West Lafayette, Indiana
May 11, 2009
Soil-applied herbicides can knock
out a farmer's toughest weeds, but the chemical products are no
match against soggy fields, said Bill Johnson,
Purdue University Extension
weed specialist.
A wet early spring has made it difficult for crop producers to
use soil-applied, or residual, herbicides to their fields ahead
of planting. Producers should not push the envelope on herbicide
treatments while fields are holding too much water, Johnson
said.
"If it's dry enough to plant, it's dry enough to spray," Johnson
said. "We don't plant our crops when soil conditions are such
that we're dropping the seed into mud, so we don't want to apply
our herbicides onto muddy ground either.
"We run into challenges when we apply to fields that are too
wet. One, we're going to leave ruts in the field. Two, all
herbicides are labeled such that they cannot be applied to
standing water anyway. If we want to get the most out of our
soil-applied herbicides, we need to put them on in conditions
similar to those in which we would plant the crops."
As their name suggests, soil-applied herbicides are sprayed on
the soil before, during or just after crops are planted. While
too much water nullifies their effectiveness, most residual
herbicides need some soil moisture in order to work properly.
The herbicides are activated by moisture within the soil and are
then absorbed by weed seedlings. Weed growth stops or is
stunted, leading to plant death shortly after it emerges.
"A residual herbicide will have activity in the soil anywhere
from about one to six weeks after application, depending on the
product that you use," Johnson said. "So what you essentially
get, then, is herbicide activity that you don't really see
because you have fewer weeds coming up. The weeds that do emerge
from that residual system are going to be a little bit easier to
control with your post-emerge herbicides."
Excessive wetness isn't the only challenge to using soil-applied
herbicides. Johnson said farmers should be careful to:
- Choose the right residual
product. "You have to know which weeds you're going after,"
Johnson said. "The most effective residual herbicides are
going to be matched to the weeds species that are in that
field. For instance, if you have a giant foxtail weed
problem, using a broadleaf herbicide is not going to be of
much value in that field. You want to use a herbicide that's
active against grass weeds."
- Apply at the proper
carrier volume. Johnson said glyphosate is one of the few
foliar herbicides that works well at carrier volumes of 15
gallons per acre or less, which is why it is often tank
mixed with residual herbicides. Contact herbicides, such as
paraquat or Gramoxone, require of 15-20 gallons per acre, he
said.
- Avoid applying in windy
conditions to prevent spray drift.
- Rotate between products
and weed control practices to minimize the development of
herbicide resistance. "Over 90 percent of our soybeans and
over 70 percent of our corn are glyphosate- or
Roundup-tolerant crops, so it's going to be important for us
to treat that technology as an investment," Johnson said.
"We want to keep that technology around for as long as
possible, and the way we do that is to minimize selection
pressure for glyphosate-resistant weeds.
"Using the appropriate residual
herbicide on your worst weed problems and then saving glyphosate
for the post-emerge cleanup treatment is going to be the best
way to protect crop yields and slow the development of
glyphosate-resistant weeds."
For more information on weed control in agricultural crops,
visit the Purdue Weed Science Web page at
http://www.ag.purdue.edu/btny/Extension/Pages/WeedScience.aspx |
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