INTRODUCTION
The Mackay Central coast of Queensland has a rural industry based on sugar cane 135209 ha
(CANEGROWERS 1994) with cattle grazing as the second major industry in economic terms.
The district extends from the Shire of Proserpine in the north to Broadsound in the south.
The climate is tropical with a summer dominated rainfall averaging 1714 mm. Approximately
70 percent of the median annual rainfall occurs in the four months from December-March.
The one in 10 rainfall intensity is 8 mm per hour, (Institution of Engineers Australia
1987).
This high intensity combined with a large area of sodic soils results in a high potential
for
soil erosion.
Research has shown cultivation is the major determinant of erosion. Losses of 200 tonnes
per hectare from a single storm event have been recorded in a cultivated cane paddock
(Sallaway 1979). The Department of Natural Resources has estimated there is in excess of
40000 hectares where soil erosion is excessive under conventional cultivation in sugar
cane. Soil conservation measures have been implemented on only 24 percent of the
vulnerable cane area. Green Cane Harvesting in the 1996 season is currently above 60
percent of the area which will result in reduced soil losses due to the lack of
cultivation practiced in the trash blanket.
The fate of soil erosion including impact on water quality, infrastructure and the marine
environment in coastal areas is becoming increasingly scrutinised. Legislation including
the Environmental Protection Act, Environmental Management Plans and industry codes of
practice will impact on the discharge of runoff from agricultural lands where sediment,
nutrients and chemicals are present.
Tail water darns which aim to collect initial runoff are being investigated for their
effectiveness however these structures do not suit many situations. Alternative solutions
to filter the runoff and stabilise degraded areas within the Mackay District are currently
under trial with Vetiver grass (Vetiveria zizanioides) discussed in this paper.
APPLICATIONS OF VETIVER GRASS IN THE MACKAY AREA
Vetiver grass was first trialed in Mackay district in 1993 in waterway stabilisation.
Since then it has been used for batter, river bank and gully stabilisation. The grass
grows well in the tropical climate of Mackay with a planting population of 6 plants per
metre (2 to 3 slips per plant) forming a hedge in 6 months capable of trapping silt.
Batters up to 2 : 1 have been stabilised in alluvial sands adjacent to the Pioneer river
bank west of Mackay. The area was previously an active gully that had been eroding since
the turn of this century when it was constructed as a drain. Earthworks including removing
the rubbish and battering the gully head were performed in August 1995. The vetiver grass
hedges were then planted in critical areas. Grass was seeded between the rows of vetiver
to assist in erosion control. The vetiver hedges have established successfully. Three
applications of DAP were used due to the inherit lack of fertility and alluvial sandy
subsoil. A further two rows have been planted in the gully to control erosion in the
middle of the gully floor. This was due to the distance between the rows being planted too
far apart (100 metres approximately). There are many similar gullies along the major river
systems in the district that are in need similar stabilisation techniques due to active
erosion in flood and high intensity rainfall events.
FARM WATERWAYS
The majority of vetiver grass plantings in the Mackay district have occurred in on-farm
waterways to assist in erosion control in highly erodable sodic soil and in newly
constructed waterways. The landholders are satisfied with the hedges and there is evidence
of silt being trapped after flows in the waterways. The length of time for the hedge to
establish, approximately 6 months, has been a problem when high flows occur during this
period. Fertilising, watering and planting at a higher density reduces the establishment
period.
ROAD BATTERS
Road batters ( 2 : 1 grade) in granidiorite soils have been planted with three potential
hedge forming grasses to assess their effectiveness in trapping silt and stabilising the
batters. The trial was planted at Teemburra darn which is under construction west of
Mackay. A cut and fill batter were used to demonstrate the effectiveness of hedges. The
grasses used were two natives, Lomandra longifolia and Vetiveria filipes which
grow in moist areas associated with watercourse beds and the introduced Vetiver (Vetiveria
zizanioides). No significant rainfall events have occurred but dry conditions and
cattle grazing has virtually eliminated the Lomandra. The native vetiver has
survived however tends to grow prostrate while the introduced vetiver has grown erect and
is envisaged will trap silt and stabilise the bank in event of a significant rainfall
event.
FILTER STRIPS
A research trial is currently being established on two sites to determine the ability of
vetiver to trap silt, fertiliser (N P K) and pre-emergent herbicide (Atrazine). Natural
rainfall events will be monitored together with a range of land management/cultivation
techniques that are used in the Mackay district. The trial will be replicated and will use
recommended rates of fertiliser and chemicals. The land management conditions include,
bare cultivated, bare zero till, green cane harvested zero till (apply fertiliser on top
and buried as two treatments). Runoff will be collected and silt traps installed above and
below the vetiver grass hedge to enable analysis of the runoff from each treatment.
From the Mackay experience the advantages of vetiver grass are as follows:
Adapted to a wide range of soil types including sodic and acid soils.
Ability to survive fire, traffic and drought once the hedge is established.
Limited palatability to cattle once the hedge is established.
Stabilisation ability due to the extensive rooting system with only vertical roots to
reduce impact on adjacent crops and other grasses.
Vetiver continues to perform sediment trapping in unstable areas as hedge continues to
grow above silt layers.
No major pests or diseases evident to date.
Lack of weed potential due to absence of viable seed and extensive root system.
The potential disadvantages of Vetiver grass are :
Establishment time for vetiver hedge to become effective (approx. 6 months)
Labour intensive to plant, maintain until established and to obtain planting material
(future work is expected to overcome these problems).
Not tolerant of shade ( may use native Vetiver, Vetiveria filipes which grows in
many creeks in the district however this species is not as erect hence silt trapping
ability not as effective. May use in combination with Vetiveria zizanioides).
From experience in planting Vetiver grass in Mackay the following issues are critical to
achieve a dense effective vetiver hedge:
1. Use well rooted and vigorous planting material (fertilised). Each plant should have two
to three slips.
2. Plant at least 6 plants per metre with closer spacing or double row in critical areas
that require a quick hedge.
3. Plant into moist soil and water until plants are established.
4. Trim vegetative material to maximum 15 centimetres prior to planting to reduce
evaporative loss. Do not trim roots as this will reduce survival rate.
5. Provide weed control, during establishment phase ( note vetiver very susceptible to
Glyphosate).
6. Fertilise with phosphorus based fertiliser until established (e.g. DAP at 50 grams per
metre)
7. Trim hedge at least annually after flowering and hedge may be burnt once a year
provided soil moisture present, to maintain hedge vigour.
8. Clean silt trapped by hedge to reduce terracing effect in active eroding areas.
9. Plant other stoloniferous grasses between hedge rows to reduce erosion.
10. Avoid herbicide drift especially Glyphosate.
RECOMMENDATIONS
Vetiver grass hedges from experience to-date is seen as a useful stabilisation system an
potential filter system from agricultural land. The grass offers an alternative to
engineering solutions and from the successes achieved in the Mackay district, will gain
wide spread adoption.
Future work will revolve around developing planting techniques for vetiver hedges that
will reduce the cost and labour requirement. It is not envisaged vetiver grass will be
used to replace contour banks in the coastal areas, rather in specific stabilisation
problems and as a filter for agricultural runoff.
Use of vetiver hedges in the extensive cropping areas west of Mackay will also be trialed
in lieu of contour banks as a water spreading technique. Contour banks have been less than
satisfactory due to cyclonic rainfall influences in the extensive cropping areas of Nebo
and eastern Broadsound shires. Farming efficiency problems with non - parallel banks is
also a impediment to conventional contour bank systems.
REFERENCES
* Canegrowers (1995) "Annual Report, Central District", Canegrowers, Mackay
* Institution of Engineers, (1987) "Australian Rainfall and Runoff, A Guide to Flood
Estimation", Institute of Engineers, Australia
* Sallaway M.M, 1979 "Soil Erosion Studies in the Mackay District", Proceedings
of the Australian Society of Sugar Cane Technologies, Pages 125-133, Watson Ferguson &
Co, Brisbane