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achieved through the use of non-selective herbicides applied as whole paddock
treatments. Frequently though summer rainfall events are light and sporadic result-
ing reduced emergence of weed populations. In these instances, targeted spray
applications using boom mounted weed detection systems (e.g. Weed Seeker and
WeedIT) are often used in place of whole field spray treatments (Blackshaw et al.
1998 ; López-Granados 2011 ). This technology comprises optical sensors that turn
on nozzles only when green weeds are detected, greatly reducing total herbicide
use. The choice between whole paddock treatments and spot spray technology typi-
cally depends on the amount of out of season rainfall and the size of subsequent
weed infestations (Keller et al. 2014 ).
Elevated soil moisture and soil temperature levels support the rapid establish-
ment and growth of summer weeds. However, summer herbicidal weed control is
hampered by the prevalence of hot, dry conditions that reduce treatment efficacy by
interfering with the application, uptake and translocation of herbicides to plant tar-
get sites (Jursík et al. 2011 ; Lubbers et al. 2007 ; Mahan et al. 2004 ). Therefore, in
large cropping programs there are significant constraints on the effectiveness of
blanket herbicide treatments in summer fallow situations. For efficacy as well as
economic reasons, there is strong demand for the introduction and use of weed
detection and identification systems and it is likely that spot spray treatments will in
time become commonplace.
Fig. 2 Timing of routine weed control practices in annual dryland crop production systems
(Adapted from Cousens and Mortimer 1995 )
M. Wa lsh