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Post Emergence Weed
Control
by Justin Petrosino, ANR Extension Educator
OSU Extension, Darke County
June 27, 2011
This
past Friday I spent some time counting weeds at a few research sites.
The consensus is this: preemergence herbicides applied two weeks ago
are holding strong. However, plots that did not receive a PRE are
starting to get weedy. There are many fields in the county that
received a PRE a few weeks before my plots and there are still a few
fields that haven’t received any herbicide treatment. Those fields are
due for an herbicide application soon.
The
choice of which herbicides to use depends on the crop and weeds
present. If a field has marestail, giant ragweed, lambsquarters, or
waterhemp, extra measures should be taken to ensure control. If a field
does not have any of those weeds the farmer must be the luckiest guy on
earth. In Darke County resistance of waterhemp to glyphosate (the
active ingredient in Roundup, Buccaneer, Cornerstone, etc…) is
confirmed. Marestail and giant ragweed are commonly resistant to ALS
inhibitors and glyphosate, and lambsquarters is not far behind.
These weeds are also prolific seed producers. Giant ragweed can produce
5000 seeds per plant. That seems tame compared to waterhemp with large
plants able to produce upwards of one million seeds. That’s right, a
single plant can produce a million seeds! Even a small plant can
produce tens of thousands of seeds. Marestail seed is known to travel
up to 200 miles if it catches the right wind. The old adage is true
that one year’s mistake can lead to seven years of weed problems.
Choosing the proper chemicals and rates can have a major impact on
weeds. In soybeans, options for POST herbicide applications are fairly
limited. If a strong burndown/PRE was applied, a glyphosate application
may be the only herbicide needed for control. The standard
of a single pass treatment with a glyphosate product was never a good
recommendation, and it still isn’t. This holds true if large weeds or
one of our problem weeds is present. In this situation lead off with
the high rate of glyphosate plus either an ALS inhibitor like Classic
or FirstRate (if ALS resistance isn’t a problem) or a PPO inhibitor
like Flexstar or Cobra. If large marestail is present don’t expect
control.
In
corn there are more options for control. Glyphosate is not needed if
fields are scouted and a proper herbicide program is used. If
glyphosate is used and large weeds or a problem weed species is present
tank mixing with an atrazine (Aatrex) or dicamba (Banvel, Status)
product will help with control. There are also a few caveats with corn
POST products. Bleachers like Callisto and Laudis can be a little weak
on giant ragweed, especially if it is large. Adding a half pound of
atrazine per acre can increase control. Waterhemp may stand up to
glyphosate but the addition of dicamba to the tank will increase
control.
A
few simple tricks to improve herbicide efficacy include using the
proper adjuvants, testing water quality, and following label heights
for weeds. If the label calls for MSO use it, if your water hardness is
above 200 ppm consider a conditioner like AMS, and for most products
the final tank solution should be between 4 and 7. Also spray
glyphosate between 10:00am and 6:00pm.
Making sure the correct products are used along with the right
adjuvants can be difficult. One of the best resources available is the
2011 Ohio and Indiana Weed Control Guide. The guides are available here
at the Extension Office for purchase. Products are broken down by crop
with ratings on efficacy by weed. In each section there is also a brief
summary of each herbicide including mode of action, active ingredient,
rate recommendations, adjuvants needed, and formulation. One of the
most helpful sections is entitled ‘Control of Problem Weeds’. For more
information on weed management, obtain a copy of the 2011 Weed Control
Guide or contact Justin Petrosino at (937) 548-5215 or petrosino.3@osu.edu.
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