Volume
4 Issue 3
June 5, 2007
Windy Conditions (and other issues) present a
Challenge for Early Season Weed Control:
“What can be done to avoid herbicide drift complaints”
 Has
wind (or rain), good corn growing conditions and postemergence weed
control created the perfect storm? This spring a combination of good
growing conditions and a significant increase in glyphosate
application on increasing acres of Roundup Ready™ corn has run into
conflict with a very windy spring. Research by the University of
Minnesota has clearly demonstrated that corn is sensitive to
early-season weed competition. When no preemergence herbicide was
applied, a delay of four to seven days after weeds exceeded 4 to 5
inches in height resulted in a yield loss of 12 bushels per acre.
Delaying glyphosate application another seven days, when weeds were
8 to 9 inches tall, resulted in an average yield loss of 27 bushels
per acre. Timeliness of weed removal is, of course, dependent upon
weed density so in fields with a low weed seed bank the timeliness
issue will not be as acute. Therefore growers need to scout fields
for weed height and density in order to assure timely weed control
but avoid post-emergence herbicide applications when wind speeds
exceed legal label guidelines.
A simple remedy to this problem is to use a one-half rate of a
preemergence herbicide and plan to follow up with a postemergence
glyphosate application. Results from the 2004-2006 study indicated
that the best economic returns were achieved when a one-half rate of
a preemergence herbicide was followed with a postemergence (POST)
glyphosate application when corn was in the three- to four-leaf
growth stage (V3-V4). Consider the investment in the preemergence
herbicide as insurance in case you cannot get into your Roundup
Ready™ corn fields in a timely manner; in other words, the
preemergence herbicide is buying you time. In many cases the
preemergence herbicide also improves the effectiveness of the
glyphosate because weeds are more uniform in size at time of
glyphosate application.
Time for a preemergence herbicide has obviously passed but the
urgency of timely weed control in corn must be balanced by the
stewardship of safely applying your herbicide to the intended
target. The Harness™ and Surpass™ labels prohibit application when
wind speeds exceed 15 miles per hour. The Roundup WeatherMax™ label
states that drift potential is lowest between wind speeds of 2 to 10
miles per hour. Many herbicide labels do not provide specific wind
speed restrictions. The Minnesota Dept. of Agriculture regulations
state that “a person may not direct a pesticide onto property beyond
the boundaries of the target site. A person may not apply a
pesticide resulting in damage to adjacent property.”
Please consider the consequences of off-target movement of
glyphosate to non-Roundup Ready™ crops and the long-term
consequences of poor stewardship when applying glyphosate under
windy conditions. Drift reduction nozzles and drift control agents
can help reduce drift potential, but under windy conditions, drift
can still occur. Glyphosate can be applied in a coarse spray droplet
size without sacrificing effectiveness. With conventional nozzles,
dropping boom pressure or moving to larger-sized nozzles will
increase droplet size and decrease driftable droplets. In addition,
spray volume of glyphosate applications can have an impact on
increasing spray drift if carrier volume is decreased substantially.
Relatively small droplets are required for uniform coverage at spray
volumes less than 10 gallons per acre. Small droplets increase the
likelihood of spray drift. For most agronomic situations, spray
volumes of 10 to 15 gallons per acre allow for adequate coverage in
spring applications.
When the winds subside and fields dry out keep in mind that the
effectiveness of glyphosate is still top-notch and potential tank
mix partners such as Hornet™, Status™, Callisto™, and Impact™ can
still be applied to corn stages of V6, V10, V8, and 45 days before
harvest, respectively. Perhaps you will suffer some yield loss this
year from a delay in weed control but learn from the experience and
develop an effective preemergence / postemergence strategy next
year.
Jeff Gunsolus
Extension Agronomist-Weed Science
and
Liz Stahl and David Nicolai
Regional Extension Educators-Crops
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