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
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.
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 ( 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.
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.
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.
* 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