A System to
Control Perennial Broadleaf Weeds in Glyphosate Resistant
Soybeans
Jerry
Doll
Roundup (glyphosate)
has been a useful tool for perennial weed management for many
years. It has been used in soybean production systems as either
a fall or spring treatment before planting, as a spot treatment
or via selective applicators in the growing crop, and as a
preharvest treatment. Safety to the soy-beans was due to
application timing (before or after harvest or after crop
maturity) or with selective equipment (rope wick and wiper
applicators). The recent development and commercialization of
glyphosate-tolerant crops allow us to apply glyphosate to the
growing crop and maintain selectively in a manner similar to
that of other selective herbicides: the crop is able to
physiologically protect itself from the normally lethal effect
by which glyphosate kills weeds.
This
development is revolutionizing our ability to manage several
perennial weeds, especially broadleaf species with spreading
root systems like hemp dogbane, common milkweed, Canada thistle,
perennial sow-thistle, Jerusalem artichoke and field and hedge
bindweed. These weeds are persistent in all tillage systems. The
availability of glyphosate resistant crops opens the door for us
to apply the optimum rate of a highly systemic herbicide at the
ideal growth stage of the perennial weed with essentially no
risk of crop injury and we still have crop competition to
further weaken the weed infestation.
Most of
the soybeans grown in Wisconsin are planted in narrow rows to
maximize yield potential. This also enhances the competitive
advantage of soybeans with weeds by shading the soil surface
sooner than when the crop is planted in 20-inch or wider rows.
Also, many of our soybeans are planted without tillage. This is
a distinct advantage to maximize the effectiveness of glyphosate
on perennial weeds for two reasons. First, in no-till systems
weed growth is uninterrupted and the weeds reach more advanced
growth stages sooner than if tillage were performed. This means
that perennial reach the bud to early flowering stage earlier
than if tillage were done, and because this is the time when
translocation to the root is maximized, the best long term
control should result in a no-till system. Also the absence of
tillage helps ensure that the root system is intact and that
most of the below ground system will have the chance to receive
glyphosate from treated shoots.
1995 to 1998
Field Research
We have
researched perennial weed management in glyphosate resistant
soybeans since 1995 to find the most effective time and rate of
glyphosate application to control three of our common perennial
broadleaf weeds. Two trials each were done on hemp dogbane,
Canada thistle and common milkweed and the results have led us
to propose a program that manages annual weeds, 2) gives long
term suppression of the perennial species, and 3) does not
increase the number of trips over the field.
All trials
were done in grower fields in Wisconsin, were planted by the
producer, and were done in narrow row, no-till (or near no-till)
production systems. Either a burn down herbicide application or
a light disking was done to kill emerged annual weeds before
planting. Roundup was applied in 15 to 18 gal/acre of water when
the weed was in the late vegetative to bud stage or in the early
flowering growth stage. Rates of glyphosate applied were .56,
.75, 1.125, and 1.5 lb ai/acre (these rates correspond to 1.5,
2.0, 3.0, and 4.0 pints/acre of Roundup or Roundup Ultra) in
single applications and combinations of the lower rates as
repeated applications (the second application usually 7 to 14
days after the first). A summary of the weed and soybean growth
stages and planting and treatment times is found in Table
1.
Control
ratings were taken several times during the season. Weed
reinfestation ratings and were taken at the end of the growing
season and again in June of the following year. Weed populations
the year after applications were also determined.
Summary
Hemp
dogbane was most
sensitive to Roundup when in the early and full flowering
stages. Sequential applications of glyphosate that started with
hemp dogbane in the early bud stage gave control similar to that
of single applications made to plants in the early and full
flowering stages. The 1.5 and 2.0-pint/acre rates of Roundup
applied in the early to full flowering growth stages approached
eradication after a single use in glyphosate resistant soybeans.
Roundup
gave excellent control of Canada thistle at most rates
and times of application, including the higher Roundup rated
applied at the early application time. The 1.5 and 2.0-pint
Roundup rates applied to thistles in the early flowering stage
also appeared to eradicate thistles in just a single season.
Canada thistle is more sensitive to glyphosate than hemp
dogbane.
Treating common
milkweed in the late vegetative to early bud stage (30 days
after planting) was less effective than treating in the early
flowering stage. Roundup at 3.0 or 4.0 pints/acre applied to
milkweed in the early flowering stage and in split treatments
gave the best kill of treated plants. However, some milkweed
plants escaped control at each date because they emerged after
the application. Split applications were no better than a single
application in the early flowering stage.
Overall
control of the treated perennial broadleaf weeds in glyphosate
resistant soybeans was excellent and several treatment timings
and rates approached eradication. Non-uniform weed emergence was
especially noticeable with hemp dogbane and common milkweed.
Canada thistle emerged both more uniformly and earlier than the
other species studied.
Thus, the
right time to treat perennial broadleaves is at the late bud to
early flowering stage. This is often 5 to 7 weeks after planting
when the soybeans are in the V-4 growth stage (Table
1) and is later than when annual weeds would normally be
treated. However, this later timing is consistently the best
time for perennials because at this growth stage herbicide
movement from the treated foliage to the roots is maximized.
We never
decided to apply Roundup based on the calendar. The number of
days from planting until we made the early (usually bud stage)
and late (usually early flowering stage) applications show why (Table
1). The days to reach these growth stages varied
considerably across years, sites and weeds. On average, the
early applications were made 32 days after planting and the
later ones at 42 days. Soybeans at these times were approaching
the two trifoliate leaf stage for the early applications and
averaged 4 trifoliate leaves for the later timing.
Putting the
system together: annual and perennial weed
control in glyphosate
resistant soybeans
Based on
these observations and other considerations, we have arrived at
the following recommendations for perennial broadleaf weed
management in Roundup Ready soybeans. In addition, a no-till (or
near no-till) system does not disturb the weed roots before
treatment.
 Plant the crop
without tillage. Tillage delays the development of perennial
weeds while in a no-till system, the weed grows rapidly and
reaches the ideal growth stage for treatment sooner than if
tillage were done.
Apply a reduced
rate (perhaps 50%) of the recommended rate of a soil-active
herbicide with the burn down treatment before planting to
avoid crop yield loss due to uncontrolled annual weeds while
waiting to treat perennial broadleaves. Select the preemergence
herbicide(s) based on the expected annual weed population and
product price. A reduced rate is not risky because any escaping
annual weeds will be killed by the glyphosate application
targeted to the perennial weed. In fields with perennial
broadleaf weeds where tillage has been done, apply a reduced
rate of a PPI or PRE herbicide. It may take longer for the
perennial species to reach the flowering stage in these fields.
Delay the
glyphosate application until the first flowers appear on the
perennial broadleaf weed or until the weed is 24 to 30 inches
tall, whichever occurs first. Perennial broadleaf weeds do not
develop uniformly. Not all plants will flower or be at the same
height at the same time. Apply these guidelines to the most
advanced plants in the population. Our applications in the
"early flowering" stage meant that at least some
plants were flowering but most were not.
Application timing
is more important than glyphosate rate. Our research has
shown that applying Roundup at 2.0 to 3.0 pints/acre to actively
growing perennial weeds in the early flower stage gives
excellent control the season of application with greatly reduced
weed populations the next year. This illustrates the tremendous
effect of soybean competition on perennial broadleaves by not
allowing the weed to replenish the root reserves before going
dormant in the fall.
Split applications
of glyphosate are not necessary. Control from a single
treatment when perennial broadleaves begin to flower is as good
as that of repeated treatments. This means that no additional
trips through fields with perennial broadleaf weeds should be
needed as a well-timed single postemergence application
(following the burn down treatment with a residual product) will
give maximum effect on the perennial weeds.
Uniform coverage of
the weed foliage is important and difficult. As mentioned,
weed height is seldom uniform in a population of perennial
broadleaves. Select the appropriate nozzles and adjust the boom
height to cover the weed foliage as uniformly as possible.
Remember that boom height also affects the risk of particle
drift from the target area.
Monitor the
perennial weed population in following years and use an
appropriate management program if/when they again reach
threshold levels.
The Future
Our
research to date has focused on single season use of glyphosate
in glyphosate resistant crops. We now need information on the
best systems to handle perennial weeds for the long term.
Systems to test in comparison to single season use of glyphosate
are:
alternate year use of
a glyphosate resistant crop
consecutive year use
of glyphosate resistant crops (2, 3 or 4 seasons) and
alternate control
strategies in rotation crops, for example clopyralid (Stinger),
for Canada thistle control in corn in rotation with glyphosate
in glyphosate resistant crops.
Research
on the interaction of tillage with on perennial weed management
in glyphosate resistant crops is also needed. |
