Crabgrass
Management in Corn
Chris
Boerboom
Complaints about poor
crabgrass control in corn have been increasing over the last
several years and many farmers are seeking better control
options. Before reviewing control options, it is useful to
understand why crabgrass can be a problem to control. First,
crabgrass germinates at higher soil temperatures than many other
annual grasses such as giant foxtail. As a result, crabgrass
germinates later in the season when many other weeds have
generally stopped germinating. This delayed germination may
allow crabgrass to emerge after the residual activity of
soil-applied herbicides has dissipated or after a postemergence
herbicide application. Late germination may cause a serious
problem when corn does not have a good canopy to help shade out
the late flushes of crabgrass. Drought, sandy soils, and eroded
hills are conditions that can stunt corn growth, limit shading,
and allow more sunlight to reach the crabgrass. It also seems
likely that 36- or 38-inch corn rows will allow more light to
penetrate the corn canopy than 30-inch rows and favor crabgrass
growth.
A second reason for
more frequent crabgrass problems has been the increased reliance
on Accent-based herbicide programs. Accent basically has no
activity on smooth crabgrass and must be applied to very small
large crabgrass for suppression. Therefore, correct
identification of grass seedlings is important if relying on
Accent for grass control. With close inspection, crabgrass
seedlings become quite easy to identify by checking for the
membranous ligule, the short papery membrane at the base of the
leaf blade. Crabgrass is the only common annual grass weed with
a membranous ligule (quackgrass and wirestem muhly also have
membranous ligules, but are perennials). Other annual grass
weeds like the foxtails have a ligule that looks like a fringe
of hairs. There are two species of crabgrass, large crabgrass
and smooth crabgrass. Large crabgrass is hairy on the leaf
blades and stem while smooth crabgrass only has scattered hairs.
Crabgrass
Control Options in Conventional Corn Hybrids
The primary herbicides
for crabgrass control are soil-applied herbicides because there
are few postemergence herbicide options for crabgrass. All of
the acetamide herbicides (Axiom, Dual II Magnum, Frontier,
Harness/Surpass, Lasso/Micro-tech/Partner), their premixtures
with atrazine (Bicep Lite II Magnum, Bullet, Guardsman, Harness
Xtra, LeadOff, Surpass 100), Prowl, and Eradicane give good to
excellent crabgrass control when applied before crabgrass
emergence. They will not control emerged crabgrass (or other
emerged grasses) even though many of these herbicides are also
labeled for postemergence application. When applied
postemergence, another herbicide i required to kill
emerged grasses and then these soil-active grass herbicides
would provide residual control of future flushes. Because
crabgrass germinates later in the season than foxtail, some
crabgrass may escape control after these herbicides have
dissipated in the soil by mid-season. By that time, corn should
have developed a good canopy and should limit the growth of late
season escapes.
Once crabgrass has
emerged, herbicide options become limited in conventional corn
hybrids. Postemergence-applied Bladex will kill most crabgrass
shorter than 1.5 inches, but may injure corn under cold, wet
conditions. Bladex should be tank mixed with a residual grass
herbicide to control subsequent flushes. Unfortunately, Bladex
is being phased out of the market and the 1 lb ai/a rate may be
too low for the desired level of control. Postemergence-applied
atrazine does not control crabgrass unless the crabgrass has
just one or two leaves and atrazine will not control subsequent
flushes of crabgrass.
Accent, Accent Gold,
Celebrity, Basis, and Basis Gold (the"Accent herbicide
family") are postemergence herbicides that control many
annual grasses, but are generally poor on crabgrass. To repeat,
smooth crabgrass is tolerant to herbicides in the Accent
herbicide family and smooth crabgrass will simply escape
control. Large crabgrass can be suppressed or controlled by some
of these herbicides. However, if large crabgrass has grown
taller than one inch, control is greatly reduced. Basis Gold
tends to control crabgrass better than the other herbicides in
the Accent family because it contains atrazine. These herbicides
may provide acceptable control when applied early postemergence
and tank mixed with a residual grass herbicide. For example,
Basis Gold or Accent Gold will control small foxtails, suppress
or control emerged large crabgrass and the tank mix partner will
control subsequent crabgrass flushes. An early application,
preferably before the crabgrass has started to emerge, is very
important with this type of program. Herbicides in the Accent
family have short residual activity and will not control late
flushes of crabgrass alone. With the limitations of the
postemergence herbicides in conventional corn hybrids, it is
easy to see why soil-applied grass herbicides offer a better
control option.
Cultivation is another
control option for crabgrass that has emerged in corn. In fields
with conventional hybrids, it may be the only reasonable control
option, especially if the crabgrass is taller than two inches.
Even when using soil-applied grass herbicides, fields should be
scouted for escaping crabgrass and followed with cultivation as
necessary.
Crabgrass
Control Options in Herbicide-Resistant Corn Hybrids
Crabgrass can be
controlled postemergence with Roundup Ultra in Roundup Ready
corn, Liberty in Liberty Link corn, Lightning in IMI corn, and
Poast Plus in Poast Protected corn. Roundup Ultra will give
excellent control of emerged crabgrass. The label states that
crabgrass up to 18 inches tall can be controlled, but it should
be controlled much earlier to avoid competition and yield loss.
Roundup Ultra lacks residual activity so a tank mixture with a
residual grass herbicide would aid in the control of subsequent
flushes of crabgrass. Otherwise, a follow-up cultivation may be
needed to control late emerging crabgrass. Liberty is labeled to
control 4-inch tall crabgrass at 28 oz/a. The timing of Liberty
applications is important because control of larger crabgrass
will be reduced. Liberty also lacks residual activity and would
benefit from either a residual grass herbicide tank mix partner
or a follow-up cultivation. Lightning is labeled to control
3-inch tall crabgrass. It has given good control and has
reasonable residual activity, although some crabgrass may escape
in some years. Poast Plus is the final postemergence option and
is excellent on crabgrass, but also lacks residual activity.
This group of postemergence herbicides will control crabgrass
postemergence. However, the specific herbicide-resistant corn
hybrid must be planted in order to use the associated herbicide.
Therefore, none of these herbicides are a rescue option in
fields planted to conventional hybrids. Rather, these herbicides
can only be used in fields when it is a planned program and the
resistant hybrids have been planted.
Summary
Of all of these
options, the preemergence or preplant incorporated herbicides
will offer the best crabgrass control for most farmers.
Remember, it's best to plan for crabgrass control at or before
corn planting because control options are often limited after
crabgrass has emerged. |
