Fight Resistant Weeds With Fall, Spring Attack
STONEVILLE, MISS.
Research is backing producers’ intense efforts
this fall to attack glyphosate-resistant
Italian ryegrass using a variety of methods
in attempts to limit the damage this troublesome
weed can cause.
In 2005, Italian ryegrass resistant to the commonly
used herbicide glyphosate was first identified
in the state. Since then, it has been found
in 31 Mississippi counties and is widespread
throughout the Delta. This glyphosate-resistant
weed emerges in the fall and grows throughout
winter and early spring.
“Italian ryegrass has a wide range of adaptability
to soils, and it is highly competitive for
nutrients, water and sunlight,” said Jason
Bond, a researcher and Extension weed scientist
working out of the Delta Research and Extension
Center in Stoneville.
“Plants that are emerging now can pose a serious
threat to spring burn-down programs if allowed
to grow until spring,” Bond said.
Mississippi Agricultural and Forestry Experiment
Station work began in 2005 at the Delta
Research and Extension Center to determine
how to manage glyphosate-resistant Italian ryegrass.
To date, researchers have screened 46
different residual herbicides for their effectiveness
against Italian ryegrass. In 2010 and 2011,
researchers tested the effectiveness of a two-phase
approach: fall residual herbicide treatments
followed by spring burn-down
applications, where a nonselective herbicide is
applied to fields before planting.
Populations of glyphosate-resistant Italian ryegrass, such as this
weed photographed in Washington County in 2010, have been found
in 31 Mississippi counties. (Photo by MAFES/Tom Eubank)
“Management of glyphosate-resistant Italian
ryegrass requires a multifaceted approach,”
Bond said. “Herbicide options are limited, and
Italian ryegrass has a history of rapidly developing
resistance to multiple herbicide
chemistries.”
Plans must include fall control measures such
as a residual herbicide or tillage, he said.
“The take-home message is that glyphosate resistant
ryegrass management must include
both fall and spring control tactics,” Bond said.
Tom Eubank, assistant research and Extension
professor in Stoneville, said a fall attack
must be made on glyphosate-resistant Italian
ryegrass.
“It’s usually a lot easier to kill any sort of weed
when it’s small,” Eubank said.
A program that uses a non-glyphosate herbicide
plus a residual herbicide tends to be the
best management choice in the fall because this
treatment kills the emerged weeds and keeps
later ryegrass from emerging.
The primary reason why Italian ryegrass is difficult
to control is that it is a clump-forming
grass. Herbicides that rely on contact with foliage
often do not reach all areas of the plant to
provide sufficient control, and systemic herbicides
are a better choice for control of this weed.
“Italian ryegrass is resistant to up to three
modes of action, which refers to the process or
activity within the plant that a herbicide interrupts,”
Eubank said. “You should try to use an
integrated pest management system. The more
things you can do to try to control the weed, the
less likelihood it will develop resistance to a particular
herbicide.”
A weed can develop resistance as it is repeatedly
exposed to the same chemical means of
control. For example, when Italian ryegrass is
sprayed with glyphosate, the majority is killed,
but a plant or two may survive. Those that survive
multiply, and the next generation has a few
more plants that survive the next glyphosate
burn-down. In time, the population that remains
is completely resistant to this once-effective
chemical.
“The integrated pest management program we
recommend uses fall residual herbicides to help
reduce the overall population and numbers,”
Eubank said. “Fall tillage can also reduce weed
numbers, but it is generally not as effective as
residual herbicides. Producers should come
back in the spring or late winter with an alternative
herbicide program that attacks the plant
using a different mode of action.”
Eubank said a diversified herbicide program
is important to continue the fight against
glyphosate-resistant weeds and prevent others
from developing. Δ