Agriculture Reference
In-Depth Information
topping in the year prior to cropping wheat, fol-
lowed by shallow autumn cultivation before wheat
sowing the next year, can provide rigid ryegrass
control similar to that by selective herbicides.
However, this system has greater complexity and
requires effective integration of grazing pressure
to prevent weeds that escape nonselective herbi-
cides from setting seed (Table 12.9).
Opportunities exist to integrate different weed
control tactics in a continuous cropping system to
complement use of selective herbicides. A delay
in seeding of wheat can achieve greater recruit-
ment from the seed bank of rigid ryegrass (Gill
and Holmes 1997). These presowing weed cohorts
can then be killed by the application of glyphosate
or paraquat as resistance to these nonselective
herbicides is still rare in southern Australia.
However, the effectiveness of this tactic is depen-
dent on rainfall, which can be erratic, and wheat
yield penalties up to 20% can occur when seeding
is delayed 3 to 4 weeks past the optimum date
(Anderson and Sawkins 1997; Hocking and
Stapper 2001).
Other weed management tactics that are being
adopted by growers include higher crop seeding
rates for improved crop competition with weeds
(Lemerle et al., 2004). Crop-topping is another
practice that has become important for weed
population management for growers engaged
in continuous cropping. In this technique, para-
quat or glyphosate is sprayed to kill weed seeds
when weeds are at the fl owering to soft dough
stage. Crop-topping is an expansion of the tech-
nique which was originally developed for use in
pastures (spray-topping) and is now registered for
use in pulse crops. If used at optimal timing, seed
production of rigid ryegrass can be reduced by
around 90% when using glyphosate or paraquat
(Gill and Holmes 1997). However, weed seed-set
reductions are often between 70% and 80%
because of later-than-optimum timing of crop-
topping treatments to avoid crop damage. Conse-
quently, the weed seed bank in the subsequent
wheat crop in the rotation is considerably
reduced.
Other control tactics that have found some on-
farm adoption include capture of weed seeds
during the harvest operation as well as autumn
burning of crop and weed stubble to kill weed
seeds. Walsh and Parker (2002) showed that 75%
to 85% of the seed entering the harvester can be
captured in the chaff cart. However, the effective-
ness of this technique can be strongly infl uenced
by the level of weed seeds shed prior to harvest
due to strong wind events and delays in the timing
of harvest.
Stubble burning is an old method of weed
management that can provide variable levels of
weed-seed kill depending on the level of fuel
present (i.e., residue load) and the level of seed
burial. Recently there has been farmer adoption
of a technique in which stubble and weed seeds
are concentrated in windrows during harvest.
Burning of these windrows not only results in
greater effectiveness of the burn due to high fuel
load, but it also reduces the erosion risk associated
with whole-fi eld burning (Walsh and Powles
2007).
Table 12.9
Effectiveness of different weed control tactics
on the management of annual ryegrass in southern
Australia.
Effectiveness in Annual
Ryegrass Control (%)
Weed Control Tactic
Autumn
Stubble grazing
<
20
70 (20-95)
a
Autumn burning of stubble
Autumn cultivation to stimulate
weed germination
25 (10-40)
Delayed sowing to kill weeds
40 (10-70)
Winter options
Crop species, cultivar, seed rate
10-50
(seed set reduction)
Weed kill with selective
herbicides (herbicide
selection based on resistance
status)
90 (70-99)
Spring
Spray-topping/crop-topping
85 (50-95)
Green manure crops (sprayed
with glyphosate)
90 (70-95)
Hay cutting
80 (65-90)
Weed seed capture at harvest
60 (40-80)
by chaff carts
Source:
Adapted from Gill and Holmes (1997).
a
Values in parentheses represent the range of control
achieved.
