Michigan State University Extension
Michigan Soybean Information - FACT9706
11/16/00

Using Reduced Rates of Postemergence Herbicides in Soybeans


Soybean Facts Winter 1997

Michigan State University Extension
This research and fact sheet funded by Michigan
Soybean Promotion Committee Making Your Checkoff Pay Off

K.A. Renner and J.A. Mickelson
Department of Crop and Soil Sciences
Michigan State University

The use of reduced rates of postemergence herbicides
to control weeds in soybeans is one strategy being
researched at Michigan State University to increase
soybean production profitability. This fact sheet
provides guidelines for growers who are interested
in applying reduced rates of postemergence
herbicides for weed control in soybeans.

Narrow Rows Provide Weed Control and Soybean Yield
Advantage. Planting soybeans in narrow rows rather
than wide rows (30 inch) results in quicker closure
of the soybean canopy. A closed canopy suppresses
later emerging weeds and results in better season
long weed control. Soybeans planted in narrow rows
can more efficiently utilize sunlight, water and
nutrients because the plants are spaced
equidistantly. This can increase soybean yield.
These advantages can increase the effectiveness of
reduced rate postemergence herbicide programs.

Growers Must Identify Weed Species and Choose
Optimal Herbicides. It is important to identify
weeds so that the most optimal herbicide or
herbicide tank mixture can be applied. The
susceptibility of a given weed species to two
herbicides can vary greatly. Even if both herbicides
will control the weed at a labeled rate, one
herbicide may be more active. At a reduced rate this
more active herbicide may provide good control while
the less active herbicide will fail. The activity of
herbicides can be judged by looking at the maximum
size at which the weed is labeled for control.
Herbicides labeled to control larger weeds may be
more active on the weed than a herbicide that will
control only a small weed.

Early Application Will Not Provide Season Long Weed
Control. Smaller weeds are more susceptible to
herbicides than larger weeds, so reduced rates of
herbicides will be more successful if applied to
small weeds. However, this requires that the
postemergence herbicide application be applied very
early in the growing season. Research has shown that
an early application of reduced rates of
postemergence herbicides controls weeds present at
the time of application, but a large number of weeds
germinate after the postemergence herbicide
application and are not controlled. Early
postemergence applications have resulted in poor
weed control and low soybean yield unless a second
application of reduced rates of postemergence
herbicides is made (split application). It is best
to apply reduced rates of postemergence herbicides
at or just prior to a standard postemergence
herbicide application timing unless a split
application is planned.

Keep Adjuvants at Full Rates.

Any additives which are recommended to be applied with a
herbicide should be applied at the full rate even when
applying reduced rates of the herbicide.

When Applying Less Than Labeled Rates, the
Manufacturer is Not Liable for Failures. It is not
illegal to apply less than labeled rates of herbicides,
but the grower assumes all risk involved.

Do Not Apply Reduced Rates of Herbicides if Weeds
are Under Moisture Stress. Weeds respond to moisture
stress by producing a thicker cuticle. This can greatly
reduce the amount of herbicide absorbed into the plant.
Weeds growing under moisture stress become very difficult
to control even with full rates of postemergence
herbicides.

Effectiveness of Reduced Rates of Postemergence
Herbicides in Soybean *,**,*** (Part I)

Weed Response to Applications of
1/2x Rate
----------------------------------
Post- 1/2x Cockle- Jimson- Lambs- Black
emergence Rate/A bur weed quarters Night-
Herbicide shade
----------------------------------------------------
Basagran 1 pt G G F N
Blazer 3/4 pt P F P F
Cobra 3 oz P F N F
Reflex 1/2 pt N P P F
Classic 1/6 oz G F N N
Pinnacle 1/8 oz F F G N
Pursuit 2 oz G F P E
Galaxy 1 pt G G F F
Storm 3/4 pt F F P F
Basagran 1 pt + G G F F
+ Cobra 3 oz
Basagran 1 pt + G G F F
+ Reflex « pt
Basagran 1 pt + E G F E
+ Pursuit 2 oz
Pinnacle 1/8 oz + G F G N
+ Classic 1/6 oz
Pinnacle 1/8 oz + F F G F
+ Blazer 3/4 pt
Pinnacle 1/8 oz + F F G F
+ Cobra 3 oz
Pinnacle 1/8 oz + F F G F
+ Reflex « pt
Pinnacle 1/8 oz + G F G E
+ Pursuit 2 oz
Pursuit 2 oz + G F P E
+ Blazer 3/4 pt
Pursuit 2 oz + G F P E
+ Cobra 3 oz
Pursuit 2 oz + G F P E
+ Reflex l/2 Pt

- - - - - - - - - - - - - - - - - - - - - - - - - -

Effectiveness of Reduced Rates of Postemergence
Herbicides in Soybean *,**,*** (Part II)

Weed Response to Applications of
1/2x Rate
----------------------------------
Post- 1/2x Redroot Common Smart- Velvet-
emergence Rate/A Pigweed Ragweed weed leaf
Herbicide
----------------------------------------------------
Basagran 1 pt N P G F
Blazer 3/4 pt F G F F
Cobra 3 oz F F N F
Reflex 1/2 pt F G P F
Classic 1/6 oz E F G F
Pinnacle 1/8 oz E P G F
Pursuit 2 oz E P F G
Galaxy 1 pt F F G F
Storm 3/4 pt F F F P
Basagran 1 pt + F F G F
+ Cobra 3 oz
Basagran 1 pt + F G G F
+ Reflex 1/2 pt
Basagran 1 pt + E P G G
+ Pursuit 2 oz
Pinnacle 1/8 oz + E F E G
+ Classic 1/6 oz
Pinnacle 1/8 oz + E F G F
+ Blazer 3/4 pt
Pinnacle 1/8 oz + E F G F
+ Cobra 3 oz
Pinnacle 1/8 oz + E G G F
+ Reflex 1/2 pt
Pinnacle 1/8 oz + E P G G
+ Pursuit 2 oz
Pursuit 2 oz + E F F G
+ Blazer 3/4 pt
Pursuit 2 oz + E F F G
+ Cobra 3 oz
Pursuit 2 oz + E G F G
+ Reflex l/2 Pt
----------------------------------------------------

E = Excellent, G = Good, F = Fair, P = Poor, N = No
Control

*1/2X rates applied in a split application of 1/4x
at E. POST followed by 1/4x at POST result in better
and more consistent weed control.

**1/2X rates of Assure II, Fusilade DX, Fusion,
Option II, Poast, Poast Plus or Select will contact
Giant Foxtail. However, do not tank mix these
herbicides with Pursuit as poor grass control will
occur. See labels or Table 2L in E-434 for labeled
tank mixtures.

***Always use full rates of recommended adjuvants
with reduced rates of postemergence herbicides.

Copies of the Soybean Research Fact Sheet are available
from:

Michigan Soybean Promotion Committee
http://www.michigansoybean.org

MSUE County Office
http://www.msue.msu.edu/msue/ctyentpg/

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