Management of dryland wheat
W.K. Anderson, A. Impiglia

Winter-dominant rainfall areas contribute a major part of the world's dryland wheat production. The progenitors of modern bread wheat (Triticum aestivum) and durum wheat (T. turgidum var. durum) were first domesticated in the winter-dominant rainfall, or Mediterranean-type, environment of the Fertile Crescent (Zohary, 1969). Wheat is extremely well adapted to endure in such an environment, where it's productivity and stability of production is exceeded only by barley (Fischer, 1989). In the Mediterranean basin, a sequence of crops is often found with bread wheat in the wetter areas (more than 450 mm average annual rainfall), durum wheat in the medium rainfall areas (250 to 450 mm) and barley in the drier areas (less than 250 mm) as the dominant crop (Impiglia and Ryan, 1997). In the other parts of the world where winter rainfall prevails, factors such as local economic conditions, soil types and disease incidence result in a different crop/rainfall sequence, but wheat is most often the dominant crop.

The main characteristics of the environment that wheat growers face in developing their farming systems in dryland, winter-rainfall areas are:

• variable and often deficient rainfall (McWilliam, 1981);

• variable length of the growing season depending on soil water storage capacity (Cooper et al., 1987), timing of the opening and closing rains and winter temperatures;

• dominance of alkaline, calcareous soils that often have a high clay content (Kassam, 1981);

• low and often declining soil organic matter content associated with long-term mechanical cultivation (McDonald and Fischer, 1991; Heenan et al., 1995);

• soils that are commonly responsive to phosphorus and nitrogen fertilizers;

• periodically devastating outbreaks of diseases and insect pests.

Typical farming systems that have developed for dryland wheat production in the areas where winter rainfall predominates have been characterized by strategies that reduce risks such as:

• the use of mechanical or chemical fallow;
• the integration of livestock with cropping;
• dry sowing with variable depth of seed covering;
• the use of moisture-seeking seed drills;
• the use of reduced tillage;
• retention of crop residues;
• the use of legumes that supply nitrogen to the subsequent wheat crop;
• adjusting inputs of fertilizers and other inputs depending on timing of the opening rains;
• storing seed of more than one maturity type to suit variable seasonal conditions.

The challenge for the agronomist and farm manager in the dryland, winter-rainfall environments is to apply knowledge of climatic, edaphic, biological and economic factors to devise a system that optimizes grain yield, yield stability, grain quality and long-term viability. This chapter is not an exhaustive review of the literature that has contributed to the management of the dryland wheat crop in winter-rainfall areas, but an attempt has been made to point out how: (i) the findings of basic, strategic research are related to the development of management practices for the wheat crop system; and (ii) some current practices will need to be modified if the farming systems of the dryland, winter-rainfall areas are to become sustainable.

THE ENVIRONMENT

Winter-dominant rainfall environments, often referred to as Mediterranean-type environments, are located on the west coasts of the continents in both hemispheres between latitudes of about 25° and 40°. The largest area is in North Africa, West Asia and southern Europe around the Mediterranean basin, and smaller areas are found in the Pacific Northwest area of the United States, in Chile, Southern Australia and South Africa. Altitudes in the areas where crops are grown vary from sea level to over 1 000 m.

Between about 70 and 100 percent of the annual rainfall in the winter-dominant rainfall areas falls during the growing season, which extends from sowing in the late autumn until harvest early in the following summer. The average annual rainfall totals can be over 1 000 mm in some areas, but the majority of the wheat crop is produced in areas that receive less than about 600 mm annually. Variability of rainfall amount and distribution during the growing season is a characteristic of these environments.

Depending on elevation and distance from the sea, the mean minimum temperature of the coldest month varies from about -5° to over 10°C. Mean maximums during the grain maturation period can be as high as 30°C, with occasional daily maximums over 40°C.

The growing season for wheat varies from about 100 to over 250 days in the countries of the Mediterranean basin (Kassam, 1981; Smith and Harris, 1981). A similar range of growing seasons can be found in the other winter-dominant rainfall regions. Timing of the opening rains is variable but can often be a guide to the length of the growing season (Stewart, 1989, quoted by Harris, 1991). This association also holds for some parts of the wheat belt in Western Australia (K.J. Young, personal communication, 1997) and is used by farmers to plan their choice of cultivar, weed control and fertilizer strategy.

Kassam (1981) and Frere et al. (1987) have made detailed analyses and classifications of the climate of the Mediterranean region. The implications of the environment for crop production have been discussed extensively, often with the aid of crop models, by Smith and Harris (1981), Harris et al. (1987), Harris (1991), Harris and Goebel (1991) and by Kassam (1988) for the higher elevation areas of the Mediterranean basin. The overall conclusions that have been drawn from these studies are that both low and high temperatures have an almost equal influence to limitations of water supply in shaping the cropping system, the appropriate management practices and the choice of genotypes.

Soils found in the winter-rainfall areas are often dominated by clays and other fine-textured soil types (McGarity, 1975; Kassam, 1981). However, coarser textured sands, gravels and texture contrast soils (sands over clays) are also used for wheat production in some areas (Hamblin, 1987). Most soils in the winter-rainfall areas are low in organic matter, either naturally or as a result of prolonged cultivation, and are alkaline in reaction.

MANAGEMENT PRACTICES

Tillage

One of the major trends in dryland farming is towards reduced tillage. The use of mechanical fallow is declining in the countries bordering the Mediterranean Sea due to increasing pressure on land use (Impiglia and Ryan, 1997). In Australia, the use of chemical fallow and sowing with some form of minimum disturbance system is becoming more widespread (Fischer, 1987). For example, in Western Australia the majority of the wheat crop is direct-drilled, and over 30 percent is sown with zero-tillage systems (J. Blake, personal communication, 1998). The trend is similar in all of the other winter-dominant rainfall areas.

Nevertheless, bare fallow has been a useful tool for conserving water, mineralizing soil nitrogen, providing a phytosanitary period for disease control, reducing weed populations and sowing at the optimal time. The disadvantages include loss of production every second year, inefficient storage of water and soil structural decline through loss of organic matter and exposure to erosion. Hamblin (1987) and McDonald and Fischer (1991) have published extensive reviews of the effects of tillage on soils and crops in Australia. The general conclusion in Australia appears to be that on balance the disadvantages outweigh the advantages of cultivated fallow under many conditions.

However, Bolton (1981) reached the opposite conclusion from data collected in the Pacific Northwest where the yield advantage for fallow-wheat over continuous wheat exceeded 1 tonne/ha at several sites. In the Central Anatolian Plateau of Turkey, Guler and Karaca (1988) concluded that fallow was only advantageous where the soil depth was greater than 90 cm. They also concluded that fallow when the annual rainfall exceeded 410 mm did not increase wheat yields in the fallow-wheat system above those obtained from continuous wheat. At lower elevation in the Negev Desert, Amir et al. (1991a) concluded from a series of experiments over ten years that the advantage of fallow disappeared in seasons when rainfall exceeded 250 mm. Fisher (1962) made a similar finding in Western Australia where experiments indicated that yield increases of wheat crops following fallow were unlikely in rainfall zones that received more than 350 mm.

Water storage resulting from fallow is related to soil texture, soil depth, surface cover and the method used (mechanical versus chemical). In Southern Australia, French (1978a) found that additional storage from mechanical fallow compared to a non-fallow soil preparation varied from 0 to 125 mm. He concluded that fine-textured soils and good rainfall in the preceding winter before the start of the fallow period were associated with the higher water storage. The yield increases following fallow averaged 335 kg/ha compared to continuously cropped soils (French, 1978b). Increases in soil nitrogen after fallow in the same study were not correlated with grain protein percentage in the following wheat crop (French, 1978c).

Another factor that can affect soil water storage for use by the subsequent crop is surface cover from previous crop residues. It has been concluded that, although surface evaporation is not greatly affected, straw mulch allows deeper infiltration of water and greater storage of water during the main period of soil water recharge (Fischer, 1987). A summary of experiments in Australia (Kirkegaard, 1995) has shown that retention of crop residues rarely increased grain yields of the following crop and that increases in soil organic matter were only slowly evident. However, recent experiments in the Negev Desert (Amir and Sinclair, 1996) have shown that grain yields were increased in a continuous wheat system using zero-tillage and finely chopped straw (8 cm) compared to continuous wheat without the straw mulch. The straw mulch did not increase yields in the wettest year when 380 mm were received in the growing season. In another study, where crop water use varied from less than 200 mm to over 400 mm, it was concluded that the yield responses to reduced tillage, stubble retention and fallow compared to continuous cropping were dependent on water availability and to a lesser extent on nitrogen availability (O'Leary and Connor, 1997).

Fallow can also provide a phytosanitary break against root diseases. Amir et al. (1991b) attributed the yield gains to control of nematodes (Pratylenchus mediterranea and Heterodera avenae) and take-all (Gaeumannomyces graminis) in their experiments, since they were unable to measure any increases in plant available water after fallow in eight out of ten years.

Hamblin (1987) has reviewed the detrimental effects of cultivation on loss of soil organic matter, surface crusting, compaction and exposure to erosion by both wind and water. The practice of long, bare fallow by mechanical means is probably not sustainable in the long term. The fact that fallow in some form has probably been practised in the Mediterranean basin for over one thousand years has probably been possible because the soils were initially both chemically fertile and physically stable, soil disturbance was minimal until the advent of steel ploughs and tractors, and much of the weed control in the fallow period was through animals rather than by cultivation (Cooper et al., 1987).

The use of chemicals to control weeds during the fallow period and retention of crop residues on the soil surface can overcome a lot of the detrimental effects of bare, mechanical fallow. However, water infiltration is likely to be less in undisturbed soil and soil loss greater (Cooke, 1985), weeds tend to develop resistance to herbicides and retained crop residues can be a source of disease inoculum for the subsequent crop.

In summary, water can be stored during a fallow period in fine-textured soils where the root zone is greater than about 1 m. This water can contribute to increased yield of wheat where the annual rainfall is less than about 250 to 400 mm, depending on the soil type and environment. The benefits of fallow can also be attributed to increased nitrogen supply for the wheat crop, reduced incidence of root diseases, reduction in the weed seed bank in the soil and more timely sowing. The practice is unlikely to contribute to the viability in the long term of dryland, wheat-based farming systems unless it is practised with minimal soil disturbance and retention of crop residues.

The use of alternative practices to long fallow has become more widespread (McDonald and Fischer, 1991) and continues to increase. Some advantages of the alternatives are:

• pasture and grain legumes in the cropping sequence can lead to wheat yields as high as in a fallow system (Fisher, 1962);

• in a pasture-wheat system, increased nitrogen for the wheat crop leads to high wheat yields and grain protein (Mason and Rowland, 1993);

• the use of other non-legume crops can provide disease breaks (Angus et al., 1991);

• herbicides can be used to control weeds after the opening rains thus allowing better use of seasonal rainfall through more timely sowing.

Adoption of these practices can be impeded where farm size is small, availability of resources is restricted, credible local advice is lacking, or where price subsidies influence choice of crops.

Various methods of sowing crops with minimum disturbance have become common in some winter-dominant, dryland areas. Kirkegaard (1995) has summarized the results of experiments in Australia. The effect of direct drilling (one cultivation operation during sowing) on grain yield compared to conventional tillage methods of crop establishment (two to four passes with disc and/or tyned implements before sowing) ranged from slightly positive to slightly negative. Improvements in soil conditions, including increased nitrogen and water availability and reduced soil strength, were often associated with the direct-drilled treatments in these experiments. The other advantages of reduced tillage appear to be reduced fuel use and improved timeliness of sowing.

More recently, zero-tillage systems using knife or flat disc openers and non-residual, knock-down herbicides have increased. The advantages that are claimed include reduced fuel costs, increased flexibility of sowing time, reduced moisture requirements for seeding, a firmer ground surface leading to less likelihood of lost time in wet years, improved soil fertility and improved weed control. Not all of these claims have been substantiated experimentally, but current research is addressing most of them. However, the system as currently practised is dependent on herbicides and is thus susceptible to the development of herbicide-resistant weeds.

If zero-tillage systems are to fulfil their promise of adding to the long-term viability of cropping systems in dryland areas, the problem of weed control must be solved in a sustainable way that minimizes the risk of developing herbicide resistance.

Cropping sequence or rotation

Crop rotation has been practised in the winter-dominant rainfall region around the Mediterranean basin since Roman times at least (White, 1970, quoted by Cooper et al., 1991). Cooper et al. (1987) have reviewed the systems in use in the West Asian and North African region. Cereal-fallow, cereal-grain, legume-fallow, cereal-pasture and cereal-oilseed-legume rotations are common to most parts of the dryland, winter-dominant rainfall areas. As discussed above, wheat is the dominant cereal crop but continuous wheat systems are becoming less common.

Fixed rotations are common in the more traditional areas around the Mediterranean basin, but more flexible cropping sequences driven by fluctuations in the prices of wheat and other products are likely to be found in other areas. Some examples are:

• price subsidies make a sequence of four or more durum wheat crops attractive in southern Italy (N. Di Fonzo, personal communication, 1996);

• increases in the value of animal products can lead to substitution of barley for wheat in some areas (Cooper et al., 1987);

• unfavourable prices for animal products can lead to higher cropping intensity (Knopke et al., 1995);

• field peas are a substantial component of the system in the Pacific Northwest as is fallow;

• the area of canola has increased in systems that include grain legumes in Southern Australia in response to buoyant prices in the 1990s.

TABLE 25.1
Legumes included in rotations increase wheat grain yield and protein concentration in 250 to 350 mm rainfall areas, Western Australia

Source: Shackley, 2000.

Crop rotation is one of the principles underpinning long-term viability of dryland farming systems. It is practised to control diseases, weeds and insects, to improve soil fertility (mainly from the inclusion of legumes that fix nitrogen), to spread the risk of crop failure and to stabilize income. It has been held that crop rotation in arid regions, such as the Negev Desert (average annual rainfall less than 250 mm), is not feasible (Amir et al., 1991b). However, in parts of Western Australia where rotations with grain or pasture legumes are practised, wheat yields and grain protein percentages can be markedly improved compared to continuous wheat (Table 25.1) or fallow-wheat (Fisher, 1962). Increased wheat yields following grain legumes have been associated with residue nitrogen from the previous crop (Asseng et al., 1998), and the role of legumes in the farming systems of Mediterranean areas has been extensively reviewed (Osman et al., 1990).

The addition of a wider range of crops to the sequence can complicate farm management. Each crop will have a maximum frequency to reduce disease. For example, field peas should not be grown in the same land more than one year in three to reduce the incidence of black spot (Barbetti and Brown, 1993), and canola has an advisable frequency of one year in four to reduce blackleg in Western Australia (Barbetti and Khangura, 1997).

When animals are included in the system their requirement for feed from pastures, fallow weeds, crop residues and grain can also complicate management. Animals exert considerable pressure on the soil, which can result in compaction, increasing soil bulk density and reducing hydraulic conductivity (Willatt and Pullar, 1983), and can leave soil bare and exposed to erosion. In addition, damage to soil structure can result from mechanical operations when the soil is either too dry or too wet. Considered together, these problems demonstrate the need to investigate the use of periodic soil renovation by techniques such as deep tillage combined with the use of gypsum, direct drilling and additional organic matter (McDonald and Fischer, 1991).

The use of crop rotation, including pastures for livestock, is an essential component of wheat production systems in most parts of the dryland, winter-dominant rainfall areas. On current evidence, it is difficult to envisage a sustainable system that does not include some aspect of crop rotation or some more flexible cropping sequence that contributes to disease, weed and pest management, soil fertility and income stability. Such a system could also incorporate soil renovation, retention of crop residues and reduced or zero-tillage.

Seed and sowing

The importance of seed quality or vigour, in respect of size, nutrient composition, absence of mechanical damage, disease and sprouting, has long been recognized for crop establishment, vegetative growth and grain yield (Heydecker, 1969; TeKrony and Egli, 1991). Adequate plant populations are desirable for maximizing grain yield and for providing adequate competition against weeds, and they can be influenced by depth of sowing, seed-soil contact, water and temperature, as well as seed attributes. The timing of seedling emergence has been shown to influence final grain yield (Gan et al., 1992), and it has been proposed that the preemergence processes are important only because they influence the time of seedling emergence (Benjamin, 1990).

The source of seed for sowing can have a vital impact on establishment and yield. Both seed size and protein content have been found to affect seedling vigour in wheat (Ries and Everson, 1973). Seed phosphate concentration can also influence dry matter accumulation of the seedlings (Bolland and Baker, 1988).

Coleoptile length of wheat also influences the appropriate sowing depth and subsequent seedling establishment, crop vigour and yield. The semidwarf wheat cultivars commonly have shorter coleoptiles and thus require shallower sowing (Whan, 1976). However, seed size also has an effect, with smaller seeds of the same cultivar having shorter coleoptiles (Cornish and Hindmarsh, 1988). High temperature, both during seed maturation and in the seedbed, also reduces coleoptile length. One study using eight wheat cultivars has shown an average reduction in length of 1 cm for every 2.6°C rise in ambient temperature (Radford, 1987).

Despite the fact that seed quality can not always be shown to influence grain yield in the field, the use of plump, undamaged seeds from crops that have adequate nutrition and have been stored under cool, dry conditions is another way for dryland wheat growers to reduce risks. Faster emergence, more flexibility in sowing depth, more vigorous early growth and better competitive ability with weeds are some of the potential benefits.

Sowing time and choice of cultivar

The adaptation of the wheat crop to the variable, dryland environment of the winter-dominant rainfall areas has been principally a matter of matching the maturity pattern of cultivars to the length of the growing season. Such matching presumably occurred in the Fertile Crescent during the domestication of the wheat crop. In other winter-dominant rainfall areas, understanding of the influence on plant development of major genes for photoperiod sensitivity (Ppd genes) and cold requirement (Vrn genes) led to dramatic increases in wheat yield and adaptation (Pugsley, 1983). The use of appropriate genes to produce cultivars that flowered after the last damaging frost on average and before the last effective rain was largely responsible for the adaptation of the wheat crop to dryland conditions in Australia. The Ppd and Vrn genes have been used to delay flowering in longer season environments, thus using the season better and avoiding losses from frost and sprouting.

Matching the cultivar to the dryland environment, however, implies more than the manipulation of genes that regulate maturity. It has been demonstrated that dry matter produced at flowering is proportional to kernel number and grain yield, and for every environment there will be an optimum production of dry matter at anthesis that will optimize interception of solar radiation and post-anthesis water supply and maximize grain yield (Fischer, 1979). It is clear from this analysis that if flowering is moved earlier than the optimum time for a given environment, dry matter production will probably be less than sufficient to maximize grain yield, or if it is moved later, water supply for grainfilling will be deficient, kernel size will decrease and yields will also be reduced. On the other hand, if dry matter is increased for a given flowering date through more vigorous growth, especially in the immediate pre-anthesis period when kernel number is being determined (Fischer, 1985), grain yield can be increased.

In translating these findings into practical advantages for farmers, two conditions are required. Firstly, the optimum flowering period for each area should be estimated (e.g. Hadjichristodoulou, 1987), and secondly, the grower should have available a range of cultivars of differing maturity from which to choose depending on the timing of the opening rains. The importance of early sowing has been emphasized for the Pacific Northwest (Bolton, 1981) and for the Mediterranean countries (Cooper et al., 1987). Many authors have similarly demonstrated the extent of yield increases from early sowing in Southern Australia (Syme, 1968; Kohn and Storrier, 1970; Kerr et al., 1992). In addition, a method of estimating the optimum flowering period to maximize grain yield using cultivar x sowing time experiments has been described (Anderson and Smith, 1990). Storing cultivars of more than one maturity group can also extend the sowing opportunities (Anderson et al., 1996), spread the workload at seeding, better utilize the variable seasons and increase productivity without reducing grain quality.

Matching cultivar maturity to the sowing date is a key element for maximizing wheat grain yields in dryland, winter-dominant rainfall regions. Where long-, mid- and short-season cultivars are available to the farmer, the appropriate cultivar can be selected according to the sowing time, thus increasing productivity and reducing seasonal risks.

Sowing the wheat crop earlier can alter other crop responses and crop management practices. The response of early-sown crops to nitrogen fertilizer is often economic, but that of later sown crops is not (Anderson et al., 1995; Anderson et al., 1997). Higher yielding, semidwarf wheat cultivars may depend for their yield advantage on being sown at the optimum time (Anderson and Smith, 1990) as well as on their response to other improved inputs (Table 25.2).

Improvements in wheat grain yield have resulted from combining improvements in both cultivars and agronomic practices. New cultivars may either respond better to improved agronomic inputs or else require them to express their yield advantage.

TABLE 25.2
Improved agronomy and improved cultivar both increase yield of wheata

^a Data are from seven sites in 1987 and 1988 in Western Australia where rainfall in the growing season varied from 237 to 338 mm.

^b Sown June, 0 kg/ha N, 50 kg/ha seed.

^c Sown May, 50 kg/ha N, 70 kg/ha seed.

Lastly, the trend towards earlier sowing has been assisted by the increased use of herbicides that allow weeds to be controlled without the need for multiple cultivations before sowing. In Western Australia, since the mid-1980s earlier sowing has been accompanied by a rise in the incidence of herbicide resistance in several major weed species. Methods of reducing the use of residual herbicides to permit earlier sowing, such as control through grazing, hay cutting or green manure cropping in the previous season, need further investigation if early sowing is to be sustainable.

Nutrition and fertilizers

Grain yield responses to nitrogen (N), phosphorus (P) and potassium (K) are common in the dryland, winter-rainfall areas. Responses to nitrogen are variable depending on soil type, season and crop rotation (Russell, 1967; McDonald, 1989; Mason et al., 1994). In general, the optimum nitrogen application is greater on coarser textured soils, where losses from leaching can be higher, and less on clay soils, but this can be modified by the use of legumes in the rotation (Table 25.3). Control of root diseases through crop rotation can influence wheat yield response to nitrogen (Rowland et al., 1988), and the combination of legume rotations and clay soils can make the use of applied nitrogen unnecessary for yield improvement (Anderson et al., 1995).

TABLE 25.3
Soil type and legume rotation affect economic optimum nitrogen application rate and grain protein percentage, Western Australia^a

^a Data are from over 30 experiments using noodle wheat in the 250 to 400 mm rainfall zone.

^b Nitrogen rate required to obtain a yield response of 5 kg/ha of grain for 1 kg/ha of N.

^c N optimum is economic optimum nitrogen application.

^d Sand over clay or texture contrast soils.

Source: Anderson, unpublished data, 1998.

Nitrogen fertilizers are most often effective when applied at sowing, but where the yield expectation is high and the season is longer, split applications between sowing and up to the early boot stage can also increase grain yield and protein percentage (Mason, 1975). However, recovery of applied nitrogen by dryland wheat crops is commonly less than 50 percent (Fillery and McInnes, 1992) thus making economic returns risky in some situations. The optimum amount of nitrogen fertilizer to apply to the wheat crop depends on the supply from soil organic matter and previous legume fixation, the losses through leaching and volatilization, and the demand from the crop. All these factors can be calculated for a given crop, but in practice factors such as availability of fertilizer and cash, the relative return from other production inputs and the perceived risks of the environment are all modifying influences. The decision is seldom made on purely biological grounds in dryland environments.

Crop responses to phosphate fertilizers have been reviewed by Matar et al. (1992). In most of the dryland, winter-rainfall areas, regular applications of phosphate fertilizers are essential for wheat production owing to widespread deficiencies and high phosphorus fixation in many soils (Russell, 1967; Harmsen, 1984; Impiglia and Ryan, 1997). Soil tests can often be used in the diagnosis and treatment of phosphate deficiency for wheat production (Peverill et al., 1999).

Deficiencies of potassium for wheat production are not widespread in the dryland, winter-rainfall regions but are likely to become more prevalent where production is intensified (Ryan et al., 1997). The impact of marginal potassium deficiency can be exacerbated by the use of high rates of nitrogen fertilizer in coarser textured soils, which may reduce grain yield and hectolitre weight and increase small grains (Table 25.4).

TABLE 25.4
Effect of potassium and nitrogen fertilizers on wheat yield and grain quality^a

^a Data are from an experiment in Western Australia where the soil potassium test (0 to 10 cm) in bicarbonate extract was 34 mg/kg.

^b N = nitrogen; K = potassium.

Source: Anderson et al., 1992.

The trace element deficiencies found most commonly in the winter-rainfall areas are those associated with alkaline soils-zinc, manganese and iron. Of these, zinc and less often manganese are most serious in wheat crops. Copper deficiency is also found, especially in Southern Australia. Boron toxicity is quite widespread in some alkaline, clay soils on several continents. Aluminium toxicity is less common and associated with acid, sandy soils. Diagnosis of nutritional deficiency and toxicity can often be made by plant analysis (Reuter and Robinson, 1997).

Correction of trace element deficiencies when they are recognized is most economically achieved with small additions to the base fertilizer. Manganese and zinc added to superphosphate fertilizer can last in most soils for ten or more years. Foliar sprays can also be used but their effect is short-lived. Tissue tests, and sometimes soil tests, can be useful in diagnosing deficiencies of trace elements. Managing toxicity is probably best addressed by using tolerant cultivars. Fortunately, good sources of resistance to both boron and aluminium toxicity are available in wheat.

Adequate crop nutrition based on soil and tissue testing where appropriate and an understanding of nutrient supply, losses and demand are essential features of efficient, dryland cropping systems. The risks of applying fertilizers to the wheat crop are complicated by factors such as weeds, diseases, sowing time, variable rainfall, the availability of fertilizer and credit, and interactions with other nutrients. In practice, the decision as to the amount to apply is most often the result of balancing several of these factors.

Fertilizer placement in the soil can be critical for adequate uptake by the crop without damage to the roots. In general, it has been shown that both nitrogen and potassium fertilizers should not be mixed with the seeds to avoid delayed germination and emergence or even death of the seedlings (Mason, 1971). Damage is worst when soils are drying (Carter, 1967). Placement of the fertilizer in relation to the seed can also influence uptake and crop response. In general, placement in a band below the seed and at some distance from it achieves the best results with nitrogen and potassium fertilizers (Nyborg and Hennig, 1969), but mixing phosphate fertilizers with cereal seeds at sowing can be beneficial in severely phosphate-deficient soils (Loutit et al., 1968). Surface placement is less efficient since it may increase fixation of phosphorus fertilizers, favour volatilization losses of nitrogen fertilizers and in any case be less available when the surface dries out. Deep placement often has advantages over shallow placement for this reason (Jarvis and Bolland, 1990) but may not be compatible with minimum tillage systems of sowing.

Plant population

It has often been considered that accurate control of plant population, or seed rate, in winter cereals is not essential because of their capacity to tiller profusely. This is probably true where other factors are more limiting for crop yield, but in situations where the aim is to use all the rain that falls during the season, choosing the optimum population becomes more important.

The critical factor determining optimum population appears to be the yield level. Data from a large number of experiments using the criterion that the optimum is reached when an extra kilogram of seed increases the grain yield by only 10 kg/ha were used in Western Australia to estimate the optimum population (Anderson et al., 1991). It was found that the optimum plant population increased as the maximum yield of the experiments increased, such that about 50 plants/m² were required for each tonne of expected yield up to about 4 tonnes/ha. This can be related to rainfall zone and average seed size as in Table 25.5.

TABLE 25.5
Seed rates to achieve specified plant populations for wheat of various seed sizes, Western Australia^a

^a An establishment of 60 percent is assumed for the low rainfall zone, 75 percent for the medium rainfall zone and 85 percent for the high rainfall zone.

The yield response to plant population reaches a plateau after the optimum is reached and does not generally decline unless very large populations are used. Excessively high populations may increase water use before anthesis in water-limited environments thus reducing water availability during grainfilling, grain size and possibly yield (Fischer and Kohn, 1966a).

The actual relationship between optimum plant population and yield level in any environment will vary with such factors as the amount of solar radiation received, the quality of the seed sample, the soil conditions, the rainfall received, the sowing time and the presence of weeds. The nature of the flat yield response to increasing plant population, after the optimum population has been reached, makes erring on the high side of the optimum a relatively low-risk strategy.

The optimum plant population of wheat is proportional to the yield level such that where higher grain yields are expected, increased seed rates should be used to ensure that the plant population is not a limiting factor.

Management of weeds

Weeds often impose the most significant costs to many wheat crops, either through the yield loss from competition between the crop and weeds, or in the costs of applying appropriate control measures. While herbicides provide effective tools to manage weeds in many crops, the changes in weed spectrum that have occurred with herbicide resistance and the development of new cropping systems show that weeds will exploit any opportunities available within the crop (Bowran, 2000).

Management to reduce weeds prior to the crop has considerable benefits for both wheat grain yield and quality (Table 25.6). The negative impacts of weeds on water storage, disease and insect transfer, crop competition and contamination of the wheat grain sample are well known. Reduction of the weed seed bank in the seasons before cropping by cultivation, herbicides, grazing, hay cutting or green manure will reduce the risks of crop losses from weeds and allow earlier sowing without delays due to pre-sowing weed control operations.

TABLE 25.6
Weed control in the year before cropping increases wheat yield and protein percentage

Source: Thorn, unpublished, 1992.

The impact of weeds in the wheat crop is greatest when weeds are not controlled early in the life cycle of the crop. While wheat is one of the most competitive crops (van Heemst, 1985), early competition from weeds that occurs when the wheat plant is producing tillers can reduce the supply of nitrogen, water and light to the crop, all of which are critical factors for tiller production. Weed competition that occurs during tillering, usually in the first 10 to 50 days after sowing, is most detrimental to grain yield. Competition occurring later in the life cycle can affect grain numbers and grain weight, but usually has a smaller effect in yield reduction.

The extent of yield reduction is a function of weed density, since lower weed densities will remove less nutrient and may even be out-competed by the crop. Trial evidence (D. Bowran, personal communication, 2000) suggests that where grass weeds, such as wild oats (Avena spp.) or brome grass (Bromus spp.), are present at similar densities to the wheat crop and are not removed, yield losses of 15 to 25 kg/ha/day of competition may occur. Removal in the first five weeks after emergence (or before emergence if a residual herbicide is used) is thus advisable.

The level of crop yield reduction is a function of the species, the density of the weeds, the density of the crop, when the weed emerges relative to the crop and when the weed is removed from the crop. Table 25.7 shows the average yield loss from a number of different grass species in wheat crops when the weed emerges with the crop.

TABLE 25.7
Yield loss with increasing density of grass weeds in wheat sown at 50 to 60 kg/ha

Source: Modified from Poole and Gill, 1987.

The data in Table 25.7, from extensive field studies, show that weeds can produce severe reductions in yield even at quite low densities. The implication is that where a farmer establishes wheat at 125 to 150 plants/m² and a weed such as ryegrass (Lolium spp.) emerges at the same density, then the potential yield foregone is about 25 percent. However, increasing the plant population of wheat can considerably reduce the yield loss (Table 25.8).

TABLE 25.8
Effect of wheat plant population in the presence or absence of ryegrass on wheat yield, ryegrass population and ryegrass seed production

^a Presence of ryegrass = +R; absence of ryegrass = -R.
Source: Fee, unpublished data, 1996.

Fertilizer applied to the crop can promote weed growth and seed production and may thus be lost to the crop. Increasing crop density has a major effect on reducing these competition factors. Where weed density is low, increasing crop density will be a major factor in reducing competition from weeds and limiting weed seed production for a following crop.

Weeds may impose other costs in wheat production. Weed seed contamination that reduces grain quality can be significant if a lower premium is obtained or if the crop has to be cleaned after harvest. Weed seeds that have the potential to be toxic pose particular problems as they may be difficult to identify or to clean from the crop.

Weeds not controlled before the cropping season need to be controlled by some means prior to sowing wheat if practical. Herbicides applied whilst weeds are small can be effective and are widely used. Soil moisture is conserved, soil structural damage is reduced and time is saved compared to the use of cultivation. Glyphosate-based herbicides provide a wide spectrum of weed control, can be mixed with other herbicides for enhanced knock-down of weeds that are hard to kill and are relatively low cost. Where weeds are small, the bipyridyl herbicides, paraquat and diquat, also offer excellent knock-down weed control. The 'double knock' system of using glyphosate followed by bipyridyls five to ten days later provides faster desiccation of weeds prior to seeding and improved control of weeds that are hard to kill with glyphosate (Bowran, 2000).

Cultivation is a useful tool where weed germination is early and drying conditions follow. Cultivation, if not too deep (2.5 cm maximum), can stimulate weed seeds to germinate on subsequent rain, and these can then be killed with a knock-down herbicide or with further cultivation. Cultivation is a useful tool for some weeds that are hard to kill with knock-down herbicides.

Other non-chemical solutions can also be used strategically. Early grazing can be an extremely valuable tool for reducing weed populations, reducing blockages in seeding machinery and minimizing the amount of herbicide required prior to seeding. Burning of stubble kills both weed seeds and germinated weeds, and where wheat is sown after wheat, will assist in leaf disease control. Where weed seeds have been concentrated into harvester trails, it may be possible to kill 80 percent of seeds prior to opening rains if a 'hot' burn can be achieved.

In many dryland, winter-rainfall areas, chemical control of weeds once the crop has been sown is a widespread practice. Both broadleaf- and grass-specific chemicals are available, many of which have a residual action. The effectiveness of such systems is highly crop, weed, soil and weather specific and will not be discussed in detail in this chapter. Two aspects of residual herbicide use should be mentioned, however. Firstly, the principle of alternating the chemical groups to delay or avoid herbicide resistance in the weed populations is highly desirable. Secondly, herbicide-resistant crops can be used to reduce the population of some weeds that are not resistant to the particular herbicide. This can give the farmer a chance to put other solutions in place. Neither principle should be relied upon as the sole, long-term strategy for weed control in a wheat cropping system since there is a high likelihood that resistance will still develop.

Weed control systems that integrate chemical and non-chemical methods have the best chance of reducing weed competition and maintaining profitable crops. Such systems may include non-residual chemicals, cultivation, highly competitive crops and cultivars, green manure and hay crops, grazing and even burning of crop residues where appropriate.

Disease management

Leaf and root diseases have played a major part in shaping wheat production systems in dryland, winter-rainfall areas. Grain yields have seldom been high enough to warrant chemical spraying for disease control on a routine basis as in more humid areas. This has meant that breeding for resistance or tolerance and the use of agronomic methods have been more widely used. Breeding for resistance has been more successful for addressing the threat of the leaf diseases, such as the rusts (Puccinia triticina [syn. P. recondita], P. striiformis and P. graminis) and Septorias (Leptosphaeria nodorum and Mycosphaerella graminicola), but cultural methods have been more useful for reducing the impact of root diseases, such as take-all (G. graminis var. tritici), dryland root rot (Fusarium graminearum) and Rhizoctonia root rot (Rhizoctonia solani). Management of virus diseases, such as barley yellow dwarf virus (BYDV), have largely been approached through control of the aphid vectors, since sources of resistance are not common.

An exhaustive review of the diseases common to the dryland areas and their control is not attempted here. Some of the cultural methods used as part of an integrated disease control strategy are discussed. Root diseases that are transmitted to the wheat crop from alternative grass hosts have been successfully controlled by the use of 'break' crops, such as pulses, and non-legume broadleaf crops, such as canola. The biomass of grasses in the preceding pasture phase has been related to the incidence of take-all (MacNish and Nicholas, 1987), and the use of herbicides for grass control can be effective in decreasing take-all and increasing the yield of wheat (Table 25.9).

TABLE 25.9
Relationship between grass weed control, take-all and wheat yield

Source: After MacLeod et al., 1993.

The management required to reduce disease incidence and yield losses due to take-all can be summarized as (MacNish, 1980):

• reduction in grasses preceding the wheat crop;
• use of non-cereal 'cleaning' or break crops prior to wheat;
• use of the ammonium form of nitrogenous fertilizer;
• application of sound crop management including weed control and fertilization.

The incidence of take-all has been shown to be sensitive to soil pH (Murray et al., 1987), with wheat crops growing on soils below a pH of about 4.8 (in calcium chloride, CaClhaving greatly reduced symptoms. This response to pH is thought likely to be associated with differences in nutrient availability to the plant (Glenn and Sivasithamparam, 1991). Zero-tillage sowing systems (Wong and Southwell, 1987) and seasons that receive higher spring rainfall (Roget and Rovira, 1991) have been related to an increased incidence of take-all. Decreased incidence has been associated with burning or early incorporation of stubble and grazing of the preceding pasture (Murray et al., 1991).

Root rots caused by R. solani have been associated with alkaline soils and with reduced or zero-tillage systems in dryland, winter-rainfall areas, although the effects of reduced tillage on the pathogen can be fewer with the use of short-term chemical fallow (Roget et al., 1987). Dryland root rot, caused by F. graminearum, by contrast has been associated with lower rainfall areas but was not reduced where fallow of up to 18 months was practised (Klein et al., 1990).

Management of root diseases of the wheat crop should be focused on crop rotation, on weed control in the previous season and on tillage practices that reduce carry-over of the pathogen.

Control of leaf diseases has been approached more often through breeding for resistance and the application of fungicides than through cultural means. However, the trend towards retention of crop residues on the soil surface has been associated with increased incidence of leaf diseases, such as tan spot caused by Pyrenophora triticirepentis (Rees et al., 1981). Similarly, the increased incidence of the Septoria blotch diseases caused by L. nodorum and M. graminicola has been associated with earlier sowing (Murray et al., 1990) and increased grain yields associated with earlier sowing (Loughman and Thomas, 1992). If early sowing and retention of residues are to remain permanent features of the wheat cropping system, increased emphasis must be placed on other control measures, such as fungicides and breeding for resistance.

Diseases such as the rusts (leaf, stem and stripe) and virus diseases such as BYDV are carried over from one season to the next on plants that survive over the summer in moist areas. Given the difficulty of controlling this green-bridge, management of these diseases will probably depend on breeding for resistance and fungicides, or some combination of these. Loughman and Thomas (1992) have shown that fungicide sprays can give an economic return through control of the Septoria diseases in high-yielding crops. This result depended, however, on the resistances of the cultivars used and the combination of diseases present. In addition, it has been observed that the severity of Septoria tritici blotch is dependent on the average susceptibility of the cultivars grown in an area (Murray et al., 1990).

Management of leaf diseases should include the selection of resistant cultivars where available, the strategic use of fungicides on high-yielding crops and the disposal of previous wheat crop residues prior to sowing of the new crop.

Insect pest management

Generally, insects do not pose a major threat to wheat in dryland areas, but this may vary from region to region, and damage can be severe in some seasons. In general, the chances of insect damage are greater in better seasons and in higher yielding crops. Many insects can be found in the wheat crop, but most of them cause little or no damage. Miller (1987) has listed the major species found in wheat crops in West Asia and North Africa. Pest monitoring and the use of economic thresholds are important tools for the initiation of pest management procedures.

Aphids can be important pests of wheat, and yield losses can approach 30 percent in wetter conditions (Miller, 1987). Their feeding can cause serious damage, and in addition, some species can act as virus vectors. Rhopalosiphum padi, R. maidis, Sitobion avenae, Schizaphis graminum, Metopolophium dirhodum and Diuraphis noxia are species commonly found throughout the world. Aphid populations can build up over the summer in dryland areas, especially following unseasonal rains, and an autumn flight can occur in addition to the usual spring flights. Control of the autumn infestation is particularly important for reducing the impact of viruses on grain yield. Aphids have many natural enemies, such as lacewings, ladybird beetles and diseases, which are important natural control factors. Therefore, selective chemicals should be used in chemical control of aphids, and sprays only applied when necessary.

The sunn pests are a complex of species of stinkbugs, the most important of which is Eurygaster integriceps. Stinkbugs are of major importance in West Asia and North Africa. They feed on cereals, grasses and some weed species and can cause losses to leaves, shoots, stems and grains. However, the most serious losses are caused by the feeding of nymphs and adults on the milky stage or mature grain, leading to a reduction in baking quality (Javahery, 1995). Although natural biological control plays a very important role in reducing sunn pest populations, the emphasis has been on chemical control using organophosphate insecticides, but the severity of the outbreaks (Miller and Morse, 1996) has not been reduced by long-term pesticide use.

The Hessian fly, Mayetiola destructor, is one of the most destructive pests of wheat and other cereals. Massive outbreaks have occurred in North Africa (especially in Morocco) and the United States, and losses can be as high as 50 percent. Heavy infestations can result in a thin stand, stunting and lodging, leading to reduced yields. Late seeding can significantly reduce the severity of this pest (Anonymous, 1969), and ploughing the crop residues can also be effective, but neither measure is considered suitable for the farming systems of some countries. Insecticides are not fully effective and may be expensive (El Bouhssini et al., 1988). Host plant resistance is thus important in reducing the abundance of Hessian fly (Roberts et al., 1979).

Wheat stem sawflies, Cephus spp., can cause up to 80 percent crop loss and are periodically severe in Syria, Turkey and Jordan (Miller, 1987). Up to 40 percent of stems can be girdled by the pest in an out-break, leading to reduced kernel weight and kernel number (El Bouhssini et al., 1987). Seed treatment with carbofuran was not fully effective in controlling the pest in experiments in Morocco (El Bouhssini et al., 1987), and agronomic methods, such as delayed sowing, rotation and deep ploughing, are not considered appropriate. Solid-stemmed cultivars are much less affected and have been suggested as the basis for breeding resistant cultivars (Rashwani, 1985). Significant progress in this regard has been made in both bread and durum wheat for Mediterranean conditions (N. Nasrellah, personal communication, 1996).

Locusts and grasshoppers can also be considered important threats for the wheat crop in localized dryland areas. Red legged earth mite (Halotydeus destructor) and lucerne flea (Sminthurus viridus) can also cause damage to seedling crops in Australia.

The management of damage to wheat crops from insect pests begins with frequent observation. Monitoring of pest numbers can reduce the unnecessary use of pesticides but improve the chances of obtaining a response to the use of selective chemicals where economic thresholds have been established. Breeding for resistance to pests such as sawflies, aphids and Hessian fly is an important component of integrated pest management systems.

WATER-USE EFFICIENCY AND LIMITING FACTORS

It is often assumed that the most limiting factor in dryland wheat crops is water. In the absolute sense, this is true but in practice many factors limit the efficient use of water in yield production. The concept of limiting factors, discussed early in the century by Blackman (1919) who drew on earlier German work, has been the guiding principle for agronomists and farmers in devising cropping systems. In most cases, more than one factor limits yield, and improvements have come from bringing together all the factors that are recognized as limiting in a given situation. Synergisms between the factors often operate such that the response to two factors applied together is much greater than the response to the same two factors applied individually.

The idea that water-use efficiency is the key to yield improvement in dryland agriculture is clearly valid, but the ratio of grain yield to water used (evaporation plus transpiration) is most often affected more by yield obtained than by water used. In other words, any treatment that increases yield while using the same, or a similar, amount of water will have a higher water-use efficiency. This is clear when it is observed that low-yielding crops use about as much water as high-yielding crops in dryland environments (Fischer and Kohn, 1966b; Cooper et al., 1983; Anderson, 1992).

The dependence of water-use efficiency on correcting the limiting factors other than water supply has been elegantly demonstrated under Southern Australian conditions by French and Schultz (1984a, 1984b). Applying the principle that grain yield is linearly proportional to water used so long as other factors are not limiting, they showed that wheat yields and water-use efficiency were often restricted by management factors, such as a lack of weed control, inappropriate sowing time and inadequate crop nutrition. It has further been demonstrated that dryland wheat crops in Western Australia can approach or equal the rainfall-limited potential yield given the appropriate combination of agronomic practices (Table 25.10).

TABLE 25.10
Yield increases due to agronomic inputs^a

^a Data are from experiments at Northam, Western Australia, on a clay loam soil in 1987 and 1988 when the seasonal rainfall was 327 and 328 mm (Anderson, 1992).

^b Potential yield, which is calculated as seasonal rainfall minus losses of 80 mm multiplied by a transpiration efficiency of 20 kg/ha/mm (D. Tennant, personal communication, 1999), was 4.96 tonnes/ha.

^c Sown June, tall cultivar, 0 kg/ha N, 40 kg/ha seed.

^d Sown May, semidwarf cultivar, 80 kg/ha N, 80 kg/ha

In practice, the combination of improved agronomy and improved cultivar can increase grain yield through reducing water lost to the crop and increasing the proportion of water available to the crop after anthesis. Losses of water occur more through soil evaporation in dryland, winter-rainfall environments than through run-off or deep percolation. The efficiencies of both water and nitrogen use can thus be improved (Table 25.11).

TABLE 25.11
Agronomic efficiencies of wheat production^a

^a Data are from seven experiments in 1987 and 1988 in Western Australia where rainfall in the growing season varied from 237 to 338 mm.

^b June sown, tall cultivar, 0 kg/ha N, 50 kg/ha seed.

^c May sown, semidwarf cultivar, 50 kg/ha N, 70 kg/ha seed.

Source: Anderson, 1992.

Appropriate agronomic practices will increase the proportion of total water use that passes through the plants and in so doing will increase grain yield and water-use efficiency. It is thus suggested that if farmers concentrate on improving crop management, improved water-use efficiency will follow.

Where all of the seasonal rainfall is not used by the crop, the remainder can eventually add to the water-table. In such situations, a rising water-table can bring dissolved salts to the surface where evaporation can result in dryland salinity. It is thus doubly important that the farmer and agronomist strive to use all of the rain that falls during the growing season if the cropping system is to achieve sustainability.

MANAGEMENT FOR GRAIN QUALITY

As markets for wheat become more conscious of quality and less sensitive to price, premiums paid for particular grain quality have increased. Premiums are largely paid on attributes that can be readily ascertained at the point of delivery to the buyer or market place. Grain protein, hectolitre (or test) weight and small grain sievings fall into this category. The other factor over which the grower has control is choice of cultivar. Through various mechanisms, the market specifies, directly or indirectly, the suitability of cultivars for particular products and end uses. It is also important to recognize that a grain quality character, such as flour yield, can be as important as grain yield itself where the grain is not sold but is consumed by the producer. Durum wheat grown for consumption by the farm family is often not the highest yielding type as grown for sale and profit, but is the local landrace grown for taste and quality for traditional recipes (Tutwiler, 1995).

In situations where increasing grain yield by agronomic means is risky, or not possible due to inadequate water supply or unavailability of other inputs, it may be preferable to change the production system to increase the price received. In such cases, profitability can be maintained without undue production risks, and unsustainable cropping intensity is not likely to be necessary. This discussion will concentrate on the grain qualities that can be easily measured at delivery and for which growers are likely to receive price premiums. In the future, it is likely that it will be possible to assess other qualities and that growers will receive premiums for a wider range of parameters that affect milling, dough and end product quality. Additional quality traits are especially important for durum wheat, such as vitreousness, carotene content and gluten strength, and are influenced by management to a greater or lesser extent (Impiglia and Ryan, 1997).

Soil type

Most farmers have a limited choice of soils on which to grow the wheat crop. An under-standing of the grain quality types that are most appropriate on particular soils is a useful management tool, nevertheless, for designing profitable cropping systems. Where legume rotations are practised on the clay and clay loam soils in Western Australia, it is easier to produce the higher protein that is required for hard wheat. In contrast, on the coarser textured soils it is easier to manage for the medium grain protein percentages that are required for noodle wheat (Table 25.12).

TABLE 25.12
Effect of soil type on success rate of achieving various grain protein levels from two sets of field experiments following grain or pasture legumes

^a Sand over clay at <50 cm.
^b Loamy sand to at least 1 m.
Source: Anderson, unpublished data, 1997; Kerr, unpublished data, 1996.

Legume nitrogen

Nitrogen accumulated from fixation by legumes prior to the wheat crop can have a vital effect on the production of high grain protein. Most crops on clay loam soils that followed pastures with over 50 percent legume content and few grasses, or field peas, had protein of 13 percent or more and suffered no yield penalty compared to crops in a continuous wheat or non-legume rotation in Western Australia (Anderson et al., 1995). Crops with a good legume pasture history did not require nitrogen fertilizer to achieve 13 percent grain protein percentage in that study. Similar responses of grain protein and related dough properties to legumes in the rotation have also been recorded in other winter-rainfall areas (Lopez-Bellido et al., 1998).

Manipulation of agronomic practices within a site (nitrogen fertilizer, cultivar, sowing date) can change the grain protein percentage by 1 to 2 percent. In contrast, rotation with medic (Medicago spp.) pasture increased protein by about 5 percent for a given grain yield level compared to continuous wheat (Anderson et al., 1998).

On coarser textured soils, suitable for the production of medium protein grain, legume rotation can have a similar impact (Table 25.13).

TABLE 25.13
Legume history affects grain protein percentage and the probability of obtaining protein in the range acceptable for noodle wheata

^a Data are from 18 experiments over four years in Western Australia.
^b Sand over clay at <50 cm.
^c Loamy sand to at least 1 m.

Source: Anderson, unpublished data, 1997.

In the case of soft wheat suitable for biscuits and cakes, there is a requirement for grain protein that is less than about 9.5 percent, higher protein percentages leading to reduced prices. However, failure to meet the standard required for such grain in experiments in Western Australia has been found to be as much related to excessive small grain screenings as to inappropriate protein level (Anderson and Sawkins, 1997). Small grains were occasionally produced in excess of the allowable standard due to the combination of late sowing, high applied nitrogen and high seed rate, but were most often associated with high soil fertility following long clover pasture phases.

Grasses

Grasses have a large requirement for nitrogen and if not removed either prior to the crop year or during the season, reduce the amount of nitrogen available to the wheat crop. Unless soil nitrogen supply is abundant, this often results in reduced grain protein percentage as well as reduced grain yield (Table 25.14).

TABLE 25.14
Grass weeds reduce grain yield and protein percentage

Source: Anderson et al., 1995.

Sowing time

Delayed sowing increases grain protein in many cases but reduces grain yield so that it is generally uneconomic to delay sowing purely to increase grain protein (Anderson et al., 1996). Later sown crops, in addition, are often more at risk of producing excessive small grain screenings. There is also a tendency for cultivars that are sown outside their optimum time, either too early or too late, to have lower hectolitre weight. This effect can be associated with increased disease incidence in early-sown crops and water stress in late-sown crops.

Plant population

Increasing plant population within the normal commercial range appears to have no serious deleterious effect on grain quality (Anderson and Sawkins, 1997). This is particularly important for wheat growers using higher seed rates as a means of improving the competitiveness of their crops against weeds.

The price incentives that are available for grain of the appropriate quality for particular end uses are almost as important for profitability as yield itself. Most of the available evidence confirms that the production of high-quality grain does not entail practices that are in conflict with the production of high yields. Selection of cultivars that are acceptable to the market, selection of appropriate combinations of rotations and soils and adjusting fertilizers, weed control and plant populations for maximum yield according to the rainfall zone are all practices that should lead to good quality. Profits can be maintained by improving quality without overusing practices that may lead to increased soil acidity, herbicide resistance and soil structural decline. Quality has thus become linked to sustainability.

The available evidence confirms that seasonal effects have a major influence on variation in grain quality. However, there is an increasing realization that good management can substantially improve the probability of achieving the grain quality that brings a price premium. Growers thus do not have to accept the grain quality that is dictated by the season, but can adopt management practices that provide a buffer against seasonal variation.

ACKNOWLEDGEMENTS

Helpful advice and permission to use data in preparing this chapter was received from Dr D. Bowran, Mr C. Fee, Dr N. Nasrellah, Dr M. Barbetti, Dr D. Tennant, Mr C. Thorn, Mr J. Blake, Dr N. Di Fonzo, Mr I. Rowland, Dr G. van de Klashorst, Ms F. Berlandier, Mr K. Young and Ms N. Kerr.

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