cotton - Greenmount Press

Managing glyphosate resistant
weeds in summer cropping systems
■ By Graham Charles, NSW DPI
THE 2012–13 summer will see a new chapter unfold for
many cotton growers, as well as other farmers in the
northern cropping areas. It might be a good cropping
season, but it will almost certainly be a season of frustration,
with more and more weed control failures following glyphosate
applications.
This prediction is easy to make, given the alarmingly large
number of failures that were reported last summer, with weeds
such as awnless barnyard grass, feathertop Rhodes grass,
windmill grass and flaxleaf fleabane being very difficult to
control, even with a double-knock of glyphosate followed by
something like paraquat.
Unfortunately, glyphosate tolerant and resistant weeds are
now a reality in the cotton system, with the number of confirmed
cases of resistance escalating last season and many more
unreported cases observed. This hasn’t happened due to a failure
with the Roundup Ready Flex Crop Management Plan, or a failure
in the plan’s implimentation, but due to a failure in the farming
system, and primarily in the summer fallow component of the
system. The problem has been over reliance on glyphosate as the
primary weed control tool for too long in zero-tillage systems.
zero-tillage has been adopted on much of the cropping area
since the 1980s or early 1990s, with some fields now in zerotillage
for over 20 years and a few over 30 years.
The continuous use of glyphosate for summer weed control over
this period has placed very strong selection pressure on weeds and
the result has been a rapid increse in the emergence of glyphosate
resistant and tolerant weeds over the past couple of years.
Even where fields have been well managed or don’t have a
long history of glyphosate, all too often resistant weeds have
been introduced from an extenal source. Fleabane and windmill
grass are readily moving across the landscape in strong winds,
easily moving from property to property, and machinery such as
headers are notorious for introducing unwelcome weeds.
In hindsight, the emergence of a weed with resistance to
glyphosate was inevitable, although many experts at the time
maintained that resistance could never occur due to the mode
of action of glyphosate. Today, there are 23 different weeds with
resistance to glyphosate in 20 countries.
Interestingly, resistance is generally occurring in different
species in each situation. For example, Palmer amaranth has
developed resistance in the US and is a major problem there, but
no amaranths with resistance to glyphosate have been found in
Australia to date.
Nevertheless, this has been a wake-up call. Resistance to any
herbicide can occur if it is overused.
The summer fallow
Much of the success of the northern cropping system has
been due to the adoption of zero-tillage fallows, often with
retained stubble. These fallows have been relatively easy and
inexpensive to maintain with glyphosate, but will be much more
expensive and difficult to maintain once they become dominated
by glyphosate resistant weeds.
Many farmers are now moving to products such as Flame to
reduce the pressure on glyphosate and control the more difficult
Glyphosate resistant windmill grass is the latest in the series of resistant weeds and is becoming obvious around many cotton
fields. The resistant windmill plants on this head ditch have been burnt - off by herbicide but not killed.
44 — The Australian Cottongrower December 2012–January 2013