LEY PASTURES - THEIR FIT IN CROPPING SYSTEMS

| Date: 02 Sep 2010

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Introduction

Ley pastures have been an integral part of farming systems in southern Australia for many years. Yet with the declining fertility and productive capacity of many cropping soils in Queensland and northern NSW there is an increasing need to develop successful ley pastures systems in these regions. Ley pastures in crop rotations have the ability to reverse declining soil health and structure, increase soil fertility and nutrient cycling, improve livestock productivity, and reduce environmental problems like soil erosion and deep drainage.

The term ‘ley pasture’ means different things to different people, and can include a variety of annual or perennial species, legumes or grasses used in short or long-term rotations. Hence, there are a range of combination of rotation length and combinations of species available. Table 1 provides a broad summary of the types of ley pastures available in the sub-tropics and their benefits and non-benefits in cropping systems and the pasture species that are most suited to each type of pasture.


Benefits and roles for ley pastures – a summary

Improving nitrogen for subsequent crops
Using legumes in ley pastures will increase soil fertility and can increase grain quality and yield in subsequent crops (depending on seasonal conditions). Forage legumes leave more nitrogen for subsequent crops than grain legumes, because for grain legume much of the fixed nitrogen is removed in the grain. Total nitrogen accrual depends on
• legume DM production (N fixed = approx. 1.5 % DM)
• Soil N status (legumes use soil nitrate before beginning to fix their own)
• Grazing management (N losses with grazing)

Legume dominant pastures will fix the most total nitrogen (typically 40-60 kg N/ha/yr), efficiency of N accumulation decreases as legumes recycle mineralized N from previous years. The accrual rate is often lower but the efficiency of nitrogen fixation higher in mixed grass/legume pastures as legumes are forced to fix N. Deep-rooted perennial pastures (e.g. perennial grasses, lucerne) will also catch nitrogen that has leached below the root zone of crops and return it to the surface where it can again be accessed by crops. 
 

Rebuilding soil organic matter and improving soil structure

Soil organic matter has declined under annual cropping systems, which has also seen soil structure, infiltration rate, and soil biological activity decline.  Ley pastures are the most effective way of improving soil organic matter. However, legumes pastures alone have little effect, a grass-based pasture is needed to substantially increase soil OM. Grass-based ley pastures have been shown to increase soil organic C (0-10 cm) by 0.02-0.1% C per year; faster on heavier vertosols than on lighter textured soils. At the same time, accumulation of soil organic matter is a slow process and requires long pasture phases and the largest and fastest increases in soil OM mainly occur in the surface layers (0-2.5 cm).

By increasing soil OM, pastures also improve soil structure and infiltration rate and reduce surface crust formation, along with surface and sub-surface porosity. In some cases infiltration rate has increased by 4-fold after 5 years of grass-based pasture. However, these benefits are rapidly lost again if cultivation is resumed, but may persist longer with no-till cropping systems. Although, it is believed (but not yet shown) that regular phases of ley pastures could maintain or restore the soil condition in the cropping systems (Fig 1)
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Control and management of weeds

Ley pastures can also provide weed management benefits through competition and via grazing by livestock both of which can reduce seed set and run down the seed bank of some weeds. In particular, the seed bank of palatable annual weeds (e.g. wild oats and barnyard grass) can be significantly reduced (e.g. Table 2), but lower palatability weeds (e.g. mustard) or those with long lasting seeds (e.g. turnip weed) are less effectively controlled. Despite this there are often problems with weeds in pastures, especially in thin stands or when they are sown into particularly weedy situations. Our understanding of how to perform cost effective and efficient chemical or mechanical control of weeds in pastures would greatly improve their adoptability for many producers.

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Providing a disease and pest break

Pastures may also benefit cropping systems by reducing disease or pest pressures, however, our knowledge of this is poor. We do know that some pastures can host some pests and diseases (e.g. lucerne and medics will carry Phytophthora root rot, some grasses can carry crown rot and common root rot) but wether or not inoculum levels are increased or maintained is unknown.


Reducing runoff, soil erosion and deep drainage

Because pastures generally provide greater levels of cover than in cropping systems they reduce runoff and soil erosion (Table 4). Improvements in soil infiltration rates after a pasture phase will also reduce runoff and erosion risk during periods of cropping. Many perennial pastures possess deep root systems that can access nutrients and water from below the root zone of annual crops. By extracting this water and using it for growth, perennial pastures maintain a drier soil profile which reduces the risk of water and nutrients being lost through drainage, which cause problems like dryland salinity and eutrophication of river systems. In particular, shallow soils with a small water-holding capacity are the most ‘leaky’. These soils are where cropping is the least profitable and pastures can have the greatest effect on reducing deep drainage (Table 3).

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Improving livestock production

Ley pastures can also greatly improve the productivity of the livestock system by maintaining higher carrying capacities, providing higher quality feed that increase animal growth rates (e.g. legumes), and by providing valuable feed during winter and early spring. Forage provided during winter (e.g. oats, medics, lucerne) also has a higher value because of low supply and quality of other feed at this time.

Difficulties with ley pastures

Pasture establishment – Establishment failure or thin stands has a high cost to profitability of the pasture phase, mainly in terms of lost production. While pasture establishment can be difficult, poor results often occur due to inadequate attention to seed bed preparation, poor quality of seed used, or sowing at an inappropriate time, rate and depth. The likelihood of success is greatly improved when pastures are sown with a similar approach and precision to sowing crops.

Removing pastures & refilling the soil profile – some pastures can be difficult to remove with herbicides alone (e.g. lucerne), however, it is important to remove these as residual pasture plants can compete strongly with crops.  Some perennial pastures, such as lucerne, extract soil water below the lower limit for crops and so the soil takes longer to refill before a crop can be sown. Thus, these pastures need to be removed earlier (e.g. June) to allow sufficient fallow length to reduce any yield penalty in subsequent crops (Fig. 4). Other annual or short-term perennials don’t deplete the soil profile to the same extent as lucerne so require a shorter fallow to replenish water in the soil profile. Furthermore, after pastures (especially after legumes) low ground cover slow replenishment of the soil profile; cover crops may be a tactic worth exploring to overcome this problem when transitioning out of a pasture ley.

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Exploring the role for new short-term legumes

While long-term mixed grass/legume pastures (>5 years) can be used to rejuvenate marginal cropping soils, on more productive soil types shorter-term pastures may be more suited to ‘tighter’ cropping. However, we have limited knowledge of the performance of newer pasture options such as burgundy bean and sulla, which are suited to short-term rotations with crops. How they compare to existing options in terms of productivity, water use, water-use-efficiency, nitrogen balance and profitability are of particular interest, and has been the focus of recent work within the Southern Queensland Farming systems project.

Sulla (Hedysarum coronarium), is a short lived perennial temperate (winter growing) forage legume native to the Mediterranean and is best adapted to alkaline clay soils. Individual plants live for 2-3 years but it will regenerate from seed. Sulla has exceptionally high quality forage and contains condensed tannins that make it non-bloating, which offers advantages over lucerne or medics for beef producers. The condensed tannins also make sulla less attractive to insects, are reputed to be anthelmintic (reduce animal nematode and worm burdens) and can increase protein digestion, and production in sheep (and perhaps cattle). There are currently three Australian cultivars; Wilpena and Moonbi are grazing types developed in Qld, NSW and SA; and Flamenco is an upright cultivar more suited to hay production developed in WA.

Burgundy bean (Macropillium bracteatum) is also a short-lived perennial that can grow on a variety of soils ranging from slightly acid to alkaline sandy loams to cracking clays, but is particularly adapted the heavy, alkaline clay soils. Burgundy bean is capable of growth at lower temperatures than most other tropical and subtropical legumes, giving it a longer growing season. It is a subtropical equivalent of the more tropically adapted butterfly pea (Clitoria ternatea). Burgundy bean is non-bloating and extremely palatable. This means it is preferentially grazed in mixed pastures and hence, grazing must be carefully managed to prevent it being grazed out, and to allow sufficient seed set each year to maintain stand density. If allowed to seed, substantial recruitment occurs.


Production comparisons

At 4 core experimental sites (Brigalow, Roma, Condamine, Toobeah) the production of 4 summer- and 4 winter-growing forages has been compared over the past 2-3 years (Table 5 & 6). We have also conducted some on-farm sowings to explore the productivity of sulla and burgundy bean under other conditions.

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Annual grass forages, oats and forage sorghum have been more productive than the legumes in most cases, except in winter 09 where establishment of oats was poor. However, forage sorghum produces a high proportion (40-60%) of stem, and overall has lower forage quality than the legumes and oats.

First year winter production from sulla was exceptional at Roma and Boonah in 2008 due to good spring rain, but in other sites and years growth has been slow in the first year (Table 5). The second year productive potential seems higher but has not been achieved due to seasonal conditions.

Of the summer legumes, lablab production has generally exceeded burgundy bean in the first year particularly when burgundy bean establishment has been reduced (e.g. Toobeah 08/09). In subsequent years burgundy bean has been able to respond quickly after spring rains and produce significant growth before other annual forages could be sown. 
 

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Because of differences in water use between species and site.years, water use efficiencies (kg DM/ha/mm) were calculated to enable comparisons of efficiency of forage production. As was expected forage sorghum had the highest WUE of all species, as it is a C4 grass and all others are C3 species. Generally lablab appeared to have a higher WUE than burgundy bean, at least in the first year. They were both higher than lucerne. WUE of winter forages were highly variable, but sulla and snail were much higher in their second year due to improved early plant growth.

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Estimated gross margin returns

We have also attempted to compare various short-term pastures in terms of their likely economic returns. We estimated gross margins based on best current knowledge of production and input costs (Table 9). Growth and livestock prices fluctuate from year to year, hence, we also explore how sensitive returns are to these changes (Table 10).

The newer perennial forage legumes had high first year growing costs, mainly due to the higher seed costs (Burgundy bean - $262/ha and Sulla - $298/ha), which meant that they provided little gross margin return in their first year (refer to Table 9). The higher overall cost of the perennials also meant that the production necessary to recover costs was higher, although they would have multiple years to obtain these production levels (Table 9). For the perennial legumes, once costs were averaged over a 2 or 3 year phase they were generally lower than the annual crops which incurred more fallow costs (Table 9). On average, a 3-year phase of burgundy bean or Lucerne were the most profitable options, with oats, lablab and sulla having similar average returns around $100/ha/yr.

Both commodity prices and crop biomass production significantly influence gross margin returns from ley pastures, as they do for grain crops (Table 10). Returns are particularly sensitive to low forage production and in dry years gross margins can often be negative. Interestingly, returns from the perennial legumes were less affected by lower levels of forage production and they are also likely to encounter better than average years that make-up for poor years, provided they persist for a number of growing seasons
 

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In addition to the livestock production returns from forage legumes it is also important to recognise and value the contribution of nitrogen they can make to the farming system. This benefit is not realised in the forage legume, but reduces the costs of fertiliser applications in subsequent grain or non-legume forage crops. The value of this nitrogen benefit can be significant especially when prices of fertiliser inputs are high. For example in October 2008, when nitrogen was worth about $1.60/kg N, lablab producing 6 t DM/ha would have contributed $144 worth of nitrogen per hectare. Over a 3 year phase of lucerne or burgundy bean 200 kg of nitrogen would be added under average seasonal conditions, equating to about $320 of nitrogen inputs per hectare. Under current lower nitrogen prices the average value of N inputs from ley legume pastures would be about $20-50/ha/yr.

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Contact details

Dr Lindsay Bell
CSIRO Sustainable Ecosystems
PO Box 102, Toowoomba Qld 4350
Ph: 07 4688 1221
Fx: 07 4688 1193
Email: Lindsay.Bell@csiro.au

GRDC Project code: CSA00013