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OCR for page 9
The Australian Experience of Managing Herbicide Resistance
and Its Contrasts with the United States
Michael Walsh
University of Western Australia
Since the 1990s, the grain-growing regions of Australia have experienced a major
problem with herbicide-resistant weeds. This occurred because of a unique combination of
events. First, the highly productive pasture species annual ryegrass (Lolium rigidum) was
present at high densities across vast areas devoted to livestock production. Then, from the
1970s, much of this pasture land dominated by Lolium was converted to wheat fields. With
this dramatic shift from livestock to crop production, Lolium instantly was a problematic
crop weed. The newly available acetyl coenzyme-A carboxylase (ACCase) herbicides were
used widely and persistently. Herbicide use without diversity on huge populations of
genetically variable Lolium across vast areas resulted in widespread resistance evolution.
Cross pollination among resistant survivors ensured multiple herbicide resistance and the
loss of efficacy of many herbicides. This shattered the illusion of herbicide invincibility and
drove the search for integrated control strategies. Multiple resistance forced diversity in
weed-control practices and the use of both herbicide and nonherbicide tools for
sustainable weed control. For example, Lolium, like many crop weeds, relies on annual seed
production and seed maturity is synchronized with crop maturity. Importantly, Lolium
seed remains attached to the plant at the same height as the crop seed heads at grain
harvest. Modern grain harvesters are efficient at sorting weed seed from crop grain, thus
some 95 percent of Lolium seeds pass intact through the grain harvester to be returned to
the crop field in the chaff fraction, perpetuating the ongoing weed problem. Therefore,
grain harvest represents an excellent opportunity to target Lolium seed production.
Toward this, Australian farmers have developed and adopted several "harvest weed seed
control" (HWSC) systems that effectively target annual ryegrass and other weed seeds
during the harvest operation. The HWSC systems currently used in Australia include chaff
carts, direct harvest residue baling, narrow windrow burning, and the recently introduced
Harrington Seed Destructor. The use of HWSC in addition to herbicidal weed control now
9
OCR for page 10
10 NATIONAL SUMMIT ON STRATEGIES TO MANAGE HERBICIDE-RESISTANT WEEDS
has been proven to reduce Lolium infestations dramatically, clear evidence of the value of
new weed-control tools in prolonging the life of herbicides.
KEY POINTS
When ryegrass pastures were converted to wheat production in Australia,
ryegrass (Lolium) became the main weed.
Australian farmers have developed and adopted several "harvest weed seed
control" (HWSC) systems that effectively target annual ryegrass and other weed
seeds during the harvest operation. The use of HWSC in addition to herbicidal
weed control now has been proven to reduce Lolium infestations dramatically.
Annual Ryegrass (Lolium rigidum), courtesy of Macleay Grass Man
(http://freeimagefinder.com/detail/7370410186.html).
