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Scouting this Growing Season: Weeds
By Justin Petrosino, ANR Extension Educator
OSU Extension, Darke County
Its time to talk about my favorite topic in pest management, weed
control! Actually I should say management, control denotes a
reactionary measure. We want to be proactive when it comes to taking
care of our weeds. This is especially true when it comes to tough to
handle weeds like marestail. Knowing a little about our weeds lifecycle
can also help us plan a program to manage it.
Let’s use marestail as a case study. Here we have an ugly weed that can
begin to emerge as early as August and continue emergence late into the
fall. Those plants then overwinter and as wheat breaks dormancy, so
does marestail. This is complicated by stragglers that emerge in the
spring time. Marestail throws us another curve beginning in late April
by switching from being a small basal rosette to bolting into the tall
plant we typically see peaking through the soybean canopy. As if that
wasn’t bad enough we also have populations in Ohio that are resistant
to glyphosate and ALS herbicides.
How can we use this knowledge against marestail? Simple: attack early,
attack often, and attack with everything we’ve got before it gets four
inches tall. Marestail management should begin in the fall with an
application of glyphosate + 2,4-D or Canopy + 2,4-D. Come back in the
spring with a broad spectrum product like Ignite 280 SL or glyphosate
with some extra kick like 2,4-D or Sharpen. Now there is a caveat with
Sharpen, once we use it the label restricts using another PPO herbicide
for 30 days. That eliminates another key element to our program, PRE
herbicides with residual activity. Out of all of our best PREs Canopy
DF is the only product that does not contain a PPO. So it will work
with Sharpen, but add metribuzin. If you are not using Sharpen in your
burndown other products lines like Valor and Authority have great
residual control of marestail utilizing PPO herbicides. Another proven
spring program is paraquat + 2,4-D + metribuzin. It is possible to have
a one pass burndown + PRE treatment one week before soybean planting.
Just make sure to use the appropriate rate of 2,4-D. That of course can
be found on the label.
Now can we use POST herbicides to control marestail? Reading over
research by Dr. Loux’s crew shows that utilizing Ignite 280 SL POST on
marestail can be beneficial IF we utilized fall control, a good
burndown with a PRE, AND marestail is four inches tall or less. The
point of marestail management is to keep it under control until the
soybean canopy closes. Marestail is a poor competitor if it emerges
under a closed canopy.
The other weed we can’t seem to control is giant ragweed. In soybean
many of the same spring burndown programs we use for marestail control
work for giant ragweed. However we need to use PRE product containing
the ALS herbicides chlorimuron or cloransulam-methyl. These can also be
found mixed in with some of our PREs for marestail. We do have better
POST programs for giant ragweed with Ignite 280 SL or glyphosate, if
you are not fighting glyphosate resistance. However to make these
programs work giant ragweed should be four to six inches tall for
Ignite or six to ten inches tall for glyphosate. In corn adding
atrazine to other broadleaf herbicides can typically control giant
ragweed.
In general we want to observe a few simple rules in weed control.
First, we want to start clean with a good burndown. By slacking off
here you may set yourself up for an automatic 16% yield loss from
marestail. Second, we want to stay clean until the soybean canopy
closes by utilizing PRE and POST herbicides. To speed canopy closure
you can drill beans or plant in 15s rather than 30s. Third, we want to
scout and evaluate. Monitor what weeds are in your field throughout the
season and see what weeds escape your program. Programs may need slight
tweaks here and there or a major overhaul after being implemented for
many years. For more information on weed control consult the 2011 Ohio
and Indiana Weed Control Guide (bulletin 789) or contact Justin
Petrosino at (937) 548-5215 or petrosino.3@osu.edu.
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