| About 5300 hectares of
cotton is currently irrigated using centre pivots and lateral move machines
in Australia. This represents approximately four per cent of the irrigated
cotton crop in an average year — significantly more than is irrigated
by drip irrigation systems (3100 hectares).
While centre pivots have been used in the Australian cotton industry
for more than 20 years, recent interest in the performance and operation
of both centre pivot and lateral move machines resulted in the Cotton
Research and Development Corporation commissioning a scoping study to
better understand the issues surrounding the use of these. The study
was conducted during 2001 and involved a face-to-face survey of more
than 80 per cent of the 36 cotton growers using centre pivots and lateral
moves.
INDUSTRY OVERVIEW
Centre pivots and lateral moves are currently installed in each of the
major cotton producing areas from Emerald in Queensland through to Hillston
in NSW as well as in experimental areas in other states. The majority
of machines used in the cotton industry are centre pivots (76 per cent)
while the remainder are lateral moves. The average size of the centre
pivots is about 70 hectares while the average size of lateral move machines
is 165 hectares with the largest single machine irrigating 267 hectares.
Centre pivots and lateral moves in the cotton industry have been commonly
installed on cracking clay (61 per cent) and clay loam soils (27 per
cent). Half of the growers surveyed obtain all of their water from surface
supplies while 35 per cent use only groundwater.
Over 40 per cent of the machines operate with a supply pressure of less
than 30 psi while another third of the machines operate at between 30
and 40 psi. Nearly all growers (96 per cent) used static plate sprinklers
for germination but about half of the growers (48 per cent) then used
low energy precision application (LEPA) emitters (either socks or Quadsprays)
for in-season irrigations.
REASONS FOR INSTALLING MACHINES
The main reasons cited for installing the machines were the potential
for water savings (93 per cent), labour savings (85 per cent) and reduced
crop waterlogging (73 per cent). These benefits were highlighted by
one grower who said: “I’m saving a ‘cruiser every
two years and two blokes’ wages compared to managing a furrow
system.”
Another responded saying that: “Waterlogging is a thing of the
past. If there is a front coming, I only put on one inch, instead of
two inches and see what rain I get.”
Approximately two-thirds of growers indicated that improved uniformity
of water application and the ability to automate the system was also
important while approximately half the growers were interested in increased
yield and either fertigation or chemigation opportunities.
Other issues, such as the elimination of the requirement for extensive
surface irrigation earthworks, were also found appealing. Approximately
two-thirds of the growers surveyed owned more than one machine and nearly
all of the growers (93 per cent) used their machines to also irrigate
other crops (normally grains or peanuts).
YIELD AND WATER USE EFFICIENCY
All growers reported an improvement in the crop water use efficiency
(CWUE) using centre pivots and lateral moves when compared to their
own traditional surface irrigation systems (Figure 1). The average CWUE
was 1.9 bales per megalitre of irrigation using centre pivots and lateral
moves compared to grower’s own traditional non-optimised surface
irrigation of 1.1 bales per megalitre of irrigation — a 72 per
cent improvement.
Yields per unit area were primarily influenced by management strategy
and system capacity. All the centre pivot and lateral move growers surveyed
applied less water per unit area with their machines than they applied
using their own surface irrigation systems.
Growers reported applying on average 3.1 megalitres of irrigation water
per hectare less than fully irrigated surface systems. But the survey
results were strongly influenced by the large proportion of growers
who were short of water. Growers who had plenty of available water and
an adequate system capacity achieved yields per unit area similar to,
or greater than, traditional surface irrigation.
Growers with limited available water achieved lower yields per unit
area compared to traditional surface irrigation. But these growers would
not have had enough water to fully irrigate the cropped area using surface
irrigation. The average yield for centre pivots and lateral moves was
slightly lower (0.5 bales per hectare or 6.4 per cent) than traditional
surface systems.
CENTRE PIVOTS VERSUS LATERAL MOVES
The major drivers in machine selection are capital cost per unit area,
the size of the area to be irrigated and the labour requirements. The
average cost of lateral move machines was $1800 per hectare compared
to $2000 per hectare for centre pivot machines. But anecdotal evidence
reported by the surveyed growers who use both centre pivot and lateral
move machines suggests that the labour requirement could be as much
as 80 per cent higher for lateral moves compared to centre pivots.
One grower’s response when asked why he would prefer not to choose
a lateral move machine was: “You always need to have one eye on
it, just to make sure that it is actually still going properly.”
Another responded by asking: “Do you like to sleep at night?”
While centre pivot machines often cost between 10 and 15 per cent more
in capital costs than lateral move machines on a per hectare basis,
growers indicated that the on-going savings in labour and management
costs more than compensate for the extra capital cost.
Approximately half of the growers who irrigate with centre pivot machines
were planting and cultivating in circles so that they could use LEPA
emitters for in-season irrigations. Most commented that while the transition
from ‘square’ farming wasn’t easy, they now prefer
farming in circles.
DESIGN ISSUES
Over half of the growers surveyed (56 per cent) indicated that they
would like to make changes to the design of their machine for future
installations. A wide variety of design issues were identified by growers
ranging from problems with system capacity, operating pressures, field
slope, soil type and sprinkler packages.
The broad perceptions of the performance of centre pivots and lateral
moves in the Australian cotton industry are closely related to design
and management problems associated with some of the first machines used
in the industry. Early centre pivots were successfully sold to growers,
particularly in central Queensland and northern NSW, with little understanding
of the crop requirements and the necessary system capacities required
for each region.
One grower claimed pivots were originally presented as 50 hectare machines,
but in some instances up to three additional spans were sold increasing
the irrigated area to 100 hectares without any change in pumping or
system capacity. This sales technique significantly reduced the cost
per hectare, but gave growers little chance of crop success (when measured
in bales per hectare) with system capacities at roughly half the local
peak crop water use.
Unfortunately, many growers still have machines designed or managed
with capacities substantially below peak crop water requirements. While
cotton can handle continuing small deficits, other grain and legume
crops that are commonly grown under these same machines need capacities
to match the relevant peak crop water use.
More than three-quarters (79 per cent) of centre pivot and lateral move
growers reported experiencing some wheel rutting problems, with most
indicating that it was only a problem in the first few years of operation
due to inexperience and poor machine design.
The majority of growers indicated that wheel rutting and bogging were
no longer major problems in their irrigation management. A wide range
of machine modifications and management practices are currently being
used by growers to successfully reduce the incidence of wheel rutting
and bogging.
AGRONOMIC ISSUES
Development costs
A significant number of growers (83 per cent) installed machines onto
new country without levelling or drainage — representing a significant
cost saving over traditional surface irrigation systems.
Reduced waterlogging
The ability to apply smaller volumes of water in a single irrigation
and so make better use of in-crop rainfall was also cited as a major
benefit of these machines. One third of growers typically applied less
than 15 mm in a single pass while another third of growers applied less
than 30 mm.
The lack of irrigation-induced, temporary waterlogging often associated
with traditional surface irrigation practices means that crops under
these machines are not often held back as much as surface irrigated
crops. This is believed to be the main reason for approximately 20 per
cent of centre pivot and lateral move growers experiencing excessive
rank growth.
So irrigation schedules and application volumes can and should be modified
to maintain a desired level of crop stress when using these machines.
Where the crop has been encouraged to grow without either moisture or
waterlogging stress, growers have reported the ability to finish crops
earlier with significant reductions in the amount of chemical used late
in the season.
Applying chemicals and fertilisers
Chemigation through centre pivots and lateral moves was routinely conducted
by 14 per cent of growers. These growers reported success using Gemstar
with one grower also using Dipel. One grower indicated that the only
chemicals used in producing a seven bales per hectare crop were Gemstar
applied through the machine and Tracer applied normally. Significant
improvements in the efficacy of Gemstar were reported when cotton was
regularly chemigated at rates as low as five per cent of label rate
with 10 millimetres of irrigation water.
Almost one-third of growers (31 per cent) indicated that the use of
these machines changed their insect management strategies. An increasing
number of growers are showing interest in setting up their machines
with the capacity to chemigate through separate spray systems that are
hung underneath the main trusses of the machine.
In these cases, growers are specifying high-speed electric motors and
gearboxes with large diameter tyres so that they can obtain high machine
speeds and an ability to apply the chemicals quickly with their machine
over the entire field.
Forty-five percent of growers had applied fertiliser through their machine
with 38 per cent reporting a decrease in the total fertiliser applied
to the crop compared with applications for traditional surface irrigated
fields. More than two-thirds of growers (69 per cent) indicated that
they had decreased their pre-season fertiliser application.
Growers also indicated that irrigating with a centre pivot or lateral
move provided an increased ability to time the application of fertiliser,
improving both the management of labour, machinery and water as well
as the efficiency of fertiliser uptake.
OTHER ISSUES
Other issues covered in the full report include:
• Sprinkler and LEPA systems;
• Design and managed system capacities;
• Wheel rutting and bogging;
• Field drainage;
• Crop germination;
• Scheduling and crop management;
• Economics of centre pivots and lateral moves.
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