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

Irrigation management
Terraced contours
Lasered terraced contour systems (with bankless supply
channels contained within the contour bay) are being
adopted in the Murrumbidgee Valley to suit both rice and
winter crop production. Significant cost savings can be
made by not changing the contour irrigation layout to a
more conventional border check layout for winter crops.
Because the contours are terraced and are designed for
very high flow rates, the waterlogging risk is greatly reduced
compared to a traditional contour layout.
Spray irrigation
Spray irrigation using centre pivots and lateral moves
is suitable for canola, provided the irrigation system is
designed to meet demand during spring and provided there
is sufficient output to match evapo-transpiration (ET) levels of
around 8 mm per day. In practice, most centre pivots will be
designed for a summer crop of 12 mm ET per day or better
so when planning multiple circles do not overstretch the
capacity of the irrigation system. Water application rates and
application frequency need to match the crop requirements
and the infiltration characteristics of the soil to ensure there
is no waterlogging or moisture stress.
Soils that disperse on wetting are generally not suited
to spray irrigation systems because the water output of
the irrigator exceeds the soil’s capacity to take that water
in. Stubble retention has enabled much improved water
infiltration rates under centre pivots, particularly where soils
are prone to dispersion.
Water quality is important with spray systems, as salty
water applied direct to foliage can damage plant tissues.
The management of irrigated canola crops is similar to
that of canola grown under high rainfall conditions in many
ways (see earlier chapters). The following information applies
specifically to irrigated canola.
Sowing,
establishment and
irrigation
Sowing time is a major factor in achieving high-yielding
irrigated canola crops. Sowing time needs to take account
of the maturity of the variety as it relates to flowering time
and pod filling; the irrigation layout and establishment
method; the soil type and plant height at flowering, which
generally indicates lodging potential.
A second critical factor is sowing depth. Most soils used
for irrigation in south-eastern Australia are medium to heavy
clays or varying soil surface structure. The objective of
sowing at a shallow, uniform depth cannot be overstated.
The small canola seedling will find it difficult to push through
a clay from depth.
On raised beds, shallow sowing and watering-up is the
preferred method of establishing the crop. Aim to sow and
water-up between 20 April and 10 May. Good drainage is
essential. This sowing time, when soil temperatures are
generally higher, ensures establishment prior to the onset
of cooler, wetter winter conditions. Sowing in the first two
weeks of April is not recommended as it can lead to crops
flowering too early and lodging.
On border check (flat) layouts, the preferred method is
to sow dry and water up on soils that have good structure
and are not likely to slake or crust. In this situation, sow
seed shallow, only 5–10 mm deep, onto a firm seedbed
base to reduce the risk of seed sinking and not establishing.
Heavy rollers can be used to break down large clods and
produce a firm, level seedbed. Always sow up and down
in the direction of the border check as this will aid drainage
throughout the year. Canola is slower to establish when
watered up on flat layouts than on raised beds. Therefore,
aim to sow slightly earlier, 15–30 April in the Murrumbidgee
valley of NSW and 20 April–15 May in northern Victoria.
For soils that are not suitable for watering-up (commonly
red soil types), the preferred method on border check
layouts is to pre-irrigate in late March/early April and
sow the seed at 15–20 mm into moisture as soon as
the soil is trafficable. Press wheels or rollers will improve
seed-soil contact and assist germination. Medium-term
weather forecasts, whilst not completely reliable, will give
some indication of the potential for rain during the time
the paddock will be drying out awaiting sowing. In some
seasons, it may be prudent to delay pre-irrigation for a
number of days, for example, if no rain is forecast for April.
Aim to sow the crop from 20 April and finish by 15 May.
Expect to use 1–2 ML/ha to establish the crop in a dry
autumn. Considerably less water will be required to establish
a crop using spray irrigation. About 30–40 mm is sufficient,
and will provide some additional stored moisture going into
winter.
Spring irrigation
Timing of the first spring irrigation is critical.
Canola’s peak water use is from stem elongation until the
end of pod filling (about 25 days after the end of flowering)
when any stress will result in yield loss. However, its quick
early root development allows for a more efficient use of
water early in spring. Because of its more advanced growth
stage, canola should be irrigated before wheat as it will have
used more soil moisture through late winter. Canola is less
tolerant of waterlogging (often caused by a combination of
irrigation and rain) than wheat in the period from flowering to
maturity.
Adequate soil moisture at flowering is crucial, particularly
at early flowering. The first irrigation should be applied at
60 per cent plant available water (PAW). This ensures there
will be no moisture stress at the critical flowering time. A
missed irrigation during flowering can halve yields. In crops
with lower yield potential and where irrigation water is
limited, one timely irrigation at early flowering will give the
greatest yield and financial response. Early spring irrigation
will maximise the number of pods which set seed and later
irrigations will maximise seed size.
Canola best practice management guide
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