Growth Regulator Herbicides For Burndown Applications
URBANA, ILL.
Control of existing weed vegetation prior to
planting corn or soybean is achieved
through the use of preplant tillage, herbicides,
or sometimes both. Several herbicides can
be used to control existing vegetation, but control
is frequently improved when there is more
than one herbicide active ingredient in the burndown
application. For example, saflufenacil (the
active ingredient in Sharpen and a component
of Verdict, OpTill, and OpTill PRO) is an effective
herbicide for burndown control of horseweed/
marestail but often needs a tank-mix
partner to enhance control of other winter annual
broadleaf and grass species.
Growth regulator herbicides, most commonly
2,4-D, are frequently included in burndown applications.
Both amine and ester formulations of
2,4-D are labeled for burndown applications before
planting, but the ester formulation is usually
preferred. The low water solubility of an
ester reduces the potential for it to be moved
into the soil by precipitation, where it could severely
injure germinating crop seeds. Also, the
ability of esters to better penetrate waxy leaf surfaces
often results in improved control of large
weeds, especially when air temperatures are
cool.
The labels of some 2,4-D ester formulations
(3.8 lb acid equivalent per gallon) allow preplant
applications without a specified waiting interval
between application and planting corn, while
other formulations require a 7-day interval.
When applied before planting soybean, a wait is
required – at least 7 days after application of 1
pint per acre (0.5 lb ae), and 15 to 30 days (the
interval varies by product) after application of up
to 2 pints per acre (1.0 lb ae). In addition to waiting
intervals, some labels indicate that no tillage
operations can be performed for at least 7 days
after application and that the seed furrow must
be completely closed during the planting operation
or severe crop injury may result. Factors
that increase the likelihood of the 2,4-D coming
in direct contact with the crop seed increase the
probability of severe crop injury. Pay careful attention
to label statements of any 2,4-D formulation
used before crop planting.
Recent conversations have indicated that some
retail applicators might be experiencing difficulty
acquiring enough 2,4-D to meet the need
for burndown applications. Substituting an
amine formulation of 2,4-D for an ester formulation
is possible, but remember that there are
some significant differences between these formulations
(see Table 1):
• As mentioned, an amine formulation does
not penetrate the leaf cuticle as easily as an
ester formulation, and higher application rates
of amine formulations are sometimes needed to
provide the same level of control as lower rates
of ester formulations.
• Amine formulations are less volatile but more
water soluble than ester formulations and are
moved into the soil more easily following precipitation.
• Movement of 2,4-D into the soil increases the
likelihood of crop injury if the seed or germinating
seedling comes in contact with the herbicide.
The interval between application of 2,4-D
amine and soybean planting is longer (15 days
for 1 pint or less, 30 days for more than 1 pint)
than the interval with ester formulations.
Dicamba is another growth regulator herbicide
sometimes included for control of existing vegetation
before planting. Several commercially
available products contain dicamba, but not all
products are specifically labeled for application
before crop (especially soybean) planting. In
comparison with 2,4-D, dicamba provides more
control of chickweed, henbit, and prickly lettuce;
comparable control of dandelion; slightly less
control of horseweed species; and less control of
mustard species.
Application rates used in burndown applications
can impact the interval between application
and planting. For example, Clarity can be
applied at up to 16 fluid ounces per acre before
planting corn (use only 8 fluid ounces on coarse
soils or medium- and fine-textured soils with
less than 2.5% organic matter), with no interval
between application and planting. However, before
planting soybean, the 16-ounce rate would
require a minimum accumulation of 1 inch of
precipitation and a 28-day waiting interval between
application and planting. An 8-ounce application
rate still requires a minimum
accumulation of 1 inch of precipitation, but the
waiting interval is reduced to 14 days. These intervals
must be observed before planting soybean
or crop injury might occur. Δ
DR. AARON HAGER: Associate Professor, University
of Illinois