For today’s top stories, visit
SN Today at www.sciencenews.org
University in Corvallis chronicles
the emergence of herbicide-resistant
hybrid goat grass. It is evolving the ability to tolerate the herbicide imazamox
in fields planted with wheat that has
been bred to be immune to the chemical. Here, the weed scientist explains,
resistance traces not to herbicide overuse, but to the spread of resistant genes
in the pollen of wheat, the weed’s distant
cousin, through interbreeding.
The irony, Mallory-Smith says, is that
farmers have found the imazamox-resis-tant wheat such a high performer that
many actually apply little or none of the
herbicide to which it’s immune.
Applying the weed killer would
knock out much of the goat grass invading farmers’ fields. But left untreated,
the goat grass survives to accept pollen from the wheat — and in so doing,
creates hybrids that incorporate its
cousin’s resistance. Although farmers
can target the hybrid goat grass with
other herbicides, doing so risks killing
the wheat as well.
Weeds immune to one herbicide
will generally also prove insensitive to
others that employ the same mode of
action — usually the chemical’s disrup-
tion of an essential enzyme function in
the plant. For some major crops, weeds
have already countered most com-
mon modes of poisoning, notes Patrick
Tranel of the University of Illinois at
Urbana-Champaign.
Back Story | RESISTANCE MOVEMENT
The International Survey of Herbicide Resistant
Weeds monitors the amount of crop acreage worldwide that is affected by chemical-resistant plant
pests. Wild oats tops the list, but in the United
States the biggest problems are common water
hemp, Palmer amaranth and horseweed, says survey director Ian Heap. Palmer amaranth may be the
most serious threat to crops, he says, because it’s
immune to so many different classes of herbicide.
Wild oats
Common water hemp
Horseweed (mare’s tail)
Ryegrass
Palmer amaranth
Wild (red) rice
SOURCE: I. HEAP/ WEEDSCIENCE.ORG
Million hectares impacted
4. 9
3. 2
1. 8
1.6
1. 4
Growing menace In recent decades, hundreds of weed types have developed resistance
to common agricultural herbicides. The problem
has kept growing even with the introduction of
new chemical formulations designed to attack
weeds via novel biochemical strategies.
exaggerating when I say, at least for soy-
beans, we’re on the verge of running out
of options.”
Stephen Powles of the University of
Western Australia in Perth notes that in
his country, some weedy ryegrass spe-
cies “can be resistant to seven different
herbicide types, meaning there are
almost no herbicides which still work.”
Herbicide makers have encour-
aged farmers to embrace the mantra
KISS — for “keep it simple, stupid,” says
Jonathan Gressel of the Weizmann
Institute of Science in Rehovot, Israel.
He and other weed scientists now argue
that only the opposite approach will
stall the growing evolution of herbicide
resistance in weeds. Scientists advocate
rotating crops on a given field, applying
several different herbicides and con-
sidering the use of additional physical
treatments such as burning or light till-
age. Gressel would also go so far as to
recommend quarantining fields where
resistant weeds initially turn up.
Unfortunately, Owen says, farmers
think they can continue using simpler
herbicide-centered strategies “as they
wait for industry’s next silver bullet.” But
novel herbicides are not on the horizon,
he says, so a diverse approach to weed
management must be. s
ARTHUR CHAPMAN/WIKIMEDIA COMMONS
www.sciencenews.org
