Residuals 101:
5 Things Every Farmer
Should Expect from a
Residual Herbicide
Residuals 101: 5 Things Every Farmer Should Expect from a Residual Herbicide
Introduction
Across the country, residual herbicides have become the standard recommendation of leading researchers
and a key component of sound farming practices. University research has proven that adding a residual to a
herbicide program aids in keeping fields clean longer and free of yield-robbing weeds to improve overall profit
and resistance management practices. Put simply: a residual herbicide is a smart and valuable investment for
today’s farmer.
Yet with several products on the market today, it is important for farmers to know what to look for in a
residual herbicide. Many researchers assess residuals based on one or more of the five areas of performance:
1. Length of Residual
2. Application Timing Flexibility
3. Broad Spectrum Weed Control
4. Resistance Management
5. Crop Rotational Flexibility
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Residuals 101: 5 Things Every Farmer Should Expect from a Residual Herbicide
1
Length of Residual
Experts agree residual control of tough
weeds is a critical component of a good
preemergence herbicide as well as a strong
overall herbicide program. An effective, long-lasting
residual product will mean less competition for the
crop, higher yield and a better return on investment
on a farmer’s overall weed control program.
Length of Days Residual Provided
Number of days between PRE and POST applications (4”–6” tall weeds)
Days:
0
10
20
30
40
50
Valor® XLT (3 oz)
Gangster® (1.8 oz)
Valor (2 oz)
Boundary® (1.5 pt)
Authority® Assist (5 oz)
Authority MTZ (10 oz)
Prowl® H20 (2 pt)
The longer a residual material can hold off yield-robbing weeds, the
greater value it will bring to a farmer’s operation and, ultimately,
bottom line. Additionally, a long-lasting residual keeps fields cleaner
longer, giving farmers and crops a jump start.
Source: K. Bradley, University of Missouri, 2008.
Length of residual varies by weed, soil texture, soil pH, organic
matter present, rainfall and herbicide rate.
Yet some herbicides that are marketed as “residuals”
may have limited residual control and may become
ineffective as the season evolves and tough weed pressure
increases. With products that range from a few days of
residual control to as long as several months of control,
it is important for farmers to know what time frame of
control to expect from a preemergence herbicide.
University of Missouri Weed Scientist Dr. Kevin
Bradley said, “With residuals, the longer the control
the better. I think two weeks of control is probably
not enough out of any herbicide to really even justify
spraying. Four weeks is a realistic amount of control
to expect from a residual—six weeks would be great.
You should get at least a month of residual so that your
program can include one preemerge herbicide and one
postemerge spray and you’re done. If you only get two
weeks’ control, you are probably going to have to spray
a postemerge twice.” Dr. Christy Sprague, weed scientist
at Michigan State University, agrees that “a herbicide
should ideally last at least four weeks to be deemed
as a residual.”
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Residuals 101: 5 Things Every Farmer Should Expect from a Residual Herbicide
2
Application Timing Flexibility
Percent Control 28 Days After Application
90
80
70
60
50
40
30
20
10
Giant Foxtail
Valor® XLT 3 oz
Giant Ragweed
Valor XLT 3 oz
+ Sencor® 4 oz
Scepter® 2.8 oz
Valor XLT 3 oz
+ Sencor 6 oz
Scepter 2.8 oz
+ Sencor 4 oz
Source: Ohio State University Trial Data, 2008.
Researchers also note that tillage should not be
performed after a residual is applied, as it will eliminate
residual activity.
Farmers should, therefore, not assume that all
residuals are created equal in terms of length of control.
“Four weeks is a realistic
amount of control to expect
from a residual—six weeks
would be great.”
Depending on geography, crop selection,
weather trends or even personal preference,
farmers may have a variety of timing needs for
their herbicide program—be it a fall/winter application
or a spring burndown. Application flexibility lets
farmers apply the product based on their seasonal
timing needs. A highly flexible preemergence residual
will allow a farmer to apply any time from post-harvest
to preemergence.
The most popular time to apply a preemergence
herbicide is in the spring, several days or weeks before
planting. Advocates of this practice argue this application
timing allows for the longest period of residual
weed-killing activity to last through the planting season.
In recent years, however, many farmers have opted
to apply a preemergence fall or winter application
(October–February, depending on the geography) to
rid fields of nutrient-robbing winter weeds that may
also harbor pests and diseases. Among a variety of
winter annual weeds controlled by post-harvest-applied
residuals, Purdue University Weed Scientist Dr. Bill
Johnson said, “Fall applications are a great idea for
dandelions, chickweed, henbit and purple deadnettle.”
Another benefit of this “clean spring” approach
is the ability to ensure that a preemergence herbicide
is applied before poor springtime weather conditions
occur. Many farmers experienced this situation firsthand
during recent springs, when long periods of rain kept
them out of fields well into what should have been the
spring burndown and planting months. Conversely,
farmers who had applied residuals in the fall already
had a head start on fighting weeds and were able to
plant as soon as fields were dry.
—Dr. Kevin Bradley, University of Missouri
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Residuals 101: 5 Things Every Farmer Should Expect from a Residual Herbicide
Farmers who rely exclusively on burndown applications may find
themselves stuck if springtime conditions do not cooperate with
targeted application timing. A flexible residual product allows a
farmer to apply it at various points throughout the season, including
fall and winter months, helping extend the window of possible control.
Finally, many farmers see labor management
advantages to applying herbicides during slower times
of the season versus the busy springtime months.
When choosing the timing of an application, a
farmer must again consider the length of residual
control a product can deliver. For example, if a product
can only provide two solid weeks of residual control,
that product may be limited to only immediate
pre-planting applications.
In summary, a residual product’s ability to be
applied at a variety of points throughout the season
will allow for not only greater flexibility in a management
program, but also overall value and return on investment
from the residual material.
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Broad Spectrum Weed Control
The primary reason for using any herbicide
is to control weeds. Therefore, it is imperative
that farmers look for a residual that will
control the weeds that are problematic within their
specific geographic location.
“It’s important to focus on the weeds that certain
residuals control better than others. Some weeds are
more problematic in certain areas. For example, in
Michigan you have common lambsquarters, which
ends up being a continual issue that farmers are trying
to control. Whatever the problem weed may be for the
area, it is important to find a residual that is going to
help control that issue,” Sprague said.
Bradley agreed that each farmer’s weed issues should
be analyzed to ensure that the farmer is choosing the
herbicide that will control specific problematic weeds.
“Residual weed control really should be about
what weed problems that person is facing—whether it
be resistant weeds, waterhemp or even giant ragweed,”
Bradley said.
Additionally, using a residual that has broad
spectrum weed control, and even offers performance
on grasses, will ensure that more of the specific
targeted weeds are controlled, rather than just one
or two. Ultimately, it will be more economical and
timely for the farmer to use a residual that is able to
control several weed species, including grasses, with
little to no breakthrough pressure.
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Resistance Management
Not long ago, “resistance” was a threat most
farmers assumed was happening in someone
else’s field. However, resistant or even tolerant
weeds are becoming a major issue in weed control across
most of the country. Many researchers suggest that
controlling these weeds with a comprehensive herbicide
program—including a residual and other modes of
action—before they grow too large is the best fight
against resistance.
Johnson explained, “In dry climates weeds can start
robbing crops of yield when they reach about the six-inch
height. Every day that we delay our postemerge weed
management strategies due to weather, equipment
failure or other circumstances can result in about a 1.2
bushel per acre per day yield loss in corn and one third
of a bushel per acre per day yield loss in soybeans.
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Residuals 101: 5 Things Every Farmer Should Expect from a Residual Herbicide
By applying a residual herbicide, we can extend the
window for putting on a postemergence herbicide to
reduce yield losses and also reduce the exposure of our
worst weed problems to glyphosate. I think that’s the key
to resistance management—simply not exposing weed
species to the same mode of action numerous times.”
“Residuals are our number one line of defense
against glyphosate-resistant waterhemp in Missouri.
We have more success at reducing the total population
of waterhemp in a field with a preemerge herbicide,
rather than attempting to treat a tremendous number
of glyphosate-resistant waterhemp all at once with only
a postemerge. With an effective preemerge foundation,
there just won’t be as many weeds to treat with a
postemerge. The numbers are stacked more in your
favor,” Bradley said.
“By applying a residual
herbicide, we can extend the
window for putting on a
postemerge herbicide to
reduce yield losses and also
reduce the exposure of our
worst weed problems
to glyphosate.”
—Dr. Bill Johnson, Purdue University
Percent Weed Control with
Residual Herbicide in Soybeans
(Preemergence application listed)
0
20
40
60
80
100
Authority® Assist (5 oz/A)*
Authority First (3 oz/A)*
Authority MTZ (12 oz/A)*
Boundary® (1.5 pt/A)*
Gangster® (2.4 oz/A)*
Prefix™ (5.29 pt/A)*
Valor® XLT (3 oz/A)*
Envive™ (2.50 oz/A)*
*Postemergence application includes Touchdown Total®, AMS
Foxtail
Ragweed
Velvetleaf
Lambsquarters
Pigweed
Wild Mustard
Rating date: June 30, 2008.
Data obtained from Michigan State University Weed Science Research Program.
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Residuals 101: 5 Things Every Farmer Should Expect from a Residual Herbicide
5
As a result, many researchers agree that rotational
flexibility is an increasingly key component to consider in
a residual herbicide. “It’s important for you to make sure
that the residual you choose will allow you to rotate to
certain crops,” Sprague said.
Crop Rotational Flexibility
Crop rotation is widely regarded as one of
the best ways to keep fields healthy. There are
many benefits to rotating crops, including pest
control, disease prevention, soil nutrition availability and
overall weed control. Therefore, a good residual must be
designed to work well with a variety of possible crops a
farmer may introduce into his or her crop rotation plan.
However, many farmers are also finding that even
the best-laid crop plans can change unexpectedly and at
the last minute. Erratic springtime weather has caused
numerous farmers to abandon one planned crop for
another when spring planting (or in many cases,
re-planting) was driven late into the season. Add to
that uncertainty several seasons of wildly fluctuating
market prices, and many farmers are shifting their
crop plan even days prior to scheduled planting.
Conclusion
All residuals are not created equal. But by understanding
the specific needs and weed pressure of an individual farm
operation and by analyzing residuals based on a few key
factors, farmers can make an educated decision on which
residual herbicide to choose to maximize their profit and
minimize risk.
Researched and written by archer>malmo, Memphis, TN.
Contact 800-682-5368 for more information.
Length of Residual—Rotational Interval (months)
Valor® XLT Gangster®
Alfalfa
Valor
Authority®
Canopy®
Optill™
Pursuit®
Plus
Scepter®
Sencor®
Sharpen™
12–18
12
12
12
10
4
10
18
4
4
Corn, Field
10
9
1
10
10
8.5
9
10
4
0
Cotton
10
9
1
18
10
18
18
18
8
1.5
Drybeans
12
9
4
12
12
4
4
11
12
1
Peanuts
18
9
0
NL*
18
4
0
11
18
4
Rice
10
9
1
NL*
10
40
40
Spring
8
4
Sorghum
12
9
1
10
12
18
18
11
12
0
Soybeans
0
0
0
0
0
0–1
0
0
0
0–1
Wheat
4
3
1
4
4
4
4
3
8
0
pH Restriction
Y
N
N
N
Y
N
N
N
Y
N
Please check the label for other crops.
*Not labeled
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Residual Control Worksheet
When meeting with your local agricultural dealer, use this checklist to outline key attributes you find important when
making your residual herbicide selection.
Residual Control
To fight tough weeds, I am looking for at least ______ weeks’ residual control.
*Reminder: Most university researchers recommend at a minimum 4 weeks’ residual control.
Recommended product:___________________
Proven length of control: __________________
Application Timing Flexibility
I am interested in the following residual herbicide applications (check all that apply):
☐ fall or winter application
☐ spring burndown
☐
☐
layby (cotton only)
other ____________________
Broad Spectrum Weed Control
I have the greatest issues with the following weeds
(check all that apply):
☐ annual grasses
☐ annual nightshades
☐ Carolina geranium
☐ chickweeds
☐ cocklebur
☐ common ragweed
☐ cressleaf groundsel
☐ cutleaf eveningprimrose
☐ dandelion
☐ field pennycress
☐ giant ragweed
☐ henbit
☐ kochia
☐ lambsquarters
☐ marestail
☐ morningglories
Showing resistance?
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
Residual Control Worksheet
Broad Spectrum Weed Control (cont.)
I have the greatest issues with the following weeds
(check all that apply):
☐ Palmer pigweed
☐ pigweed
☐ purple deadnettle
☐ Russian thistle
☐ shepherd’s-purse
☐ smartweed
☐ sunflower
☐ teaweed
☐ velvetleaf
☐ waterhemp
Showing resistance?
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
☐ Yes ☐ No
Resistance Management
I have suspected or confirmed resistance issues within my fields.
☐ Yes ☐ No
I am interested in using a different mode of action, such as a PPO-inhibitor, in my herbicide program.
☐ Yes ☐ No
Crop Rotation
I will be planting—or have the possibility of planting—the following crops on my fields:
☐ soybeans
☐ corn
☐ cotton
☐ wheat
☐ sorghum
☐ peanuts
☐ rice
☐ drybeans
☐ alfalfa
☐ other _________________________