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2012a; Powles 1986 ; Tucker and Powles 1991 ), and brome grass (Owen et al.
2012b). Resistance in wild radish and annual ryegrass populations continues to
increase in frequency and distribution where in the majority of populations, resis-
tance extends simultaneously across many herbicide chemistries (multiple herbi-
cide resistance) (Owen et al. 2014 , 2015 ). The management of multi-resistant
populations of these species is now a major focus across most cropping regions of
Australia.
The consequence of herbicide resistance is that once highly effective herbicides
are no longer of any use. The widespread occurrence of multiple resistant weed
populations and the subsequent loss of essential herbicide resources, threatens the
sustainability of conservation agriculture. Globally, the herbicide industry has
shifted focus from herbicide discovery programs to gene trait development.
Combined with the increasing difficulty in identification and development of new
herbicide molecules, the introduction of new herbicide modes of action has ceased
since the 1990s. Since that time, introduced herbicides have all focussed on existing
modes of action. Thus in the absence of new herbicides, producers for the foresee-
able future will be relying on existing herbicide technologies. With herbicide resis-
tance continuing to erode these herbicide resources, Australian crop producers are
losing the ability to effectively control weeds in conservation cropping systems.
Thus the sustainability of the current and any future herbicide resources is depen-
dent on the development of alternate, highly effective weed-control technologies.
3 Weed Seed Development
Bitter experience with herbicide resistant annual weed populations has resulted in
the widespread acceptance that it is necessary to target weed seed production wher-
ever feasible. The recognition that, the major proportion of an in-crop annual rye-
grass population results from the previous season’s seed production, (Gill and
Holmes 1997 ; McGowan 1970 ; Monaghan 1980 ; Pearce and Holmes 1976 ; Reeves
and Smith 1975 ) has led to a focus on minimising weed seed production. Crop-
topping, where non-selective herbicides (glyphosate or paraquat) applied over a
crop to target weed seed production, was developed following the successful use of
this practice in pasture phases (spray-topping) to control weed seed production (Gill
and Holmes 1997 ; Mayfield and Presser 1998 ). In situations where the crop has
matured while weed seeds remain immature there is a period of time, ‘window of
opportunity’ during which weed seeds can effectively be targeted without incurring
crop yield loss. Weed seed production is frequently reduced by up to 90 % for a
range of weed species including annual ryegrass (Gill and Holmes 1997 ; Steadman
et al. 2006 ), wild radish (Walsh and Powles 2009 ), Vulpia bromoides (Leys et al.
1991 ), barley grass (Powles 1986 ) and brome grass (Bromus spp.) (Dowling and
Nicol 1993 ). However, the reproductive phase of annual ryegrass is strongly influ-
enced by seasonal conditions and often the period of seed maturation overlaps that
of the crop (Aitken 1966 ). Frequently then the timing of crop-topping treatments is
M. Wa lsh