mare's tail infests Southeast Missouri fields
PORTAGEVILLE, Missouri, August
4, 2004, University
of Missouri: Mare's tail, a
familiar nemesis for Missouri farmers, has
reappeared with a new resistance to glyphosate-based
herbicides, a University of Missouri weed
Mare's tail, also known as horseweed in
the Delta region, "isn't a new problem,
but until recently, glyphosate controlled
it," said Andy Kendig, weed science
specialist at MU Delta Research Center in
Portageville. "Now, we see fields where
everything is burnt down except horseweed.
It's really erupted over the past two years."
Glyphosate is the active ingredient in
Roundup and other herbicides, upon which
some farmers rely almost exclusively. "Some
of the mare's tail problem is due to glyphosate-only
burndown treatments," Kendig said.
"The good news is that we have a couple
of very good treatment options. The bad
news is that it doesn't always get done."
MU researchers recommend a March application
of an herbicide such as 2,4-D or Clarity,
he said. "These two herbicides are
essential to control mare's tail - or primrose
or several other troublesome weeds. Even
if there are a few late-germinating horseweeds
that escape, this application is still needed."
Without the pre-plant burndown application,
he said, "you have to go with mediocre
cleanup options. There are only a few choices,"
including "old-fashioned tillage."
[Also reported at http://deltafarmpress.com/news
See also additional story: "Weed
control could be circle of truths"
creeping their way around popular herbicide, new UGA
August 24, 2004, University
of Georgia: Morning glories are beloved mailbox
flowers all over rural America, but to farmers, they
are something else: a noxious weed that can lower yields
and choke harvesting combines. For some 30 years, however,
the herbicide glyphosate has kept morning glories quite
effectively out of farm fields.
Now, for the first time, however, researchers at the
University of Georgia have identified morning glory
families that are tolerant to glyphosate – noxious
vines that could cause problems for the country's farmers.
"Our study suggests that serious and immediate
consideration should be given to developing regional
strategies for managing the evolution of tolerance in
morning glories," said Regina Baucom, a doctoral
student at UGA who directed the research.
Baucom and UGA assistant professor of genetics Rodney
Mauricio co-authored the study, which is being published
this week in the Proceedings of the National Academy
of Sciences. The research was funded by the National
Science Foundation and a research grant from Sigma Xi.
"Our interviews with farmers in the Southeast
suggest that morning glories can tolerate applications
of glyphosate," said Baucom, "and, in some
cases, increasing concentrations of the herbicide have
been required to control it."
Such an increase in tolerance to the chemical gives
researchers a unique opportunity to study the evolutionary
genetics of a novel trait and may help them and others
slow the rate of evolution of tolerance in morning glories.
What Baucom and Mauricio found was that, in at least
one natural population of morning glories they studied,
there is a substantial genetic variation for tolerance,
meaning that the "evolutionary door" is wide
open. For evolution by natural selection to succeed,
there must be genetic variation with a population and
a significant selective force. This study is a case-in-point
of evolution by selection – human-mediated evolution,
similar to the evolution of bacteria resistant to antibiotics.
"Given the continued presence of glyphosate, the
number of tolerant individuals should increase within
the population over time," the scientists reported,
"as might the overall level of tolerance of the
population." The fact that glyphosate is a relatively
recent tool in the fight against weeds led the scientists
to conclude that the tolerance trait in this wild population
was naturally occurring – not caused by use of
The presence of genetic variation, however, does not
in itself guarantee that tolerance to glyphosate will
evolve. The requirement also exists of "net selection"
for tolerance, and it is acted upon by two components:
fitness costs and benefits. The "benefit"
of being tolerant must outweigh any sort of "cost"
of being tolerant, much akin to the theory of economic
In fact, this research has shown that there is positive
directional selection for tolerance to glyphosate, meaning
that by applying glyphosate, those that are tolerant
to the herbicide produce more seeds than those that
are susceptible (given that susceptible individuals
either die or produce almost no seed). Perhaps more
key for the farmer, however, is the finding that in
an environment devoid of glyphosate, tolerant families
produce many fewer seeds or offspring than susceptible
families. This is evidence of a fitness cost of tolerance,
and this information can be used in managing or controlling
the further evolution of tolerance in morning glories
by arguing for not spraying RoundUp® in certain
years. Since the issues are so complex, new strategies
will have to be considered to control increasing numbers
of glyphosate-tolerant varieties.
"For glyphosate, such strategies could involve
something as simple as periodically spraying with alternate
herbicides, as long as there is little cross-tolerance
with glyphosate," said the authors. "If, however,
there is cross-tolerance with other causes of plant
damage, such as hail, herbivores or pathogens, alternative
spraying regimes may not be a viable mechanism for controlling
the evolution of glyphosate tolerance."
weed causing big trouble in Southeast
August 24, 2004, ARS News Service:
Like the plant in "Little Shop of Horrors"
a little-known weed is growing fast. Tropical
spiderwort, inconsequential for seven decades,
has recently spread in alarming proportions
in fields in Georgia, Florida and North
First detected in the United States in
the 1930s, the weed has made major gains
in Georgia, according to Agricultural Research
Service agronomist Theodore Webster of the
Crop Protection and Management Research
Unit in Tifton, Ga. Webster and his colleagues--Michael
Burton and Alan York of North Carolina State
University, and Stanley Culpepper and Eric
Prostko of the University of Georgia--are
monitoring the weed's advances.
In 1999, it was found in five counties
in southern Georgia. By 2002, 41 Georgian
counties reported tropical spiderwort was
present, and 17 listed it as moderate to
A 2003 survey revealed that tropical spiderwort
was entrenched in Georgia, affecting 52
counties, with 29 counties listing the weed
as moderate to severe. More than 195,000
acres in Georgia are infested. It's now
widespread in Florida, and has been discovered
on about 100 acres in Goldsboro, N.C.
Tropical spiderwort, Commelina benghalensis,
is now the most troublesome weed in Georgia
cotton and the second most problematic weed
in peanut. The weed competes with crops
for water and nutrients, and smothers the
crops at the same time. One reason for the
surge in the weed's growth is its resistance
to the commonly used herbicide glyphosate.
Conservation tillage [undertaken in conjunction
with the use of GM glyphosate-resistant
crops] and reduced use of soil-applied herbicides
may also be contributing to the problem.
According to Webster and his colleagues,
tropical spiderwort spread has coincided
with resurgent cotton production in Georgia.
Cotton acreage in the state increased from
about 260,000 acres in 1989 to nearly 1.5
million acres in 1995, in part due to the
success of the boll weevil eradication program.
Most cotton grown in Georgia is tolerant
to glyphosate, allowing growers to spray
the chemical on cotton crops to control
weeds. Webster and his colleagues are studying
the biology and management of tropical spiderwort
and will continue to monitor its presence
in the Southeast.