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SOLUTION 10

Legumes for saltland

 

10.5 Establishment & management

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Choosing the right species and varieties

Companion legumes play an important role in the continued vigour of grassy pastures, whether on saline or non-saline land. However, the reality for any saltland that has more than mild salinity, is that legumes will play only a minor role in the pasture and ongoing nitrogen deficiency will be common. The applicability of legumes needs to be assessed on a site by site basis as salinity levels, risk of waterlogging, soil pH and seasonal rainfall will all influence the appropriate choice.

The issues are complex, so local experience regarding which legumes have the best chance of making a significant contribution should be sought from technical experts, public or private extension providers, or from other farmers. 

Balansa and Persian clover are both annual clovers that are generally compatible with tall wheatgrass on mildly saline sites. Balansa is particularly adaptable to waterlogging, but neither regenerate well in moderately saline conditions – in this case, the clover will often perform very well in the year of sowing, but fail to reappear the following year.

The burr medics, particularly Scimitar, are early flowering and produce a relatively high proportion of hard seeds. This gives them a major advantage over other legumes in low rainfall areas. Of  the balansa clovers, Frontier matures 2-3 weeks before Paradana and about 5 weeks before Bolta. This means that Frontier can set seed in many environments before salinity levels increase in spring, while Paradana and Bolta are only able to set seed in very long growing season areas. Newer Persian clovers, including Nitro Plus, Persian Prolific and SARDI Persian are earlier flowering than older cultivars, but are not as early flowering as Frontier. Jota Melilotus albus is very late flowering and will only set seed in very long growing season areas.

The annual clovers are often best direct-drilled the year after tall wheatgrass or puccinellia establishment to ensure the perennial grasses have established well before the more vigorous annuals are introduced – there is a hierarchy of importance here, with the grasses ‘essential’ for a long term pasture, and the legumes ‘desirable’. Similarly, these clovers can be direct drilled into established stands of puccinellia (in particular) or tall wheatgrass (more difficult because of its clumpy nature) to give them what often turns out to be a one-year clover and soil nitrogen boost.

Strawberry clover can be a useful companion legume with tall wheatgrass in long growing season areas, but it has a dislike for highly acid soils.

Lucerne can be successfully established on land with low to moderate salinity provided there is only limited risk of waterlogging. From this point of view it is well suited to land which has been protected by adequate surface drainage. Like strawberry clover, it is sensitive to highly acid soils. In reality, the most important role for lucerne in dryland salinity is in reducing recharge because of its deep rooting and its ability to use large amounts of soil water. This may be ‘upslope’, or around the edges of saline areas.

Burr medic is suitable for moderately saline soils, but like lucerne it is intolerant of prolonged waterlogging.

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Site preparation

While not ‘legume specific’, the following steps are all important prior to the sowing of a saltland pasture.

As part of the site assessment prior to saltland renovation, take a soil sample (top 10 cm) in summer/autumn and get it tested for salinity and major nutrients. It is important to also test for salinity in winter and spring for salinity to get a seasonal picture of the salinity profile, particularly as legumes tend to be more salt-sensitive than the companion grass or shrub. With the seasonal salinity profile decisions can be made about the suitability of the site for legumes. If the site is highly saline in summer but fresh in winter then balansa clover is suitable. If the site is moderately saline throughout the year, strawberry clover may be the most suitable option. The nutrient results from the soil test are also important because the legumes tend to have a higher phosphorus requirement than most salt tolerant plants and therefore add to the expense of renovation if the site has low P status. The P fertiliser rate to maintain a productive legume stand is approximately 1kgP/dse depending on the Colwell/Olsen P.

Fencing to control grazing pressure is required, as pasture is often sown into soil that soon becomes very wet and vulnerable to pugging by livestock. Temporary electric fencing may be sufficient, particularly for small areas where separate grazing is impractical.

Remove excess water if waterlogging is likely to present a problem at sowing. Diversion or reverse interceptor banks can reduce the movement of runoff water onto the area, (but care should be taken that these banks do not go through sodic soil that might give way and lead to erosion), or shallow drains can assist the movement of surface water off the site. When designing drainage systems it is important (and possibly a legal requirement) to consider the impact of disposal on downstream biodiversity and landholders and ensure there will be no harm done. See also Managing Surface Water.

Control weeds (especially the annual grasses such as sea barleygrass), by spray topping the previous spring. It may still be necessary to kill germinating weeds prior to sowing with a knockdown herbicide.

Lightly cultivate or scarify the soil prior to the break of the season (if planning some leaching of salt with the opening rains. With both cultivation and herbicide use, the time without vegetation should be minimised to reduce the capillary rise of salts to the surface.

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Weed and pest control

The main weeds at sites where legumes are likely to be able to make a contribution are capeweed and annual grasses such as sea barleygrass. These should be sprayed in the spring before sowing to reduce the seedbank. Annual legumes might have to be sown a year after perennial grasses such as tall wheatgrass or buckshorn plantain, because the vigour of the annual can suppress growth of the tall grass seedlings, particularly on areas of mild salinity.

Red-legged earthmite (RLEM) is a significant threat to all legumes, particularly as young seedlings, and must be controlled during pasture establishment. Bare earth treatment with bifenthrin (Talstar®) immediately following sowing is likely to control RLEM for up to 5 weeks to allow successful establishment. Pastures should be monitored for signs of RLEM, lucerne flea and aphids in spring and in subsequent seasons and controlled with appropriate insecticides if necessary. This approach used regularly in the same paddock could lead to pesticide resistance, but this is unlikely in a grazing situation.

Seed can be treated with omethoate, but this provides only limited protection, albeit at the very critical time immediately following germination.

Timerite® is an online tool developed by Australian Wool Innovation and CSIRO that provides the optimal spray date for the effective control of RLEM for your local area, thereby reducing the risk of carryover to following years. By spraying on your Timerite® date in spring, optimal control is achieved in the following autumn when annual legumes are most susceptible.

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Sowing

Pasture mixes

Balansa clover, strawberry clover, Persian clover, Melilotus albus and lucerne are all generally sown on saltland in combination with a grasses such as tall wheatgrass, puccinelliatall fescue/phalaris or sub-tropical grasses. Balansa clover, sub clover and burr medic are often included in an under-storey, shotgun mix including annual and sometimes perennial grasses. The grasses help to balance the pasture by extending the grazing season and taking advantage of variations in soil condition across the paddock. The legumes contribute valuable nitrogen to the grasses and protein to the grazing animals.

Under favourable conditions balansa and Persian clover will out-compete young tall wheatgrass seedlings, particularly as sowing after an opening rain has flushed some salt from the topsoil. These clovers are not particularly salt-tolerant and this initial flushing of salt greatly enhances their dominance. Therefore, under low salinity conditions, balansa clover is often not sown with wheatgrass in the first year, but rather a year or two later.

Typical recommended mixes for high rainfall areas (eg. >500 mm) include:

  • Summer ECe: <5 dS/m: TWG, tall fescue*, strawberry and balansa clovers**.
  • Summer ECe: 5-10 dS/m: TWG, strawberry and balansa clovers.

For lower rainfall areas a typical mixture would comprise burr medics and balansa clover. Barrel medic could be included for alkaline soils.

[*Victorian trials have shown that Resolute and Advance tall fescue will germinate and persist at  summer ECe levels up to approximately 8 dS/m, losing up to 50% of productivity at this higher salinity level.

** The early-flowering Frontier balansa has the advantage that it can set seed before salinity levels escalate in spring.]

Seeding rates

State agencies and others have developed fact sheets with seeding rates for single species, but these need to be adjusted for saline conditions.

Local experience will determine most suitable species and seeding rates but a typical pasture mix with legumes on a mildly saline site might include 4-5 kg/ha tall wheatgrass with medics at 2 kg/ha, clovers at 0.5-2 kg/ha, lucerne at 2 kg/ha and phalaris at 2 kg/ha. However, the best local mix might contain sub-tropical as well as temperate grasses and in drier regions it is likely that saltbush will be in the mix, possibly replacing all the perennial grasses. It should be noted that sub-tropical grasses and saltbush require spring sowing, whereas the optimum sowing time for the other species is in autumn. Saltbush should also be sown prior to legumes and other companion species on difficult or more saline sites.

Establishment costs for a mixed pasture are about $300/ha for seed, cultivation, herbicide and fertiliser, plus a further cost for fencing which will depend very much on the size of the saline area. In addition there is the opportunity cost associated with the establishment downtime, but in most cases the opportunity foregone on unimproved saltland is very small.

Because of the large number of possible combinations, it is best to seek local advice regarding both species and cultivars to include, and the rates for the individual species. Legumes must have a suitable rhizobium to enable nitrogen fixation - as saltland has often not carried legumes before, or at least not for a long time, it will generally be important to inoculate the seed before sowing.

Time of sowing

Options for sowing times are largely dependent on weather condition and the state of the paddock.

Many of the difficulties in establishing legumes are similar to those associated with other pasture species on saline sites. Because many saline sites are waterlogged for significant periods, there is often only a small window of opportunity to control weeds and sow the pasture.

If possible, seeding should be done following good opening rains that are sufficient to flush salts from the soil surface. The seedling and early establishment stages are very susceptible to salinity. Dry sowing should be avoided, particularly in unreliable rainfall areas, as the opening rains may be sufficient to germinate seeds, but insufficient to flush salts from the surface, causing major seedling losses. It also reduces the opportunity for weed control. In many areas, this may mean a very narrow sowing window, as areas prone to waterlogging may get too wet for sowing very quickly.

The best options are:

  • Autumn sowing after the autumn break (in southern Victoria, before the end of April)
     
  • Spring sowing, as soon as the area is trafficable after the end of winter. This option is only feasible in areas with reliable spring rainfall and works well in years with dry winters. However, if the area is not trafficable until late spring, there will be insufficient time for the plants to establish before the onset of higher salinity levels in summer. Sowing in spring is of course not an option if an annual such as balansa clover or burr medics are to be included.
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Managing new stands – year 1

Balansa or Persian clover as a companion in tall wheatgrass or puccinellia pastures might be avoided in the first year as their vigour can suppress the grass. However, if balansa or Persian clovers are sown, stock should be removed in time to allow the clover to flower and set seed. This conflicts with the need to maintain grazing pressure on grasses such as tall wheatgrass to ensure it remains vegetative, so when seed-set is complete, crash grazing is recommended to bring the pasture down to a uniform stubble of about 10 cm. Removing the excess growth will help control weeds, encourage better leaf growth and make grazing management over summer much easier.

In lower rainfall areas, where burr medics are likely to be the dominant sown legume, grazing pressure should be markedly reduced during flowering and seed set to maximise seed production and persistence. This is also the best management for balansa or Persian clovers, if they are included in mixtures.

Grazing over summer-autumn should be conservative, aimed mainly at maximising pasture persistence, rather than maximising immediate production. For perennial grass-based pastures grazing down to about 5 cm will promote strong root development. Removal of excess thatch and legume stubble is required to enable adequate hardseed breakdown of annual legumes and provides good conditions for regeneration with opening rains.

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Managing new stands – year 2 and thereafter

Annual legumes (balansa and Persian clovers, burr medics) survive saline conditions essentially by not growing over summer when saline conditions are usually the most severe – the challenge for these annual legumes is to ensure regeneration in subsequent years. To make this strategy successful, annual legumes need to mature early enough that they can set seed for following years before salinity levels in the soil build up in spring. They also need to produce sufficient hard seed (that maintains its hardness) to ensure survival of the pasture despite a possible false break in year two. 

Annual legumes face two significant challenges every year. Firstly they have to set viable seed in the spring when soil salinity levels can be rising rapidly. Secondly, if they attempt to germinate with the opening rains, there is a serious risk that soil salinity levels will be too high for the germinating seedlings. Later in the season, the winter rains will have flushed the salt from the surface soil, making it more suitable for germinating legume seedlings, but by then, other annual species and the growing perennials will have occupied the seedling niches. The result is that many annuals produce very poorly in their second and following years. Research is being conducted to understand seedling interactions with soil surface salinity and rainfall to enable development of legumes better able to resolve this issue.

Perennials such as strawberry clover and lucerne only need to be established once, but like the annuals, they need an appropriate inoculant to ensure the nitrogen fixing potential of the plant. An additional constraint is that unlike annuals, they do not set seed and die off to avoid the often very high summer soil salinity levels.

The main risk to perennial pasture species is always over grazing, generally as a consequence of set stocking. Grazing management of mixed pastures will need to be carefully considered where pasture species have different seasonal growth patterns. Allowing seed set for legumes whilst preventing loss of pasture quality through rank growth of perennial grasses, such as tall wheatgrass, involves compromises which can cause deterioration of the pasture. For saltland which is more fragile than normal land, some form of rotational grazing is recommended – it is usual to leave a greater residual (ie the pasture left in the paddock when the stock are withdrawn) than for non-saline pastures so that groundcover is kept high and surface soil evaporation is kept low. The grazing management system must also account for the needs of the various pasture components – aerial seeding species such as balansa clover and burr medics need to have minimal grazing during flowering and seed set. Once annual legume seed set is complete (which depends on the cultivar) the stand can be crash grazed to remove excess growth and grass stems, which will have started to run up to flower.

An annual fertiliser application is required to maintain the persistence of the legume component. A soil test every 3-5 years will help determine whether fertliser is required.

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Rejuvenating old saltland pastures

Most old stands of salt-tolerant pasture have little or no legume component. Where salinity levels allow, balansa clover seed can be broadcast with the fertiliser in early autumn. If soil conditions permit, and if rank grass is removed and some bare ground is visible between plants, balansa clover will readily germinate.

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Animal nutrition issues

Most of the nutritional challenges facing animals grazing on saltland relate to the shrub (dense saltbush plantings and saltbush and understorey) or to the grass (tall wheatgrass, puccinellia, vegetative grassestemperate grasses and sub-tropical grasses) components. Overall, legumes in saltland pastures make a significant contribution to animal nutrition because of their high protein content, their high digestibility compared to other pasture species, and because they do not accumulate salt.

An exception to the nutritive excellence of legumes is bloat - though saltland pastures with sufficient legume content to cause bloat are extremely rare. However, animals grazing saltbush-based or tall wheatgrass-based pastures usually graze the ‘other’ pasture species first and therefore might obtain a diet high in legumes when first introduced into the paddock. Also, any non-saline land within the fenced saltland paddock may contain a high legume content and be grazed preferentially.

There are some potential animal health concerns with Jota Melilotus albus, particularly if it is used to make silage. M. albus contains relatively high levels of chemical compounds called coumarins, which can be converted into a compound (dicoumarol) that can cause haemorrhaging in livestock when fermented silage is fed in large quantities. Although Jota has been selected as a cultivar with lower coumarin contents than other M. albus types, its levels are still higher than other pasture legumes. However, the risk will be reduced if animals also have access to other feed sources (live plants or supplements) to dilute possible effects.

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