farmer). Conducting a noxious weed exam could help slow the movement of problematic weeds throughout the state and minimize future weed problems.

Another issue to consider is the quality of seed in storage that was not planted this year due to weather. Storing seed in an environment where the temperature (in F) plus the % relative humidity are less than 100 (Harrington’s rule) helps to minimize the rate of seed deterioration (or loss in germination and vigor). Seed germination is an important consideration for determining seeding rate to ensure the critical final stand for yield is achieved for crops like corn and soybeans. Most seed germination percentages on a seed tag for agricultural seeds (like corn and soybeans) are valid for 12 months from the last date of the month in which they were completed, with the exception being cool season grasses which are valid for 15 months beyond the month of testing (Ohio Revised Code, Chapter 907.07). Be sure to check the seed tag for both the date of the test as well as the germination when planning seeding rates.

You can search this blog for a complete description and pictures of all of the weeds on the Ohio Noxious weed list.

Current Weed Issues II: Revised Herbicide Management Strategies for Late Planting

Source: Dr. Mark Loux, OSU Extension

We’re running about a month behind in many cases, and with respect to weeds we are a month later than normal in implementing herbicide programs.  The most important thing to know about this is that we are well into the period of summer annual weed emergence, most of which occurs between early May and the end of June, which overall shortens the period of weed control that we need and allows earlier application of POST herbicides.  There are some advantages to this – here’s what it means for those fields just planted or that will still be planted within the next couple weeks:

Because we are this late, the burndown has become a major part of what is usually our in-season herbicide program, and is taking care of a good portion of the summer annuals that residual and POST herbicides would usually control.  The big glaring issue at this time is nasty burndown situations, and we provided some suggestions for this in previous articles.  Lots of pretty yellow fields due to cressleaf groundsel.  Keep in mind that this and other winter annuals that have flowered are ending their life cycles, so they have died or started dieing on their own anyway.  Focus should be more on the large giant ragweed, lambsquarters, marestail, etc that are present.  We are also late enough that waterhemp is part of the burndown mix in come fields.  Don’t skimp.

Herbicide programs do not have to last as long in crops planted late.  When we plant in early May, we need an herbicide program that controls weeds from then until the end of June or so, which is in part why we use residual herbicides and frequently apply POST herbicides 5 to 6 weeks after planting.  Applying the POST too early in a crop planted in early May introduces the risk of poor control of weeds that emerge soon after that application, before the crop is developed enough to control them on its own.  This is much less of an issue with a late-planted crop.  Since summer annual weed emergence tapers off as we move through June, the POST herbicides can be applied much sooner after planting without sacrificing control.  Planting soybeans this late can therefore allow earlier POST applications when weeds are small – more like 3 weeks after planting.  This can help minimize carryover concerns with fomesafen, and also provide a wider window to look for the right conditions to apply dicamba (see below).

In studies of reduced-rate POST applications that we conducted a couple decades ago, planting soybeans in late May or early June allowed us to use earlier POST applications (e.g 21 days after planting) at lower rates and still maintain control.  There were just fewer weeds emerging after planting and the duration of weed emergence after planting was also shorter.  It was not possible to achieve this in early-planted soybeans – we needed either two applications at reduced rate or a later application at full rates for control.

One issue with later POST sprays is the potential for herbicide carryover from products such as mesotrione and fomesafen (Flexstar, etc), among others.  Fomesafen carryover has been rare in the state but risk increases with later applications, especially if rainfall subsequently becomes limiting.  Where glyphosate and fomesafen are being combined in a late POST application to control ragweeds, it may become necessary to replace the fomesafen with lactofen (Cobra/Phoenix) as applications move into July.  Our research indicates that the lactofen products are less effective than fomesafen in this mix by about 10 to 20%.

Another concern would be POST applications of dicamba on Xtend soybeans shifted later into the hotter weather that occurs as we move from June into July.  The consensus of the weed science community is that both of the approved dicamba formulations have potential to move via volatilization, and the risk of this would increase with increasing temperatures and increased frequency and duration of inversions.  The current long-range forecast also indicates a trend for hotter than normal temperatures as we move into mid-summer.  Current labels allow application through 45 days after planting or prior to the R1 stage, whichever occurs first.  For soybeans planted early, the 45-day limit is often the main determinant, but later planted soybeans progress through growth stages more rapidly so the R1 stage may be the more frequent limitation.  Movement of dicamba onto other types of soybeans later in summer also has increased potential to reduce yield, since long-term effects of exposure to dicamba are more severe when soybeans have flowered.  Bottom line here is that there is less weather and time suitable for dicamba application with late-season applications, and movement and injury that does occur can have more substantial impact.  Planting within the next couple weeks and looking for the right conditions to spray starting about June 21 would provide more flexibility with regard to weather and weed size than deciding to wait until about July 10 to spray when it’s hotter and weeds are already large.

Can residual herbicides be omitted in late-planted soybeans?  Maybe.  Reduced weed populations could make this more feasible, but we really hesitate to recommend it.  Omitting residuals is never the right thing to do in fields with a history of weed control problems or high weed populations, or those with waterhemp and Palmer amaranth.  One advantage of omitting residuals would less risk of antagonism with burndown herbicides in mixtures.  Applying certain soybean residuals in June can increase risk of carryover.  The effectiveness of current soybean herbicide-tolerance trait systems makes this more of a possibility, but lack of residuals generally increases risk of problems and selection for resistant weeds, and makes timing of POST herbicides more critical.  The latter point is important because with a compressed season, applicators can be required to cover a lot of acres within a short period of time.  Keeping residuals in the program allows for more flexibility overall.

In late-planted corn, residual herbicides may be effective enough to reduce need for POST herbicides.  Or the residuals could be applied early POST, after the rush to plant is over (keeping rotation guidelines in mind).  Some corn fields are already in this situation, planted without any herbicide applied yet.  The table below shows restrictions on POST use of residual corn herbicides (source – U of Illinois).  This information can also be found in the herbicide descriptions in the Weed Control Guide , and is also summarized in this PSU newsletter article and this table from the MSU weed control guide.  Reminder that use of 28% UAN as a spray carrier is prohibited for POST application of herbicides with the exception of Degree Xtra.



Managing Big, Wet Cover Crops

Source: Dr. Mark Loux, OSU Extension

Managing cover crops in a year like this can challenge even those with the most experience.  A few suggestions regarding termination of covers:

  • Increase glyphosate rates to compensate for larger size, and consider applying alone or just with Sharpen.  Mixing glyphosate with other herbicides or ATS can reduce its activity on grass covers, especially when large.  Herbicides that can antagonize glyphosate include 2,4-D, metribuzin, atrazine, and flumioxazin and sulfentrazone products.  Sharpen has not caused a reduction in glyphosate activity on grass covers in university research.  One approach would be to apply the glyphosate or glyphosate/Sharpen first, wait a few days, and then apply residual herbicides.

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