GENET archive


9-Misc: Herbicide-resistant weed plagues California

                                  PART I
-------------------------------- GENET-news -------------------------------

TITLE:  Herbicide-Resistant Weed Plagues Calif.
SOURCE: Associated Press, by Juliana Barbassa
        posted by Agbios, Canada
DATE:   10 Aug 2005

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Herbicide-Resistant Weed Plagues Calif.

PARLIER, Calif. -- Horseweed was once merely a nuisance to farmers - hard
to pull out, quick to sprout back after cutting, and capable of towering
over tractors.

Now, it's becoming a full-blown nightmare worthy of an agricultural
horror flick: scientists in California have found clusters of the weed
that are resistant to scores of herbicides, leaving farmers to fight an
increasingly formidable and costly foe.

Pete Christensen said he watched his costs soar as the most popular
herbicide became increasingly powerless to stop the weeds from choking
the grapes on his 75-acre vineyard near Selma.

About five years ago he started noticing that Roundup wasn't withering
the weed as usual. Three years later, he had tripled the concentration of
the herbicide, and had doubled the applications, but the weeds were
growing thicker than ever, rising over his vines and competing with them
for water, nutrients and sunshine.

"It was dominant in the landscape," Christensen said.

The weed, also known as mare's tail, has always been around, but it
wasn't until last month that University of California researchers
confirmed that some strains of it had become resistant to herbicides like
Roundup, posing a threat to the nation's most productive farmland.

Researchers were alarmed by the weed's rapid proliferation. Its spindly
stalks can be seen poking out of Napa Valley vineyards in the North,
along highways and pastures in the Central Valley and in Southern
California fields.

Farmers elsewhere have been dealing with resistance to the chemical
glyphosate. First found in Delaware in 2000, glyphosate-resistant
horseweed has since been found in 10 other states in the East and South.

Farmers dealing with the problem have been forced to repeatedly till
their fields, rely on weeding, or on more toxic herbicides to control the
tall, fast-growing pest.

Developing resistance to a chemical isn't unusual among plants and
animals, scientists said. What makes the horseweed adaptation such a
nuisance is how fast it reproduces and how big it grows, stretching 10 or
12 feet tall, sucking up scarce water and nutrients.

As a relative of the dandelion, each weed produces up to 200,000 tiny
airborne seeds a season on fluffy yellow flowers.

For decades, growers, gardeners and anyone looking for an easy way to
beat back weeds have relied on glyphosate. While it's inexpensive, it
works on several types of weeds, and is less toxic than other pest-
control ingredients.

Farmers planting Roundup-Ready crops such as corn, soybeans or cotton
that have been genetically engineered to survive the chemical could spray
it liberally over their entire field, killing all weeds and leaving only
their crops standing.

The herbicide's popularity may be partly to blame for breeding the
resistance, researchers said. By killing nonresistant weeds, it allows
only the survivors - those few naturally resistant plants - to thrive.

"They've created a problem by relying on one solution to solve all
problems," said weed ecologist Anil Shrestha of the University of
California's Kearney Agricultural Center.

Some scientists said the development wasn't surprising.

Systems like Monsanto's Roundup-Ready crops, which promise an easy, one-
chemical solution to the age-old problem of weed control, only work for a
short time, said Margaret Mellon, director for the Food and Environment
Program at the Union of Concerned Scientists.

"When you expand the use of an herbicide dramatically, resistant weeds
start moving in," said Mellon.

Bob Prys, a manager for the 13,000-acre Borba Farms, said the weed became
a problem just three or four years after they started growing Roundup-
Ready cotton on the 500-acre ranch. They sprayed the field, killing
everything but the cotton plants, and saving money by having to till
their fields less frequently.

Now Prys said they're relying on weeding again and adding other chemicals
to their herbicide mix - adding unexpected costs to the higher price they
pay for Roundup-Ready seed. "It's caused us to re-evaluate our Roundup-
Ready cotton," Prys said.

Monsanto researchers recommend mixing in other chemicals to eliminate the
threat before there is a problem, said David Heering, the Roundup
technical manager for Monsanto.

"At the end of the day, they'll still have fewer passes through the
fields, and fewer weed-control problems," Heering said.

The UC scientists recommended rotating crops, cultivating the land with
farm equipment, weeding, and the use of herbicides that kill the seeds in
the soil before they germinate.

Those measures will increase costs for farmers, but will prevent a more
serious and costly problem later on.

                                  PART II
-------------------------------- GENET-news -------------------------------

SOURCE: ISB News Report, USA, by Kelly M. Paulson
        files attached: aug0501-1.jpg & aug0501-2.jpg
DATE:   Aug 2005

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In biotechnology-related disciplines, the word "tolerance" is often used
as a synonym of "resistance"; for example, one might describe a Roundup
Ready® soybean as an herbicide-tolerant plant when actually it has been
genetically engineered to be resistant to the presence of a specific
herbicide (glyphosate)1.

Such sloppy semantics have eclipsed an important distinction between
tolerance and resistance recognized by evolutionary biologists. Tolerance
refers to the ability of an organism to withstand a certain amount of
damage from environmental insults without suffering the loss of fitness
that one would expect. Resistance, on the other hand, acts through a
biochemical change that blocks or reduces the action of a pesticide,
herbivore, or disease (for example), and resistant organisms hardly
notice the attack. For example, herbicide-resistant Roundup Ready soybean
plants are engineered with genes for proteins that sidestep the
destructive pathway of glyphosate, and exposure leaves them undamaged. A
plant exposed to glyphosate might evolve tolerance by producing more
seeds, or larger leaves, to compensate for the damage that glyphosate can
cause. Both resistant and tolerant organisms will enjoy higher fitness
than their non-resistant or non-tolerant counterparts; however, to
understand the dynamics of tolerant organisms over time it is important
to understand the different means by which this common ending is reached.

Although Roundup Ready soybeans may best be described as herbicide
resistant, what about the weedy species sympatric to this transgenic
crop? Might tolerance evolve in a species subject to frequent application
of an herbicide? This question has been investigated by the authors of
two recent articles in The Proceedings of the National Academy of
Sciences that discuss the development of herbicide tolerance in tall
morning glories (Ipomoea purpurea), a weedy species in the southeastern
United States1,2.

To measure tolerance, the fitness of a genotype is measured under
undamaging (control) and damaging environmental conditions1. In a
randomized block experiment, Baucom and Mauricio2 compared the relative
fitness of 32 maternal genotypes of tall morning glories under two
conditions: no herbicide applied; and RoundUp herbicide applied at a rate
of 1.121 kg/ha. To assess fitness, the investigators collected fruits and
viable seeds from the plants, and they noted the fraction of leaves
showing ill effects from glyphosate application. They regressed the
relative fitness of each genotype on the environmental condition (i.e.,
no herbicide or herbicide) and used the slope to indicate the level of
tolerance. In this analysis, a slope of zero implied complete tolerance,
which was operationally defined by the authors as the ability of the
plant to reproduce in spite of herbicide damage. For example, in the
figure below each line represents a different genotype's "fitness norm of
reaction"2 after the effect of block was removed; the x-axis represents
the two environments (0 = no herbicide; 1 = herbicide treatment). Lines
with negative slope (lower relative fitness under the herbicide
treatment) have low tolerance; positive slope indicates overcompensation;
and a slope of zero means that the genotype is completely tolerant of the
herbicide. Because the slopes are different across genotypes, this also
indicates that genetic variation for tolerance exists in these morning

Figure 1. Relationship between relative fitness and treatment environment
for the 32 maternal lines.
Residuals of fitness were used after the effect of block was removed. On
the x axis, 0 = glyphosate absent, and 1 = glyphosate present. Slopes of
the lines represent tolerance. (Source: Baucom RS & Mauricio R (2004)
PNAS 101, 13386-13390; copyright Proceedings of the National Academy of
Sciences USA)

By correlating the relative fitness under the different environments with
the overall tolerance of the genotype, the researchers assessed the costs
and benefits of tolerance.2 In Figure 2, relative fitness values under
control and herbicide-application treatments (in graphs A and B,
respectively) are correlated with tolerance.

Figure 2. Costs and benefits of tolerance.
(A) Costs of tolerance indicated by a significant negative genetic
correlation between fitness and tolerance in the presence of glyphosate
as measured by a standardized selection gradient. In both graphs, the y
axis depicts the residuals of relative fitness after the effects of block
had been removed, and the x axis depicts the level of tolerance for each
maternal line standardized to a mean of zero and a variance of one.
(Source: Baucom RS & Mauricio R (2004) PNAS 101, 13386-13390; copyright
Proceedings of the National Academy of Sciences USA)

The negative slope in A indicates that there is a fitness cost to
tolerance in a glyphosate-free environment; conversely, the positive
slope in B signifies a fitness advantage associated with tolerance under
the herbicide treatment (as one might expect). A specific example of this
fitness cost under the control condition is that the most glyphosate-
tolerant line produced 35% fewer viable seeds than the most glyphosate-
susceptible line2.

In other words, as long as there is sufficient variation in the genes for
tolerance (as there seems to be for I. purpurea), one would expect strong
selection for tolerance in the presence of the herbicide, and selection
against tolerance when the herbicide is absent.2

For risk assessors, the distinction between resistance and tolerance is
an important one, because the two traits are likely to evolve in
different ways and therefore have different effects1. In a study modeling
the evolutionary dynamics of disease resistance and disease tolerance in
a long-lived host, Roy and Kirchner3 found that resistance genes
eventually settled at a polymorphic equilibrium, whereas tolerance genes
were driven to fixation. Variation still exists in the traits governing
tolerance to glyphosate in the morning glory because Roundup represents a
relatively new selective force on this plant1. Because theory predicts
fixation of tolerance under constant exposure to a selective force3,
managers might be encouraged by Baucom and Mauricio's discovery of
fitness tradeoffs associated with tolerance when the selective force is
absent. This means that the evolution of tolerance might be delayed, in
this case, by halting application of Roundup or possibly by applying an
herbicide that acts through a different mechanism1. In the context of a
field of Roundup Ready crops, this recommendation necessarily implies
rotation to a different crop plant.

Monitoring specifically for tolerance in weedy species will be important
to better understand this phenomenon, particularly where selective forces
are high, such as in Roundup Ready fields. Further complicating future
management of tolerance is the possibility that tolerance and resistance
traits are genetically linked, so management schemes may have to account
for the evolution of both in certain species. Since the ecology of
agricultural landscapes is changing rapidly, including new selective
pressures associated with farming herbicide-resistant crops, agronomic
practices should be guided by adaptive management. As Roy cautions1, if
we don't learn how to effectively retard the evolution of resistance and
tolerance in pest species, we will compromise the efficacy of herbicides
and ultimately face a widespread agricultural problem.

Thanks to Drs. Anne Kapuscinski and Karen Oberhauser for comments on an
earlier draft of this review.


1. Roy BA (2004) PNAS 101, 13974-13975

2. Baucom RS & Mauricio R (2004) PNAS 101, 13386-13390

3. Roy BA & Kirchner JW (2000) Evolution 54, 51-63

Kelly M. Paulson Conservation Biology Graduate Program University of
Minnesota - Twin Cities Saint Paul, MN, USA


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