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

Irrigation management
Border check irrigation can be successful with good irrigation
management.
Photo: D. McCaffery, NSW DPI
Overhead spray irrigation offers improved water-use efficiency and greater
flexibility than flood irrigation methods.
Photo: M. Grabham, NSW DPI
Murrumbidgee Valley. They allow the crop to be watered up
without soil crusting problems, and to be fully irrigated in the
spring without the risk of waterlogging the crop.
Raised bed farming is a system where the crop zone and
the traffic lanes (wheel tracks or furrows) are distinctly and
permanently separated. Paddocks are laser levelled before
bed construction, with the beds running down the slope.
Some growers in southern NSW are trialling raised beds
within bankless channel terraced contour systems. In these
systems the beds are formed on a zero or near zero grade.
Soil suitability needs to be carefully assessed for the raised
bed system. Self-mulching clay soils are well suited to raised
bed irrigation. Other soils may be suitable with appropriate
management, for example, the addition of gypsum and
organic matter. Soils should be free of any compaction layers,
have good soil structure to allow water infiltration into and
drainage out of the beds, be non-slaking and non-sodic, have
a soil pH Ca
greater than 5.0 and be at low risk of developing
shallow watertables and salinity. Hard setting and crusting
soils will impede water infiltration. The selection of bed width
will depend on the soil’s subbing ability. Farming equipment
may need to be modified to suit the proposed bed width, for
example, tractor or seeder wheel spacings.
After the beds are formed, all traffic should be restricted
to the furrows where possible, to avoid compaction and,
over time, allow better water absorption (subbing) as
soil structure improves. In practice, most harvesters will
run on the beds, hence the timing of the final irrigation
should consider crop development so that the soil profile
is sufficiently dry to carry the header and minimise soil
compaction. Tyre pressures can be lowered to reduce
compaction.
Bed width varies according to soil type and is typically in
the range of 1.5–2 m. Bed widths for red-brown earths or
non self-mulching clays should be narrower, about 1–1.5 m,
as moisture may not sub to the centre of the bed if the beds
are too wide. Some growers sow twin rows closer to the
irrigation furrow, leaving an unsown gap in the middle of the
bed.
Beds are usually formed by setting listers at the desired
furrow spacing to pull the beds up in a rough condition from
a cultivated surface and then using a bed shaper to form a
flat-top bed.
Ring rollers have been used to complement bedshaping
equipment. These are made of heavy cast iron
rings independent of a main axle to allow them to adjust
to the curve of the bed. The implement is cheap to use
and principally breaks down clods and firms up the bed.
Heavy rice rollers are also used for the same purpose, but
the soil must be dry. Depending on soil conditions, these
implements are used before chemical spraying and sowing.
Disc seeders or tined seeders with narrow points are
used to sow canola on beds. Row spacing can be flexible.
Border check (flat) system
The need for adequate slope and surface drainage will
dictate the soil type used for border check layouts. Heavier
soil types, which generally do not have the desired slope,
can be successfully irrigated using the lasered terraced
contour system. In both systems, watering and drainage
need to occur in 12–15 hours. High flows of 15–20 ML/day
will be needed to achieve these relatively quick watering/
drainage times, especially on the lasered terraced contour
system. Without high flows, traditional border check in small
bays is a better option.
Flood irrigated layouts must allow the crop to be irrigated
and drained within 12–15 hours. Border check is the most
common flood irrigation method in the Murray Valley and the
Goulburn-Murray Valley of northern Victoria. Border check
irrigation is acceptable when paddock slopes are 1:1000
or steeper – preferably between 1:750 and 1:1000. Lesser
slopes, for example, 1:1500 can be used if bays are short,
so long as the crop can be irrigated and drained within the
12–15 hour timeframe.
Border check bays should have uniform slope to allow an
even rate of water advance and infiltration.
Border check systems are not suitable for sandy soils
as the top of the bay will be overwatered by the time the
water reaches the drainage end, resulting in waterlogging at
the top end, and potentially excessive deep drainage to the
watertable.
72 Canola best practice management guide