1052.4 Post Emergence Weed Control
The early crop establishment phase is the most critical for weed control in annual
crops as it is during this period that weed competition causes maximum yield loss.
It is also the period during which weed seedlings are generally most susceptible to
weed control strategies. Consequently, for many decades early post-emergence
weed control has been the focus of global herbicide companies for the development
for in-crop selective weed control treatments. However, the resultant highly effec-
tive herbicidal weed control and then subsequent resistance evolution has resulted
now in the severe circumstance of multiple herbicide resistance and limited in-crop
weed control options. The Australian experience should serve as an example of how
herbicides alone as not the solution for long term effective weed control in dryland
cropping systems.
In Australian dryland cropping systems, there are remarkably few problematic
weed species that dominate nationally. Annual ryegrass is by far the most important
(Alemseged et al. 2001 ; Llewellyn et al. 2015 ) with this competitive annual grass
abundant throughout the western and eastern Australian winter cropping regions.
Initially this species was introduced and nurtured across these regions as a highly
productive pasture plant for sheep production (Kloot 1983 ). However, the change in
emphasis from livestock to crop production systems in the 1990’s meant that
endemic and well adapted annual ryegrass populations were now a weed in cereal
dominated crop production systems. Although not previously established in the
sub-tropical northern wheat belt zone, annual ryegrass is now encroaching on this
region as well. Other major grass weeds, important across much of the Australian
grain belt, are wild oats (principally Avena fatua and A. ludoviciana) (Martin et al.
1988 ; Paterson 1976 ) and brome grass (B. diandrus and B. rigidus). The principal
broadleaf weed of Australian cropping is the highly competitive wild radish
(Raphanus raphanistrum) (Alemseged et al. 2001 ; Cheam and Code 1995 ), a seri-
ous weed in Western Australia and a localised problem throughout other regions of
the Australian wheat belt (Walsh and Minkey 2006 ). These weed species infest
cropping systems spread across a diverse range of environments and production
practices, and provide clear evidence of their genetic diversity resulting in adapta-
tion and importantly their evolutionary potential (Kercher and Conner 1996 ; Kloot
1983 ; Kon and Blacklow 1989 ; Menchari et al. 2007 ).
The combination of huge numbers of genetically diverse annual weed species
present in crop-production systems with minimal crop diversity, and strong herbi-
cide reliance has provided a remarkable example of evolutionary selection for her-
bicide resistance. Although herbicide-resistant weed populations have evolved in
many parts of the world (Heap 2015 ), nowhere else has this evolution been more
dramatic than across the Australian dryland crop production region. In Australia,
there has been widespread and frequent evolution of herbicide-resistant populations
in several weed species including annual ryegrass (Boutsalis et al. 2012 ; Broster and
Pratley 2006 ; Owen et al. 2014 ), wild radish (Owen et al. 2015 ; Walsh et al. 2007 )
wild oats (Mansooji et al. 1992 ; Owen et al. 2012b), barley grass (Owen et al.
Weed Management in Dryland Cropping Systems