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WEED OF THE WEEK - WATERHEMP
B: Would it scare you to know that there is a weed out there that
can grow 4 inches a day, produce over 100,000 seeds per plant, and
has already developed resistance to the Pursuit family of herbicides?
D: Well, there is such a weed, and for many farmers, it has already
become their worst nightmare. It's waterhemp. Now there are actually
2 different kinds of waterhemp, common waterhemp and tall waterhemp,
but they are so similar, we will just group them together for our
discussion today. We'll tell you the best ways to control waterhemp
in both corn and soybeans, but first, let's talk about why waterhemp
has gone from being a weed we saw every-once-in-a-while, to being
one of the biggest weed problems we face every year.
B: The main reason why waterhemp is so bad on many farms, is that
farmers quit using Treflan and Sencor years ago, replacing them
with Pursuit and Roundup. This led to both weed resistance and late-emerging
weeds that had no residual chemical to control them. To explain
this further, we need to talk about the difference between waterhemp
and its close relative, redroot pigweed.
D: Waterhemp is in the pigweed family of weeds, as is redroot pigweed.
The way to tell the difference between the 2 is that redroot pigweed
has very small hairs all along the leaves and stems. Waterhemp has
no hair whatsoever, which is why waterhemp is also called smooth
pigweed at times. Back 10 or 15 years ago, there was very little
waterhemp, but lots of redroot pigweed in this part of the country.
As waterhemp began to set in, most people just thought it was redroot
pigweed, which Pursuit is labeled to control. However, since waterhemp
had already developed a resistance to the Pursuit family of chemicals,
it was not controlled, instead, it flourished.
B: Since each waterhemp plant can produce over 100,000 seeds, needless
to say, after a couple years of misidentification and poor control,
waterhemp outbreaks became widespread over just the last 2 to 3
years. Now that most people know they have waterhemp, better control
plans are being put in place.
D: The important thing to remember with waterhemp is that it emerges
late in the season. Mid to late June is when most of it comes up.
Usually with weeds that germinate that late in the year, crop canopy
shades enough sunlight so the weeds don't become a problem. Waterhemp,
though, is different.
B: Waterhemp is a rather unusual plant in that it puts nearly all
its energy into above ground vegetation, leaving the roots small
and weak. All it cares about is getting quick enough top-growth
to get above the crop before it dies. Once it is above the crop
canopy, it will then strengthen its roots. As we mentioned earlier,
waterhemp can grow 4 inches a day or more. If you plant your crop
late and it is only a foot tall or so when the waterhemp emerges,
your crop can be overtaken by waterhemp in a week, easily.
D: Residual herbicides are the key to controlling waterhemp. Many
farmers with Roundup Ready crops last year had waterhemp problems
because the waterhemp emerged after the Roundup was sprayed. Once
again, we feel that Roundup Ready crops provide an excellent means
at stopping perennials in their tracks, but annual weeds like waterhemp
are usually more effectively and economically controlled with conventional
herbicide programs.
B: Speaking of which, let's discuss the best herbicides to use
so waterhemp won't bother your crops this summer.
D: In soybeans, Sencor is by far the most popular chemical used
to kill waterhemp. As long as your soil pH is 7.0 or less and your
soil is not sandy, a third of a pound of Sencor will answer your
prayers for around six dollars an acre. There are two new products
that also contain Sencor this year. Domain and Boundary can provide
the weed killing power of Sencor plus added grass and nightshade
control.
B: Treflan, Sonalan, and Prowl all offer good, but not perfect
control. The same can be said about Authority, the new pre-emerge
chemical sold by DuPont.
D: Post-emerge in soybeans, Flexstar, Cobra, Status, and Blazer
are all good, but again, not perfect. If you spray the waterhemp
when it is 2 to 4 inches tall on a hot, humid day, you may have
excellent results. However, if you use any of the post products
on tall waterhemp on a cold, cloudy day, your results could be poor.
Our best advice is to put something out pre-emerge that will get
most of the waterhemp, then use one of the post-emerge products
to clean up any escapes.
B: In corn there are a lot of good options. Axiom, which contains
Sencor is the best choice of all. On our farm we take the problem
waterhemp areas each year and apply Axiom for corn. Not that Surpass,
Harness, or Frontier would do poorly, but they won't perform as
good as Axiom. The product you will want to avoid in waterhemp areas
is Dual. Dual is definitely weaker on waterhemp than the other products.
D: Other pre-emerge products that have pretty good control of waterhemp
are Balance, Epic, and Bladex. Balance and Epic are not labeled
in Minnesota, Wisconsin, or Michigan, though, and Bladex is no longer
made. Although some Bladex is still available.
B: Post-emerge in corn, Distinct is definitely the best choice.
Although it is expensive this year at around $11 an acre, no product
is better on waterhemp.
D: By adding a half pound of atrazine to Clarity, you will achieve
results close to what Distinct will provide and save yourself over
two dollars an acre. Buctril/Atrazine or Hornet with atrazine will
give good suppression, but certainly not as good as Distinct or
the CLARITY/atrazine mix.
B: The most important thing to do to stop waterhemp, regardless
of whether it is in corn or soybeans, is to get a pre-emerge herbicide
down, then follow post-emerge with another product that will control
any escapes.
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