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Cropping systems of the temperate summer rainfall region

H. Marcellos and W.L. Felton

NSW Agriculture, Agricultural Research Centre, Private Mail Bag 944, Tamworth NSW 2340

Introduction

Cropping systems throughout Australia are faced with many management challenges in response to soil degradation, new technologies and greater economic pressures. We will deal in this paper with some of those confronting rain-fed cropping in the temperate, summer rainfall region of northern New South Wales and southern Queensland. This region extends from the Liverpool Range, through the North-Western Slopes and Plains of NSW into the Darling Downs and Maranoa of southern Queensland. Most of the region lies between the 500 mm and 650 mm isohyets, and about two-thirds of the rainfall is received between October and March. High intensity summer storms often occur during the fallow, and autumn is the period of lowest and most variable rainfall. Planting times for both winter and summer crops are highly variable, and greatly affectpotential yields. The region produces high quality, prime hard wheat. The best cropping soils range from neutral to alkaline, grey clays to black and red earths, often physically self-ameliorating due to their shrinking-swelling properties (8, 11, 27). Large amounts of fallow rainfall can be stored in these soils, for subsequent use by a crop.

Looking back

Farming expanded into drier areas

Cropping in northern NSW commenced in its south east, on red soils around Tamworth (18), and in Queensland on the eastern Downs (11). It expanded rapidly in the late 1960s as returns for grain became greater than those from sheep; a large tractor could be purchased from the proceeds of selling 3,000 sheep. Cheap scrub clearing methods, more powerful tractors, abundant casual labour, and the development of grain bulk handling facilitated the change to cropping. The area planted in the two most northerly shires of NSW, for example, increased from about 200,000 ha in 1960 to over 500,000 ha by 1970; cropping in Queensland extended from the eastern Downs westwards with the wheat area rising from about 300,000 ha in the early 1960s to peak at 1 million hectares in 1985 (5). Large areas of native vegetation like brigalow scrub were cleared between 1962 and 1975, which extended cultivation to an additional 1 million hectares (3) on soils that were initially high in total nitrogen (17).

Expansion of wheat growing in the late 1960s led to the imposition of production quotas for the 1969-70 season, which stimulated an interest in alternate crops.

Farming practices have changed

Significant changes have occurred since the 1950s in management of tillage, (allows, weeds and diseases, and crop rotation and nutrition.

Tillage by shallow cultivation with disc ploughs and scarifiers drawn by low power tractors was the norm until the early 1960s. Tyned trashworking implements were introduced during the 1970s as were higher powered four-wheel driven tractors. Introduction of the latter was complete by the early 1980s as casual labour became less available and more costly. A rule of thumb became 'a farmer could grow 2,500 acres of crop with a 250 HP tractor'. Tillage in the specialist grain areas is now mostly done with tyned implements, and reduced to between three and five workings during a short fallow. Of farms surveyed in northern NSW in 1985 (26), conventional tillage was practised on 74%, reduced tillage and herbicide on 14% and no-till 1% of the area.

Prior to the 1950s, cereal residues were almost universally burned after harvest (18) to eliminate trash, control disease, facilitate machinery use and control weeds. By the mid 1980s, less than 30% of northern NSW growers surveyed burned stubble (26); most did so immediately after harvest, but about 20% delayed burning until late summer, and 20% early autumn.

Most growers in a winter cereal system reduce crop residue during the fallow. In the most northerly part of NSW and south-west of Queensland, about one-quarter to one-third of farmers use early aggressive tillage and the remainder preferring to trashwork and maintain residue until later in the fallow when stubble is burned, and followed by cultivation which allows fertiliser N to be applied (J. Fahy, pers. comm.). Some farmers in northern NSW practice no-till and burn at the end of the fallow whereas in the south-west of Queensland perhaps a fifth would. Some growers use sheep during the fallow to control weeds, but this reduces stubble protection. In the drier areas of Queensland, less than 10% of farmers are apparently using a well managed system of tillage that retains stubble on the soil surface to control erosion (37). A survey of nearly 600 paddocks after the 1989 wheat harvest showed that by mid-January 1990, 65% had inadequate stubble cover for protection against soil erosion (Felton and Wicks, unpublished data).

Crop rotation prior to the 1960s generally meant 'continuous wheat with short fallow', with occasional spells of lucerne, or crops of oats or milo, or long fallow. Two-thirds of farmers surveyed in northern NSW during the mid 1980s included sorghum as an alternative to wheat; 28% included lucerne, 20% grazing oats, 18% sunflower and 14% barley (26). Since that time, chick peas and cotton have become major dryland crops. The diversity on the Downs is exceptional with crops such as canary seed, maize, millet, panicum, triticale, and summer growing legumes. Livestock declined on the Downs in the 1960s but there has been a shift recently towards stock in the western areas of Queensland (21), and in the older parts of NSW which represents a retiring of land from cropping. In the drier areas generally, crops are restricted to sequences involving wheat, barley, sorghum and chickpea.

A widely practised ley farming system has not been developed, although there has been a trend to medic leys in drier western areas where N fertiliser is not widely used. Lucerne plantings have increased in the older cropping shires of NSW to support more livestock.

Cropping has reduced soil fertility

The fertility of the region's soils has declined due to reduction in levels of soil organic C and total N by soil erosion and continuous farming. Keeping soil in place, and improving and conserving its fertility while maintaining economic viability have become major challenges for the summer rainfall cropping region. Profitable farmers are in the best position to make changes in line with ecological and environmental requirements.

Soil erosion is a severe problem on the Darling Downs (36), and in northern NSW (16) clue to the combination of high intensity storms, large areas of unprotected fallow and the highly erodible nature of the soils. The retention of cereal crop residues will protect soil against loss due to the momentum of intense rainfall, and is likely to increase water infiltration, reduce soil temperatures, and suppress weed growth. Sufficient residue must be produced and maintained, and cultivation eliminated for these benefits to be obtained. Erosion losses, if not reduced, can negate efforts to conserve soil organic matter.

Indications of declining soil fertility were seen in both States as early as the 1950s (18,19,25). Organic matter and its constituents, total organic C, total N and mineralisable N, declined by 19-67% (8) in six major Queensland soils cropped for 20 to 70 years. Losses of more than 40% of C and N in the soil surface were measured after 18 years of cropping at Narayen Research Station (33). Many of the region's soils now require N inputs to maintain grain yield and quality of cereals.

The loss of organic matter with cultivation is generally exponential, and soil type and cropping history have an influence (7); it will depend on the relative rates of residue addition, decomposition and soil erosion. Three broad phases can be recognised in the decline: Total N level falls rapidly during the early years of cultivation, but remains adequate for wheat growth; it continues to decline at a slowing rate to become limiting in some seasons. After a long period of cropping, N falls to a third level when inputs are needed to maintain productivity. The eastern Downs was in the early phase in the 1960s and is now generally N limited, whereas the western areas probably still have adequate N fertility (12). Economic responses to fertiliser N were not found in the newly farmed areas around Moree during the late 1960s (9), but the region is entering the third phase based on evidence from fertiliser trials during the 1980s (13).

Application of N fertiliser on the Darling Downs commenced about 1960 and has increased linearly since (12). Fertiliser was used by about half of farmers surveyed in northern NSW during the mid 1980s (26), and as might be expected, least was used in the most northerly shires.

Wheat yields are frequently lower than would be expected for the water available, and grain protein contents have fallen. ASW wheat was rarely delivered to silos along the 600 mm isohyet in NSW during the 1970s, but large amounts began to appear by the mid 1980s; the average grain protein concentration of wheat received in 1990/91 was 10%, of which little was prime hard quality. The percentage of wheat from prime hard and hard varieties being downgraded to ASW has also increased during the 1980s (4).

Managing cropping systems for the future

Management of cropping systems will continue to be strongly influenced by economic factors as growers must remain viable. Soil erosion, continuous cropping and soil fertility, interactions between crops, and nitrogen cycling and management must be addressed.

Tillage practices and crop residues

The influence of tillage practice and retention of crop residues on the productivity of winter cereals has been examined in several long-term experiments. Wheat grain yields over a nine-year period in northern NSW were similar or no tillage and cultivated treatments (Felton, unpublished data), but both were about 10% less than those obtained when stubble was burned after harvest and plots conventionally cultivated. Wheat and barley grain yields over 12 crops at Hermitage in southern Queensland were similar for four systems, no-till and cultivated, with and without stubble (23). No-tillage sorghum is, however an efficient method of production, with a yield advantage over cultivated systems of about 0.5 t/ha, associated with higher fallow moisture storage (20).

More summer rainfall can be stored in the fallow under no-till (23, 24, 30) which can lead to small increases in wheat yields in some seasons. Differences in continuous winter cereal yields between tillage and residue treatments depend therefore on interactions with other factors such as soil moisture, nitrate accumulation and antagonistic effects of their own residues. Major problems seen by growers in retaining stubble are cost and efficacy of weed control, wheat foliar disease, inadequacy of equipment and difficulty in applying N fertiliser. However, development of wheat varieties resistant to yellow leaf spot, enhancement of crop yields in rotations, and better understanding of nutrient cycling and management should assist greater adoption of no-till, stubble retained systems.

A major advance in weed control technology which will facilitate further reduction in the need to cultivate the fallow, has been the development of a weed activated spot spraying technique (14). Sensors that discriminate green plants against a background of soil or residue, control through a microprocessor, each nozzle of a spray boom. Herbicide is applied to weeds as they are detected, and not to weed-free areas. The prototype equipment has been successfully tested over 3,000 ha of fallow with savings in the area actually sprayed usually in excess of 90%, and chemical costs of less than $3/ha.

There are therefore no productivity or economic benefits of no-tillage and retention of crop residue in a continuous winter cereal context that outweigh the disadvantages. This no doubt accounts for the inadequate adoption of these conservative practices, which will not become widespread without additional financial reward, or regulation.

Can continuous cropping be sustained?

Soil organic matter declines with continuous cropping (31) and conventional cultivation due to changes in temperature, soil moisture and aeration, exposure of new surfaces for mineralisation, enhanced microbial activity and loss of soil by erosion (32). Soils in some cropping systems may reach equilibrium, in which case the rates of residue input and organic matter loss are in balance, but studies in other systems show continuing decline. Soil organic fertility can be restored by reverting cropped land to pasture, but can it be maintained, or partly restored under continuous cropping?

Long-term research is needed to evaluate the possibility that levels of soil organic matter can at least be maintained in a system in which cultivation is eliminated, crop residues are retained and crop legumes and N fertiliser are used. Tillage and crop residue management have been shown to affect organic matter and microbial activity near the soil surface. In one 13-year study in southern Queensland highest concentrations of organic C and total N were found with a combination of no-tillage, winter cereal residue retention and fertiliser N (6). In another experiment, in central Queensland involving six summer cereal crops, the combined effects of residue retention and zero tillage resulted in increases of 15% in surface soil organic C and 18% in total soil N (34) compared with conventional cultivation.

Managing soil nitrogen fertility

Although legume pasture leys can increase total soil N levels, grain farmers in this region generally rely on fertiliser and crop legumes for the inputs of N required in depleted soils.

Nitrogen fertilisation. Recommended N application rates for wheat have been sought from one- season fertiliser trials (13,35), and various approaches adopted to make results useful to growers. Good correlations can be found between grain yield and soil nitrate-N in a particular season (Table 1), but have been low when applied over sites and seasons because other factors such as available water were not taken into account. Soil tests are generally not yet available to growers. The age of cultivation of the paddock and length of the fallow have been used to predict average fertiliser requirement, but a better method is required to enable growers to predict the N needs of non-legume crops at specific times in their rotations.

Table 1. Soil nitrate at planting, and grain yields and protein contents of wheat grown in 1990 in no-tilled or cultivated main plots, following chickpea or wheat, or fallow, in 1989 at three levels of N fertility (N1-3) (Marcellos and Felton, unpublished data).

Crop legumes, Chickpeas have become the major winter crop legume alternative to cereals in this region. Grain yields of wheat following chickpeas have ranged from 51-100% higher than those of wheat following wheat, and pain protein contents have been raised by 0.3-1.0% (22). Responses have been in part due to increases in soil nitrate-N after the legume, and equivalent to those obtained with up to 100 kg N/ha of fertiliser (10).

Nitrogen fixation by chickpea is reduced as soil nitrate levels increase (10; Marcellos and Felton, unpublished data), so that the addition of new N to the soil from fixation falls to zero when soil nitrate exceeds about 100 kg/ha. The N benefit from chickpea is therefore partly due to recycling of legume residue amounting to a deferral of the N that would have been used earlier by a non-legume crop. Indeed, it may be necessary to apply additional N to wheat after chickpea in a high yielding season in order to maximise grain yield (Table 1) and to achieve high grain protein concentration.

Enhancement of N fixation by chick pea. The estimates for net addition of N from chickpea have been in the range 0-40 kg N/ha, sufficient for about an extra one tonne of wheat. For maximum N gain, this legume should be planted when soil nitrate levels are low, using agronomic strategies to maximise its dry matter production. In the longer term, plant breeding might develop varieties of chickpea capable of producing higher biomass, and possessing a N fixation mechanism that is less sensitive to suppression by increasing levels of soil nitrate.

Putting it all together

Growers must ultimately farm to keep their soil in place, and maintain its fertility at an economically viable level. The types of cropping system based on this will be shaped largely by economic factors leading to more crop and management options. The short term challenge is for growers to select the most profitable crops and improve their management of production risk for each crop in the system.

Crop choices. Different crops place different demands on soil fertility and water, produce stubbles of differing physical attributes, C:N ratios and decomposition rates, and possibly antagonism to other species, and some fix N. More research will be needed to provide information about the interactions of crops with each other, and with tillage system. The complexity of information produced may best be utilised in models and decision support systems so that consultants and growers can readily assess soil fertility and select yield or productivity goals for various crop options.

Weed management is a key factor in managing a crop rotation system in this region where weeds grow readily all year round. When cultivation is reduced or eliminated in the interests of soil conservation, weeds must be controlled by means other than cultivation; herbicides play an important role, but will not substitute for management which fails to recognise that weeds must be controlled in all stages of the system, and not solely in the current crop.

Water use efficiency and target yields. Water is the main natural resource driving crop production. Concepts of water use efficiency for grain production as developed in South Australia (15) and the USA (1) anticipate that potential wheat grain yield will increase by about 1,000 kg/ha for every extra 100 mm of water available to the crop after the first 100 mm which is allowed for evaporation. Available water is made up of fallow water at planting and in-crop rainfall. A grower may select a yield goal based on the amount of fallow water at planting and the amount of in-crop rain he is prepared to budget for. The yield goal approach is applicable to other dryland crops like sunflowers (28), and sorghum grain production (20). It will allow production risk to be better managed.

Managing crop nutrition. Although major progress has been made in understanding the N requirements of wheat, more research is needed on the nutrition of other crops, of different tillage systems, and of the fate of N fertiliser unused by the current crop. Detailed information is lacking about losses of N from cropping systems, by leaching or in gaseous form, so that management might be shaped to minimise its economic impact on growers, and the likelihood of N entering ground water.

Other incentives, Government policy and support are inseparable from those of the scientific, agricultural and public communities (29) in facilitating the management of sustainable cropping systems. Indeed, it would be easier to persuade farmers to adopt more conservative practices if financial incentives such as tax credits, investment allowances and chemical rebates were available to them (2). The question of changing cultivation practices by regulation has not yet been seriously raised in Australia.

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