12 | New Scientist | 22 June 2019
THE most damaging weed in the
UK is about to become resistant
to the main defence farmers
have against it – the weedkiller
glyphosate. Other countries
around the world are facing
similar problems, which could
decimate food crops.
Many weeds have evolved
resistance to several different
kinds of herbicides, and some
are set to become resistant to all
the herbicides used on particular
crops. These superweeds will
cause major crop losses and push
up food prices. They will also
speed up climate change and
harm wildlife as even more
land is converted for farming
to make up for the lower yields.
“It is not a matter of if but when
we are going to be losing chemical
control of these weeds,” says
Adam Davis at the University of
Illinois at Urbana-Champaign.
In Europe, it is estimated that
weeds resistant to the herbicide
glyphosate will cause yields of
wheat, barley and oilseed rape to
fall around 10 per cent, causing
a loss of produce worth around
€2 billion to farmers.
An additional 2 million hectares
of farmland would be needed to
compensate, resulting in higher
greenhouse gas emissions and a
loss of natural habitat. The latter
is the single greatest threat to
wildlife and biodiversity.
The latest worry is a plant called
blackgrass. “It can totally infest a
field to the extent you can barely
see the crop,” says David Comont
of Rothamsted Research in the UK.
Strains of blackgrass (Alopecurus
myosuroides) are already resistant
to certain herbicides, and some
are resistant to several at once.
Glyphosate is often the last line of
defence. Comont tested blackgrass
from more than 100 fields in the
UK and found that it is evolving
resistance to this weedkiller too
(New Phytologist, doi.org/c7bg).
In the US, several weeds are on
the brink of becoming resistant
to all herbicides that can be used
on certain crops – soya beans
for example. Growers of this use
eight classes of herbicides that
kill weeds in different ways,
says Davis. But some strains
of a problem plant called
tall waterhemp (Amaranthus
tuberculatus) are resistant to five
of these classes of herbicides.
“This is a very large problem,”
says Davis. “Many different
weed species in many different
parts of the world and many
different cropping systems are
developing resistance.”
Both Davis and Comont
compare the situation to the rise
of antibiotic-resistant microbes.
The cause is the same – the
overuse and misuse of antibiotics
and herbicides leads to rapid
evolution of resistance.
But this evolution can be
prevented. The key is to combine
lots of different ways of
controlling weeds so the plants
never have a chance to evolve
resistance to any one method.
That might mean rotating crops,
growing crops at a different time
of year, combining herbicides that
kill in different ways and so on.
The trouble is that farmers
who do the right thing make
less money in the short term.
Economically speaking, there
are no disincentives for overuse
of weedkillers, says Davis.
It is possible to control weeds
without synthetic herbicides, as
organic farmers do, but it is more
difficult and more expensive,
and yields are usually lower.
For instance, hand weeding is
effective, but very costly because
it is so labour intensive. ❚
THIN and crispy in spots, chewy
or underdone in others – that is no
way to serve a crêpe. It is easy to
get uneven results when cooking
these treats, but a mathematical
analysis has revealed how to handle
a pan to rustle up a perfect pancake.
Mathieu Sellier at the University
of Canterbury in New Zealand
and Edouard Boujo at the Ecole
Polytechnique in France modelled
the fluid dynamics of crêpe batter
in a heated pan. To them, a crêpe
should be uniformly thin, free of
holes and perfectly circular. That
is easier if using a wooden crêpe
spreader, but they assessed how
to achieve this just by moving a pan.
The pair modelled the variables
involved: the effects of gravity and
temperature on the batter as it
spreads, the tilt and rotation of
the pan as well as the cooking time.
They found that uniformity of crêpe
thickness depends on pan motion
and the steady increase in viscosity
of the batter as it cooks.
They say that the best method
starts with pouring batter into a
very hot pan, then steeply tilting it
so the liquid flows to the rim. The
steep angle keeps the fluid moving
as it heats up and starts to solidify.
Next, rotate the pan, while still
inclined, in a circle to coat the full
circumference of its cooking surface
with the batter. While completing
this circular motion, decrease the
tilt of the pan until it is horizontal,
and let the crêpe cook to your taste.
Then flip and enjoy.
This leads to 83 per cent more
uniformity than just plopping
the batter in the pan and letting
it spread on its own (Physical
Review Fluids, doi.org/gf3tc4). ❚
Chelsea Whyte
Maths reveals
how to avoid a
crummy crêpe
Fluid dynamics
“ Superweeds will cause
major crop losses, speed
up climate change and
harm wildlife”
Rise of weeds we can’t kill
Pest plants are evolving to resist our most powerful herbicides
Evolution
Michael Le Page
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