Managing dryland salinity: dense saltbush plantings
Dense saltbush pastures are no longer recommended, as there are cheaper and more productive options for suitable saltland. Dense plantings severely limits growth of understorey species, which often provide the bulk of feed.
The department recommends that dryland salinity management is part of a whole farm, and preferably a whole catchment, water management plan.
The dense saltbush pasture option
This option refers to saltbush in dense (>1000 plants/ha) plantations, with or without grazing.
Benefits of this option
- sheep feed – low to moderate (not taking understorey into account)
- lowering watertables – low to moderate
- reducing salt export from a site – moderate to high
- preventing wind erosion – high
- amenity – depending on the site, low to high
- fauna habitat – depending on management, low to moderate.
Disadvantages of this option
- expensive to establish
- feed value is low to moderate with no understorey
- fresh water requirement for livestock is high
- vehicle and livestock movement is restricted by density of planting.
This option suits the following conditions
Figure 1 shows the preferred conditions for dense saltbush plantings: moderate to high salt levels; low to moderate seasonal waterlogging; summer watertable 1.5 to 2.0 metres below the surface; rainfall 300 to 400 mm.
Choosing the right species for the site
Saltbush species suitable for different sites and uses are shown in Table 1.
Note: there is a newer, more palatable selection of old man saltbush (Anameka Saltbush™), but this needs to be established as nursery raised seedlings, which would be too expensive for the return in a dense saltbush planting.
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Table 1 Some characteristics of different saltbush species |
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Common Name |
Species Name |
Salinity |
Waterlogging |
Palatability |
Grazing recovery |
Suitability |
|
Old man saltbush |
Atriplex nummularia |
high |
low |
low |
high |
low |
|
River saltbush |
Atriplex |
high |
moderate |
high |
high |
low |
|
Wavy leaf saltbush |
Atriplex |
high |
moderate |
moderate |
low |
high |
|
Quail brush |
Atriplex |
high |
moderate |
high |
moderate |
moderate |
Site conditions
Measure soil salinity and depth to the watertable to characterise the site. Measures and suitability are:
- winter watertable: best, 0.3 to 0.7m below the surface; poor, less than 0.3m
- summer watertable: best, 1.5 to 2.5m below the surface; moderate, 1.0 to 1.5m; poor, less than 1.0m
- subsoil salinity (25–50cm) ECe: best, 8–16dS/m; moderate, 2–8dS/m; poor, greater than 16dS/m.
Saltbush is relatively sensitive to waterlogging and inundation, particularly if waterlogging is prolonged or occurs during periods of high temperature. Avoid regularly waterlogged sites or manage the site to reduce waterlogging.
Old man saltbush is generally more sensitive to waterlogging than river saltbush.
Prevent inundation by:
- choosing sites that have limited inundation
- using shallow relief drains
- planting the saltbush on mounds – arrange the mounds in a herringbone or fishbone pattern with the rows sloping towards the natural drainage line.
Saltbush tends to grow best on soils that are lighter than heavy clays. Direct seeding to establish saltbush is only possible on sandy/loamy soils, or where there is a sandy/loamy layer over a heavy clay.
Very acid soils may reduce saltbush growth.
- 250–450mm is preferred.
- less than 250mm has a high risk of establishment failure and reduced productivity – in this case, nursery raised seedlings are more successful.
- greater than 450mm has a high likelihood that waterlogging will be a constraint.
Saltbush is a summer grower and optimum growth occurs when daytime temperatures are in the range 30–35°C. Saltbush is usually dormant or slow growing during the colder months: river saltbush has little growth when the mean daily temperature is below 13°C; old man saltbush will have some growth in winter.
Growers have observed that old man saltbush has better tolerance to low temperature than river saltbush and will withstand quite severe frosts. Saltbush is therefore not preferred for saline sites in the cooler and/or wetter areas across southern Australia: perennial grasses such as tall wheatgrass and puccinellia are better options in these areas.
A combination of indicator plants will improve confidence in the site diagnosis, as factors other than salinity influence the species present.
Site suitability and salt tolerance of common indicator plants (from lowest to highest):
Suitable
- capeweed (Arctotheca calendula)
- annual ryegrass (Lolium rigidum)
- cotula, water buttons (Cotula coronopifolia)
- iceplant (Mesembryanthemum crystallinum)
Probably too salty and/or waterlogged
- curly ryegrass (Parapholis incurva
- puccinellia (Puccinellia ciliata)
- samphire (Halosarciaspecies)
Sites with large bare scalds may be too salty and/or too waterlogged for saltbush. Make sure that bare areas are not a result of livestock camps or overgrazing.. On its own, iceplant is regarded as a poor indicator, especially in Western Australia where it is becoming a more common weed in cropping paddocks.
Benefits of this option explained
Lowered watertable and reduced salt movement
Lowering the watertable and drying the soil surface will have 3 benefits:
- Surface accumulation of salt from capillary rise and evaporation is reduced, and this reduces movement of salt in surface water flows.
- By lowering the watertable, saltbush will reduce the amount of salt that the groundwater delivers into the root zone of shallow rooted understorey species. Reduced salt in the root zone means that a broader range of plants can grow as understorey.
- Drying out the soil profile by lowering the watertable creates a buffer zone that enables winter rain to leach salt beyond the root zone. This, too, makes the site more suitable for productive understorey species.
The buffer zone, once established, has the added benefit of providing some protection for the saltbush against waterlogging or inundation following heavy or persistent rain.
Amenity and environmental benefits
- Saltbush pastures look better than bare saline land.
- Saltbush protects the ground from wind erosion, even when the soil is bare between saltbush plants.
- Saltbush pastures reduce salt accumulation on the soil surface, which reduces flushing of surface salt into drainage lines by as much as 80%.
- Saltbushes provide habitat for small birds, lizards and other small animals.
- Soil microbial activity in the saltbush root zone (rhizosphere) is increased compared to bare saline land.
Other benefits
- Saltbush provides substantial shelter for sheep, which can improve lambing percentages and reduce mortality of off shears sheep. However, access for sheep and managers is limited by the density of planting.
- Peers and the community recognise saltbush planting as good management.
- Salty diets can increase the efficiency of wool growth by up to 20%. This benefit may come from the increased salt in the diet causing increased passage through the gut, which prevents the rapid destruction of protein in the rumen.
Value of saltbush for sheep feed
As a sole diet, most saltbush has low digestibility, relatively low metabolisable energy, high salt concentrations (up to 20% or more of the dry weight) and moderate to high crude protein (12–20%). Old man saltbush is generally more digestible, with lower salt content than other saltbushes.
Saltbush alone is usually not adequate for a maintenance diet for sheep or cattle.
The total digestible dry matter production per hectare from saltbush alone is usually low. It is easy to over-estimate the available forage supply in saltbush pastures, based on visual assessment, because of the upright habit of the plants.
Saltbush leaves are reasonably digestible but small stems make up much of the edible portion of saltbush. A 50kg sheep would need to eat more than 1000 grams of saltbush dry matter per day on a straight saltbush diet to maintain condition. This is impossible with saltbush, because the high salt concentration in the feed limits intake to about 800g, depending on the level of salt in the soil and in the plant’s leaves.
The very high crude protein content (typically 12-20%) of saltbush is valuable in summer and autumn, when green feed (and therefore protein) can be very limited.
However, about half the crude protein in saltbush is not protein at all, but nitrogen contained in a range of other compounds, primarily those that are used to assist the plant cells manage the high salt loads. This non-protein nitrogen can be converted into microbial protein in the rumen if there is a good supply of energy available. This is unlikely to be the case in dense saltbush plantings – where there is little or no understorey of high energy forage – but this can be supplied as supplementary feed, early grazing of stubbles, or dry annual pasture.
The high salt content of most saltbush limits feed intake. The feed value of high salt saltbush can be improved by feeding low salt supplements (grain and/or high quality hay). Sheep will actively select quantities of high and low salt feeds that improve the feed value of their diet.
For sheep to cope with high levels of salt in the diet, they must have access to low salt drinking water. Saline drinking water will reduce feed intake even more. In one study, feed intake of sheep on old man saltbush fell by more than half when the drinking water was replaced by water containing 1% salt (10 000ppm).
Dried annual pastures can be deficient in vitamin E, resulting in a muscle wasting disease in sheep. Old man saltbush is an excellent source of vitamin E, containing about 250mg vitamin E per kilogram of green leaf.
Managing the option and site
Establishing saltbush
To successfully establish saltbush, choose the right species, use good seed or seedlings, control waterlogging, and use the right establishment method.
Grazing management
Management of sheep in dense stands is difficult: there is poor vehicle access unless tracks are left; moving sheep as a mob is difficult because they cannot see the other animals; these are poor conditions for working sheep dogs.
Graze until most of the leaves and very fine twigs have been eaten, then remove the livestock.
Saltbush can be heavily crash grazed with sheep, but there must be sufficient recovery time. Saltbush stands are a safe place to supplementary feed sheep in autumn without risking soil erosion, and this often results in saltbush being grazed back to the fine sticks.
Constant heavy grazing will eventually weaken and kill most saltbush. Some farmers do not graze their saltbush each season, preferring to save the feed for less productive seasons. However, heavy grazing of mature old man saltbush in autumn has little reduction on the amount of edible dry matter available at the start of the following autumn compared to an ungrazed control. If left ungrazed, old man saltbush drops a lot of leaves, which is a wasted resource.
Regular (at least annual) grazing prevents individual bushes from growing beyond sheep grazing height. In rare situations, tall saltbush may need to be pruned back to about 60cm height.
Even in these dense saltbush stands, the volunteer understorey of annual grasses and herbs contributes about the same amount of edible biomass as the saltbush.
There are some rules of thumb to estimate dry matter:
- There is about 500g of edible biomass per plant on an established and healthy stand of dense saltbush.
- To supply a sheep's maintenance of about 1000g of digestible dry matter per day, a sheep will need to browse one plant with its understorey per day.
- A 10ha stand of saltbush with 1000 plants per hectare (10 000 plants) should support 500 sheep for at least 20 days (10 000/500 = 20). Supplement the saltbush with good quality hay or grain to provide adequate energy for liveweight maintenance.
Livestock water needs when grazing saltbush
The higher the soil salinity, the greater the salt concentration in the saltbush and the greater the need for fresh water and supplementary feed when grazed.
Integration with a farm business
Dense saltbush plantings are seldom a good investment if the only benefit will be feed production. However, farmers claim a range of other benefits when saltbush is considered as part of their overall farming system.
Farmer experience is that about half the value from dense saltland pastures comes from grazing to replace hand feeding, and the other half comes from the range of other benefits to the whole farm system. Dense saltland pastures:
- are an ideal holding area (or sacrifice paddock) during periods of intensive supplementary feeding
- allow deferred grazing of germinating annual pastures after the autumn break
- provide very effective shelter for lambing ewes and lambs – a downside is that it is more difficult to monitor lambing in dense stands of saltbush
- provide emergency shelter for off shears sheep – but there is the risk of saline sites becoming very wet when shelter is needed.