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Herbicide Residue Considerations for Fall Cover Crop Establishment

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).

Herbicide Fall cover crops:

safe to plant

Fall cover crops:

potential for injury

2,4 – D All grasses 30 days before sensitive broadleaves
nicosulfuron/ nicosulfuron+ rimsulfuron Fall cereal grains, ryegrass Small-seeded legumes*, mustards, sorghum
topramezone Wheat, barley, oats, rye, and ryegrass after 3 months Many broadleaves are restricted, does not have much soil activity
atrazine Sorghum species Cereals, ryegrass, legumes, and mustards
isoxaflutole Fall cereals grains Cereals, ryegrass, legumes, and mustards
mesotrione All grasses Small-seeded legumes, mustards
tembotrione + thiencarbazone Wheat, triticale, rye Small-seeded legumes, mustards, sorghum
dicamba All crops Only at high rates or less than 120 days after application
isoxaflutole + thiencarbazone Wheat, triticale, rye Small-seeded legumes, mustards, sorghum
metolachlor Almost anything Annual ryegrass or other small-seeded grasses
glyphosate All None
paraquat All None
thifensulfuron No restrictions for wheat, barley, and oats None with 45-day waiting interval
acetochlor Most crops should be fine Food or feed residues rather than crop injury a concern
tembotrione Cereal grains after 4 months Unknown; small-seeded legumes, mustards could be a problem
glufosinate All Food or feed residues rather than crop injury a concern
metribuzin Cereal grains and ryegrass Slight risk for small-seeded legumes and mustards
dimethenamid Most crops should be fine Food or feed residues rather than crop injury a concern
prosulfuron Cereal grains and sorghum are labeled, other grasses Small-seeded legumes, mustards
halosulfuron Cereal grains and sorghum after 2 mo., other grasses Small-seeded legumes, mustards
pendimethalin Cereal grains Small-seeded legumes and annual ryegrass
flumetsulam Cereal grains Small-seeded legumes, mustards, and annual ryegrass
rimsulfuron Based on short half-life, most fall cover crops should be OK None
saflufenacil All None
simazine Sorghum species Cereals, ryegrass, legumes, and mustards
clopyralid All grasses Small-seeded legumes
pyroxasulfone Most crops should be fine Food or feed residues rather than crop injury a concern
quizalofop Most broadleaves All grasses if less than 120 days or at high rates
sulfentrazone Cereals and ryegrass Small-seeded legumes, mustards, sorghum
chlorimuron Cereals and ryegrass Small-seeded legumes, mustards, sorghum
cloransulam Wheat, triticale, rye Small-seeded legumes, mustards, sorghum
imazethapyr Wheat, triticale, rye, alfalfa, clover Oats, sorghum, mustards
flumetsulam Cereal grains Small-seeded legumes, mustards, and annual ryegrass
imazamox Wheat, triticale, rye, alfalfa, clovers Slight risk for mustards
fomesafen Cereal grains Small-seeded legumes, mustards, sorghum
imazaquin Cereal grains Small-seeded legumes, mustards
clethodim All broadleaves None assuming at least 30 days
saflufenacil All None
flumioxazin All grasses Small-seeded legumes and mustards

            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

Source: Ohio State University

https://ipm.missouri.edu/IPCM/2020/3/coverCropTermination-KB/
https://ag.purdue.edu/btny/weedscience/Documents/covercropcarryover.pdf

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