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2.2 Pre-seeding Weed Control
Non-selective herbicides. The introduction of non-selective herbicides, paraquat/
diquat and glyphosate, in the 1970’s created the opportunity for effective pre-
seeding weed control eliminating the need for pre-seeding cultivations. These her-
bicides in particular greatly facilitated the rapid adoption of conservation cropping
systems based on reduced tillage and residue retention (Derpsch et al. 2010 ;
Llewellyn et al. 2012 ). In reduced tillage cropping systems, the only soil distur-
bance occurs at seeding where knife point or disc seeding systems provide some
cultivation for seed placement and fertilizer incorporation. Presently about 80 % of
the Australian crop is seeded in a one pass reduced tillage seeding system (Llewellyn
et al. 2012 ). The rapid and widespread adoption of reduced tillage seeding systems
has revolutionised global dryland cropping systems with large production benefits
realised from the conservation of soil moisture and nutrients as well as from the
timeliness of crop seeding.
2.3 At Seeding Weed Control
Pre-emergence herbicides. In Australia and elsewhere the widely adopted ‘no-till’
seeding systems utilise knife point seeding tynes with trailing press wheels that
accurately place crop seed in a protected environment at the bottom of a 15–20 cm
furrow. During seeding the knife point provides soil disturbance to depths of up to
30 cm allowing the deep placement of fertilizer below crop seed (Derpsch et al.
2014 ). The trailing press wheels compact the bottom of the furrow around the seed
to improve soil-seed contact. Post-seeding rainfall is concentrated around the seed
in the bottom furrow creating an ideal micro-climate for the germination and seed-
ling establishment. There is considerable “soil-throw” created during furrow forma-
tion with knife-point seeding tynes. This dynamic of some soil disturbance in
combination with furrow formation creates an ideal environment for crop seedling
establishment as well as the effective incorporation of pre-emergence herbicides
(Chauhan et al. 2006a, b). The removal of tillage for weed control at seeding in
combination with the widespread occurrence of resistance evolution to post emer-
gence herbicides has placed considerable emphasis on achieving effective weed
control with pre-emergence applied soil residual herbicides. This focus on pre-
emergence herbicides for weed control has however, come at a cost and is now
inevitably leading to the evolution of resistance in weed populations (Boutsalis
et al. 2012 ; Owen et al. 2014 ). Although pre-emergence herbicide resistance levels
are not yet at those for post-emergence herbicides they are impacting the ability of
producers to reliably control in-crop weeds. Residual herbicides are most effective
when they are soil incorporated and although this can be achieved by rainfall in
Weed Management in Dryland Cropping Systems