best practice management guide for south-eastern Australia - Grains ...

Canola in the low-rainfall cropping zone
6. Canola in the low-rainfall cropping zone
Trent Potter, SARDI; Neil Fettell,
John Holland, Greg Brooke, NSW DPI;
and Felicity Pritchard, ICF
Canola can provide the same rotational benefits in the lowrainfall
zone as in the more traditional growing regions of the
medium to high rainfall zones. The most important agronomic
benefits are cereal root disease control, nematode control
and management of herbicide resistant weeds. Research has
shown that the average yield benefits following wheat crops
range from 20 per cent in southern NSW and up to
54 per cent in some trials in South Australia. In the presence
of severe cereal disease these benefits could be larger.
Farms are generally much larger in drier environments.
Because of the uncertainty with seasons in these
environments, there is generally less emphasis on cropping,
more long fallowing and, often, more emphasis on grazing
and pasture. For these reasons, canola is often viewed as
an opportunistic crop rather than a permanent part of the
rotation, which it is in higher rainfall areas.
Canola is grown over a wide range of environments,
including those where growing season rainfall is low. In
south-eastern Australia these areas include: north-west
NSW, where crops are grown on predominantly stored
moisture; western areas of central and southern NSW, where
a combination of stored soil moisture and in-crop rainfall
is important; and the 300–350 mm annual rainfall zone of
Victoria and 250–350 mm zone in South Australia, where
crops are grown predominantly on in-crop rainfall. Canola
is not a particularly drought resistant crop so, in marginal
environments, techniques to reduce the risk of moisture and
high temperature stresses are critical.
While many principles are similar across the range of
environments of the low rainfall cropping zone, some factors,
such as optimal plant population and row spacing, will be
different, depending on subsoil moisture, sowing time and
growing season temperatures.
Key factors for success with canola in
the low-rainfall zone
Techniques which can be employed include:
n only sowing canola when adequate subsoil moisture
is present;
n no-tillage to conserve soil moisture;
n early sowing so that maturing crops avoid high spring
temperatures;
n choosing early to early-mid maturing cultivars;
n matching plant population and row spacing to
the situation; and
n correct nutrition to maximise water use efficiency.
These factors are discussed in detail following.
Adequate subsoil moisture or
in season rainfall
Canola plants are good at utilising subsoil moisture
provided the soil profile is free of subsoil constraints, such
as hardpans and sodic subsoils. The plant has a vigorous
tap root system and is able to extract moisture from depth,
particularly if sown early, allowing the crop some buffering
against the hot, dry conditions which can occur in spring.
In north-westen NSW, it is essential to have adequate
subsoil moisture at sowing. At least 1 m of wet soil at
sowing is critical to carry the crop through the season as
in-crop rainfall can be minimal. In western areas of central
and southern NSW, 70 cm of wet soil is recommended. In
South Australia and Victoria, crops are grown predominantly
on growing season rainfall but some growers will not sow
without some subsoil moisture. It is important to remember
that, because of the wide range of soil types, water holding
capacity of different soils is also highly variable and therefore
is more important in some regions.
Research in southern NSW showed that deep subsoil
moisture (below 1.2 m) can contribute 15 kg/ha/mm
extra yield compared to only 8–11 kg/ha/mm of growing
season rain. The soil profile must have adequate moisture
throughout and be free of subsoil constraints to allow the
crop roots to access this deep subsoil moisture. In seasons
with well above average rainfall, subsoil moisture may not
be important. Figure 6.1 shows the importance of subsoil
moisture for central and southern NSW.
No-tillage
On well-structured soils, no-tillage has been shown to
improve fallow moisture storage. Stubble of a previous crop
slows evaporation from the soil surface, and the crop can
utilise a higher proportion of rainfall. No-tillage, combined
Figure 6.1 Response of canola yield to starting soil
moisture in four comparisons at Condobolin NSW,
in 2002 and 2003
Yield (t/ha)
2.5
2.0
1.5
1.0
0.5
0
Trial 1 Trial 2 Trial 3 Trial 4
Low subsoil water Medium subsoil water High subsoil water
Canola best practice management guide
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