2019-06-22_New_Scientist

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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