Land management for crop production

R.G. Fawcett

Department of Agriculture, G.P.O. 1671, Adelaide, S.A. 5001.

Introduction

The theme for this review is "surface management of arable soils" and I intend concentrating on land management systems for the production of grain crops grown under dryland conditions.

During the 1900's to the 1920's, substantial areas of southern Australia were cleared and brought into production. Wheat was the only cereal of consequence that had an overseas market during this period and land management systems were developed for wheat production although cereals and other crops were also grown for grain and fodder. The initial development phase in the opening up of land for the production of wheat and other crops ended in the late 1920's, when 10 million hectares were being cropped. The area sown to wheat decreased during the depression of the 1930's and the early 1940's from 8 to 4 million hectares.

The continued use of long fallows, coupled with the preparation of fine seedbeds, led to extensive wind and water erosion of cropping lands in southern Australia during the 1930's and 1940's (Matheson 1978; Malcolm 1972). Legislation was enacted during this period in some States, to control erosion on agricultural and pastoral lands (Pauli et al. 1978).

The increased world demand for wool in the early 1950's and the associated high prices gave many farmers in southern Australia an alternative to wheat production as a source of income. Adoption of ley farming systems of land management during the 1950's helped to stabilize the soils against wind and water erosion (Watson 1969; Malcolm 1972; Matheson 1978).

The reduction in wool prices during the late 1950's and 1960's, and the relative stability of wheat and barley prices, resulted in farmers again turning to closer cropping rotations as a means of coping with the increasing cost-price squeeze that began during this period and of course still continues. Wheat quotas, introduced in the late 1960's, brought to an end the second period of major increases in the area of wheat cropping in Australia.

The total area for cropping increased from 8 million hectares in the mid 50's to nearly 20 million hectares in the late 60's. The areas being cropped in Australia are still comprised largely of winter cereals although significant areas of New South Wales and Queensland are sown to summer cereals and oil seed crops (Table 1). The areas sown to grain legumes are relatively small.

TABLE l. Areas being cropped in Australia ('000 ha) : average per year for the period 1971/72-1977/78 ¹

	W.A.	S.A.	Vic.	Tas.	N.S.W.	Qld.	N.T.	Aust.
Winter crops:
Cereals for grain 2	4171	2128	1703	20	3532	709	0	12263
Cereals for hay fodder 2							0	1034
Oil seed 3	14	6	14	0	29	20	0	83
Summer crops:
Cereals for grain 4	2	0	3	0	270	421	4	700
Cereals for hay fodder 4	2	0	3	0	26	62	1	94
Oil seed 5	3	3	7	0	134	119		266
Legumes for grain: (winter & summer) 6	76	18	6	1	15	39	0	155
Total	4459	2290	1853	37	4339	1612	5	14595

1 Source of data: Commonwealth Bureau of Statistics Year Books
2 Wheat, barley, oats, rye
3 Rapeseed, linseed, safflower
4 Sorghum, maize, millets, rice
5 Peanuts, cotton, sunflower
6 Navy and soybeans; cow, field and poona peas; other.

The history of cropping in Australia illustrates the dynamic nature of farming and land management systems, as farmers respond in various ways to economic pressures and market opportunities.

Estimates by Stewart 01979) of the areas of potentially arable land in Australia show that about 70 per cent of the potentially arable land in Australia has already been developed (Table 2). The conservation of these land resources, as well as those of arable land at present undeveloped, is vital if the future stability of crop production in Australia is to be maintained or improved.

TABLE 2: Arable land in the 3 agro-ecological regions of Australia (Stewart 1979)

Agro- ecological region	Potentially arable land (106 ha)	Developed arable land (106 ha)
Northern Australia	3.0	0.2
Eastern Australia	24.0	6.0
Southern Australia	49.7	44.7
Total	76.7	50.9

The effects of land management systems in relation to various aspects of crop production in the southern, eastern and northern agro-ecological regions (Stewart 1979) will now be reviewed.

2. Climate

Of the various components of climate, the distribution and amounts of rainfall, coupled with evaporative demand, determine in large measure the potential crop production under dryland farming conditions in these regions.

Estimates of the percentage probability of receiving rainfall equal

to, or greater than, the effective rain per month give useful indices of soil-water relationships with regard to crop germination and growth (Hounam 1947), together with weed growth and fallow moisture accumulation. Estimates for representative centres in the different regions are given in Table 3.

TABLE 3: Seasonal distribution of percentage probability of receiving rainfall equal to or greater than the effective rain per month, together with the annual rainfall (mm), for representative centres in agro-ecological regions of Australia'

Town	Month of Year												Annual rainfall
Town	J	F	M	A	M	J	J	A	S	O	N	D	Annual rainfall
Southern Australia
Moora, W.A.	1	3	7	13	79	100	100	91	44	6	0	0	469
Horsham, Vic	9	17	18	48	78	93	95	92	82	55	27	20	446
Wagga, N.S.W.	27	32	33	53	71	99	92	84	73	62	41	27	540
Eastern Australia
Narrabri, N.S.W.	38	38	45	52	52	80	74	63	48	48	45	44	610
Dalby, Q.	63	60	53	40	40	57	64	38	44	50	57	63	635
Northern Australia
Katherine, N.T.	100	96	80	19	1	1	0	0	2	15	54	96	890

'Estimates taken directly from appropriate Australian Bureau of Meteorology publications, or derived, using evaporation data (E, inches) published in maps, together with rainfall data(P, inches) and the formula : P/E 0.75 = 0.54 (Prescott 1951).

The seasonal distribution of the probability of receiving effective rainfall allows only winter crops to be grown in southern Australia (although restricted areas of summer crops may be produced in higher rainfall districts) while in northern Australia, only summer crops can be grown under dryland conditions.

In eastern Australia, the influence of winter and summer rains results in less seasonal variation in the probability of receiving effective rain, than in southern and northern Australia, but the estimates (Table 3) show that there can be substantial deficits in effective rainfall during any month of the year. With careful land management for fallow moisture accumulation before sowing a crop, it is possible to grow both winter and summer grain crops in this region (Hounam 1950).

Seedbed moisture

Preparation of a bare fallow by cultivation, with the aim of accumulating soil moisture, has been a traditional practice for crop production in Australia.

(a) Southern Australia

The low probability of receiving effective rain during the summer months in southern Australia means that the main opportunity for accumulating fallow moisture is in the winter and spring of the year preceding the crop. Research in Victoria and South Australia has shown that significant amounts of soil moisture can be accumulated in the heavier textured soils under a long fallow (8-10 months) but little or no accumulation occurs in the sandier soils (Tuohey et al. 1972; French 1978a). In Western Australia and southern New South Wales, long fallowing for moisture conservation is of little significance (Fisher 1962; Kohn et al. 1966). In some seasons, summer rains can give farmers an opportunity to begin the preparation of medium length fallows (5-7 months) especially in paddocks with loams and heavier textured soils that set hard when dry and are not prone to wind erosion, and when little surface trash remains (poor pasture growth and/or hard grazing in the previous season). The smaller soil aggregates normally produced on the 5-10 month fallows can help retain moisture in the seedbed (Holmes et al. 1960) and ensure that seeding can be more timely,- a critical factor in the drier cropping areas where delayed sowings can reduce grain yields.

Short fallows of about 2-4 months are commonly used in the higher rainfall cropping areas of southern Australia and are begun after the break of the season.

The results of Schultz (1972) suggest that any increase in soil moisture conservation due to trash retention will be relatively small, especially in the drier cropping areas where crop stubbles are usually less than about 2 t/ha; also seedbed moisture can be less with direct drilling systems especially in dry seasons (Reeves and Smith 1973; Wells 1979; Fawcett unpub. data).

Water repellent sands occur in Western Australia, South Australia and Victoria (King 1974) and seedbed moisture and crop establishment on these sands can be particularly variable and crop establishment reduced by up to 90 per cent (King 1977; Fawcett 1979). The problem can be overcome temporarily to allow satisfactory crop establishment, by disc ploughing to 10-12 cm after autumn rains and then cultivating, preferably during or soon after rain, to ensure mixing of wet and dry sand in the seedbed. Repellence in the seedbed

can remain higher on direct drilled areas, compared with areas cultivated during or soon after rain spells, and results in reduced crop establishment.

Under wet seasonal conditions, sowing on some soils in southern Australia can be delayed where seed beds have been prepared with conventional tillage methods whereas direct drilling can proceed on most uncultivated soils (Reeves and Smith 1973; Pearce 1979).

(b) Eastern Australia

Heavy textured soils are common to much of eastern Australia and up to 250 mm or more of available soil water can be accumulated under fallows (Waring et al. 1958; Fawcett 1975). Estimates of the depth of wet soil prior to seeding, based on measurements with penetrometers or other equipment, can assist in decision making regarding the potential crop yield (Anon. 1970), nitrogen application (Leslie and Hart 1967), and seeding rates (Fawcett et al. 1976). The fallow period can range from about 6-15 months although fallows of only a few weeks result when the practice of double cropping is used on land that has received substantial rains either immediately before or after harvest of the first crop, so that adequate soil moisture has been accumulated to justify sowing a second crop (Mc Getrick 1968). The effectiveness of weed control has a major influence on soil moisture accumulation, irrespective of the tillage method used for fallowing.

The probability of receiving adequate rain for crop germination in May or June is less than 80 per cent in the wheat growing areas so that lack of moisture in the surface 5-10 cm can often delay seeding of winter crops (Earley 1968; Kamel 1975). Adequate soil moisture can exist immediately below the dry surface layer and under these conditions, special shares with narrow points (approx. 3 cm across) may be used on tyned seeders so that the seed is placed directly into moist soil (Kamel 1975).

The use of presswheels when seeding, or a roller after seeding, gives a better seed-soil-water contact and can improve crop establishment (Anon. 1970; Kamel, 1975).

Conventional seedbed preparation (ploughing and cultivation) in northern Australia is started after sufficient rain has fallen early in the wet season (late October - November) to wet the soil to a depth of about 20 cm (Anon. 1961). Cultivation is possible during the dry season on some soils such as the lateritic red earths at Katherine (Phillips and Norman 1961) and presumably light textured soils that do not set hard when dry.

The restricted growing season (4-5 months) means that the longer the period taken for land preparation after the wet season begins, the shorter the period available for crop growth. Experience in southern Australia suggests that crops could be sown earlier in the wet season of northern Australia if tillage techniques such as commencement of seedbed preparation prior to the wet season, or direct drilling, can be adopted.

Weeds

Weed infestations in crops can be costly both in loss of income due to reductions in crop yields and in costs associated with control of the weeds by cultivation and the application of herbicides (Mears 1968; Rawson 1968; Lumb 1973; Pearce 1977). Since the 1950's, weed control methods have changed from reliance primarily on tillage to a rapidly increasing dependence on herbicides.

(a) Seasonal effects

The seasonal changes in the percentage probability of receiving effective rain (Table 3) indicate that weed growth before the wet season in southern Western Australia and in northern Australia, can be expected for only a limited period before sowing because of the sharp break from the dry to the wet season. In the remainder of southern Australia, the break in the season is less pronounced, especially in the higher rainfall areas and in southern New South Wales so that there is an increased probability of weed and pasture growth occurring during a more extended period before seeding. In eastern Australia there is a relatively high probability of weeds growing at any time of the year.

(b) Shallow cultivation

The dormancy of seeds of barley grass, silver grass, annual rye grass and brome grass is primarily a time-dependent mechanism so that in southern Australia the dormancy of these grasses is greatly reduced by May-June, when winter crops are sown (McGowan 1970; Quinlivan 1972). A shallow

cultivation to about 3 cm either before or just after the season breaks will encourage, during seedbed preparation, a more complete germination of annual rye grass (Pearce and Quinlivan, 1971) and presumably of the other grasses or weeds with a similar dormancy control. Pearce and Quinlivan consider that where cultivation of the land can be delayed for at least 3-4 weeks after the break of the season, there is no need for the preliminary shallow cultivation.

Seeds of some weeds such as wild oats and silver grass fail to germinate or emerge when buried by ploughing below the normal depth of cultivation (5-8 cm) and seeding of grain crops (Quail and Carter 1968; McGowan 1970; Paterson 1976). Weed seeds with a dormancy period of many years can still be a problem when the land is re-ploughed and viable seeds are returned to near the soil surface (Rawson 1968).

(d) Rotations and cultivation

The dormancy of wild oats (Avena ludoviciana) in northern New South Wales and Queensland is controlled primarily by temperature, so that the weed germinates during the winter when wheat and other winter crops are being grown. Changing from winter to summer cropping (e.g. sorghum), together with the associated cultivation during winter, can reduce substantially the infestation of wild oats in subsequent winter crops (Mears 1968; Rawson 1968).

The control of soursobs (Oxalis pes-caprae) in winter crops of South Australia and Victoria is hindered by the ability of the weed to regenerate from small pieces left after cultivation as well as from bulbs and bulbils (Mahoney 1978). While the application of diuron to wheat and barley can give substantial reductions in soursob numbers (Catt and Baldwin 1972; Mahoney 1976), the results can be variable and phytotoxic effects on the crop can occur, especially on lighter textured soils. Delaying the sowing of crops from May-June to July allows more cultivation and results in effective control of the weed (Fawcett, unpublished data) although the potential yield of the crop can be reduced. Mahoney (1978) reports that application of glyphosate (l.5 1/ha; a translocated herbicide) in August on pastures infested with soursobs, followed by cultivation for subsequent weed control during a long fallow (8-10 months) can reduce soursob numbers from 180 to 10 plants/m2.

(e) Rotations and pasture

Practices that substantially reduce the seed set of weeds such as barley grass, silver grass, annual rye grass and brome grass (seeds of which usually remain viable for less that 12 months; McGowan 1970; Quinlivan 1972) in the pasture phase of a rotation, can assist in controlling the infestation by the weeds in a subsequent grain crop. Examples include spray-grazing (Pearce 1972), and hard grazing, slashing, applying a desiccant herbicide or making hay when the grasses are at the flowering or early grain development stage (Tuohey 1969; Quinlivan 1972).

The reserve of wild oat seed (Avena fatua) can also be reduced substantially (l 500 to less than 15 plants/m2) by changing from multiple cereal cropping to at least 3 years of pasture together with sufficient grazing to stop seed set by the wild oats (Paterson 1976).

However reverting land to pasture and grazing does not give effective control of weeds with seeds having a long dormancy period (Rawson 1968) nor weeds that are not eaten by stock.

(f) Nutrition

Watkins (1970) reports that the application of nitrogen (50 kg ha N as urea) in January together with cultivation for seedbed preparation and weed control can increase the total number of wild oat seeds (Avena ludoviciana) that germinate, during May-July from 200-300 to 300-400 plants/m2.

(g) Soil moisture

Weed control by cultivation is most effective when followed by a warm dry spell so that the surface soil dries out and the evaporative demand is high enough to prevent survival of the weeds. When the soil moisture is relatively high and the evaporative demand low, weed control by cultivators (more aggressive soil disturbance) is better than that by trash farming equipment (e.g. blade plough, trash worker, rod weeder, Marston and Doyle 1978; Fawcett, unpublished data). In some seasons, soil conditions are too wet for cultivation to kill weeds but satisfactory weed control can be obtained by applying a dessicant herbicide (Pearce 1979).

(h) Herbicides

The wide range of herbicides both now available and continually coming onto the market, requires the farmer to consider many aspects before deciding which is the most appropriate herbicide to apply. These aspects include selectivity of the herbicide regarding effect on species or varieties of weeds and crops, the soil conditions and equipment needed for incorporated herbicides, the effect of surface trash and soil type on the action of the herbicide, residual effects on future crops, safety of the operator during application of the herbicide, and the cost/benefit ratio regarding increased crop yield, number of cultivations and the risk of erosion.

Soil structure and erosion

(a) Cropping

The results of a survey by Stoneman (1962) in Western Australia of cropped and comparable virgin soils illustrate the deterioration in soil structure associated with crop-natural pasture rotations for periods of up to 44 years (Table 4.). Such deterioration in soil structure can result in reduced germination and establishment of crops, especially in wet season (McIntyre 1955); Rowell et al. 1977). The hard setting nature of many poorly structured soils makes the timing of tillage and sowing operations more difficult (Ferns 1973).

TABLE 4: Degradation of soil physical properties associated with crop-natural pasture rotations in Western Australia. The values are averages calculated from data presented by Stoneman (1962) for 6 soil types.

Soil property	Virgin soil	Cropped soil
Organic carbon (%)	1.46	1.00
Aggregation (% <l mm)	39.9	18.7
Non-capillary pore space (cc/cc)	17.7	11.5
Bulk density (g/cc)	l.4	1.6

Tyned seeders, fitted with either narrow points (approx. 3 cm wide) or the normal 8-12 cm points have been found to be more suitable than triple disc drills for direct sowing into a wide range of soils, including soils where penetration can be a problem (Rowell et. al. 1977; Pearce 1979;

Fawcett 1979 and unpub. data).

(b) Pastures and legume crops

Studies in southern Australia have shown that 3-4 years under improved pasture are needed before there is any substantial improvement in soil structure, (Greacen 1958; Stoneman 1973), and Greacen has pointed out that long term improvement in aggregation can require much longer periods under pasture. The inclusion of grain legumes in a rotation may also improve soil structure (Boundy 1978).

In eastern and northern Australia, and southern New South Wales, the risk of water erosion on cultivated land is greatest during summer and autumn (October-April) when high intensity rain storms can occur (Anon. 1961; Adamson 1978; Marston and Doyle 1978). In southern Australia, there is a risk of wind and water erosion on cultivated sandy soils, not only during the fallow period prior to seeding but also after seeding until the crop grows sufficiently to give adequate soil protection.

Direct drilling and reduced tillage methods of cropping can help stabilise soils against wind and water erosion (Pearce 1972; Marston 1978; McNeill and Aveyard 1978), reduce surface crusting and increase soil aggregate stability and infiltration (Ellington and Reeves 1978; McNeill and Aveyard 1978).

Trash retention systems as well as strip cropping are being adopted by farmers in eastern Australia as aids to controlling water erosion and helping to stabilise crop production (Kamel 1975; Marston and Doyle 1978; Bierenbroodspot 1969). There is also an increasing interest in trash farming as an aid in the control of both wind and water erosion in southern Australia (Speedie and McSwain 1977; Fawcett and Schultz, unpublished data)

The amount of crop stubble or pasture residues available for trash farming in these areas can be inadequate for effective erosion control, especially after a poor season or on paddocks where the pastures have been heavily grazed.

Direct drilling or reduced tillage systems have a clear advantage over systems based on tillage for weed control, with regard to trash retention, and conservation tillage equipment, such as blade ploughs and trash cultivators, is needed if the land is to be cultivated and the rate of trash loss is to be reduced (Kamel 1975; Marston and Doyle 1978). In this regard, the trash remaining after grain legume crops can be less than after cereal crops so that it could be more difficult to retain an adequate surface cover after these crops, for the control of erosion.

Crops can be sown into trash by using seeders such as the triple disc drill and tyned trash seeders (row spacings of up to 25-30 cm) (Grevis-James and Kamel 1977; Marston and Garland 1978).

Trash can be incorporated into the soil by cultivating with disc implements, rotary hoes, or tyned cultivators with sufficient trash clearance. Excess surface trash can be reduced or removed by burning, grazing with stock, slashing or otherwise breaking up the stubble (McGetrick 1968; Marston and Doyle 1978; McNeill and Aveyard 1978; Fawcett and Schultz, unpublished data.

Nitrogen

The amount of nitrogen available for crop growth is dependent on soil and land management factors but the response by a crop to available nitrogen will depend in large measure on environmental factors including fallow moisture reserves (Sims 1964; Leslie and Hart 1967; Russell 1968a; French 1978b).

Cultivation and trash retention

Fallowing by cultivation has been a traditional method of increasing the level of mineral nitrogen in the soil prior to seeding (Hore and Sims 1955; McGetrick 1968; Russe 1968a; French 1978b).

There is some evidence that the amount of available nitrogen is similar for both direct drilling and cultivated seed beds (Kohn et al. 1966; Reeves and Ellington 1974; Rowell et al. 1977) but lower levels for direct drilling have been reported by Greenwood et al. (1970). The rate of release of mineral nitrogen and the overall decline in total soil nitrogen levels can be lower with direct drill systems compared with systems using a cultivated seed bed (Pearce 1977, McNeill and Aveyard 1978).

Nitrogen availability at seeding time should not be greatly affected by trash retention providing the amount of trash has been reduced to relatively small amounts by tillage and decomposition, but reduced availability can occur if relatively large amounts of trash with high C/N ratios are incorporated close to seeding time (Phillips and Norman 1961; Fawcett 1967; Marston and Doyle 1978; Fawcett unpublished data). These adverse effects could be more pronounced on soils of relatively low natural fertility unless adequate fertilizer nitrogen is applied (Marston and Doyle 1978).

Rotations

Continued intensive cropping rotations with wheat or other cereals, can markedly reduce the reserves of total soil nitrogen (Hallsworth et al. 1954; Sims 1964; Harty et al. 1966), and increase the potential use of fertilizer nitrogen (Leslie and Hart 1967; McGetrick 1968; Russell 1968b; Halse 1969).

On the other hand, widening the cropping rotation to include one or more years of legume based pastures, (medics or sub clover in southern Australia and lucerne in eastern Australia) can increase the availability and total levels of soil nitrogen (Hallsworth et al. 1954; Littler 1964; Storrier 1965; Russell 1968b; Malcolm 1969; Doyle and Holford 1978).

Maintenance of the legume component in a pasture in southern Australia can be a problem with more intensive cropping rotations and in drier seasons (Wells 1967; Quinlivan 1972). Management systems that can help to maintain the legume component in a close crop-pasture rotation include spray-grazing (Pearce 1972), re-seeding of pasture legumes (Quinlivan 1974), increasing stocking rates and the application of phosphates (Bowden et al. 1978).

The inclusion of grain legume crops such as lupins, peas or tic beans in a cropping rotation can maintain or increase the yields of a following cereal crop (Boundy 1978; Schultz and King personal communication) though the relative importance of additional soil nitrogen, control of cereal root diseases, or other factors associated with the grain legumes, needs clarification.

Pests and diseases

Land management practices can influence the incidence of some pests and diseases in crops.

The replacement of long and short fallows (8-20 and 3-4 months respectively) with direct drilling or reduced tillage techniques can increase the risk of damage to crops by cereal curculio (Desiantha caudata; Grierson and Allen 1977) and other insects such as red legged earthmite and webworm (Catt, personal communication).

Practices that can help in the control of cereal cyst nematode in southern Australia include fallowing for 8-10 months, inclusion of a legume dominant pasture, grain legumes, or oil seed crops in a cropping rotation, the use of selected cultivars of wheat or rye known to reduce the survival of the nematodes, and the use of nematocides (Meagher and Rooney 1966; Brown et al. 1969; McLeod 1973; Dube, personal communication).

Dube (personal communication) points out that the use of grain legumes and oil seed crops is limited, in South Australia at least, to areas with an annual rainfall of more than 400 mm. In drier areas, medic pastures are the only alternative non host. Any increase in the availability of soil nitrogen due to these management practices, or the use of fertilizer nitrogen, can also help to increase the yields of cereal crops in the presence of cereal cyst nematode (Brown et al. 1969).

Long fallowing and rotations can also reduce the incidence of take-all (Gaeumannomyces graminis) although the results are not consistent (Hore and Sims 1955; Taylor 1966; Chambers 1971; Rovira and Ridge, in press).

Fuel, labour and equipment

There is a general trend within the grain producing areas of Australia towards increasing size of farms together with larger tillage, sowing and harvesting equipment, and large tractors. This is one of the consequences of farmers adjusting their farm operations to meet the increasing cost/price squeeze (Billing 1970; Fisher and Ronan 1977). The large equipment can help increase the output per man and reduce costs but the amount of fuel used per hectare is not necessarily decreased (Ryan 1972).

Of the various operations needed for crop production, reductions in the number of pre-sowing cultivations offer the most scope for reducing fuel consumption, so that conventional, reduced tillage and direct drilling tillage systems are being assessed more closely with the aim of increasing the efficiency with which fuel, and other resources, are used for farm production in Australia (Wedd 1976; Ellington 1979; Johnson 1977; Pearce 1977).

Johnson (1977) reports that 182 million litres of fuel were used in 1974 for wheat production in New South Wales. Of this total, about 70 per cent (28-66 l/ha sown) were used for tillage prior to seeding, with 13-16 1/ha being used for primary tillage and 22-50 1/ha for secondary cultivations. Comparable data for the Wimmera in Victoria are much lower, being 7 and 16 1/ha respectively (Ryan 1972). Preliminary estimates for South Australia indicate that values are of the same order as those for Victoria (Fawcett and McCord, unpub. data). The types of soil and tillage equipment, number of cultivations and size of tractor are some of the factors influencing fuel consumption.

The relative use that farmers will make of herbicides and tillage can be expected to vary considerably depending on factors such as the weed species, suitability of available herbicides for weed control, the affect of the herbicides on the current and future crops, the soil type and expected cost/ benefit ratio.

Future implications

The present trend towards closer cropping rotations in southern Australia and the continuation of such rotations in eastern Australia are placing increasing pressures on the continued stability and productivity of our arable soils, as in the 1930's and 1940's.

The ley farming systems developed during the 1950's in southern Australia were adapted to relatively wide cropping rotations but modifications are needed to ensure a more reliable growth of medics or clovers in the pasture year(s) of more intensive cropping rotations, with less dependence on an early break to the season. While grain legumes may be an alternative to legume pastures as a source of nitrogen, for the control of soil borne diseases and for the maintenance of soil structure and fertility, substantial increases in the area of grain legumes would be needed to make much overall impact on cereal grain production. There will be an increasing role for the use of fertilizer nitrogen if the supply of available nitrogen from legume pastures or crops is not sufficient for crop production.

Experience to date in Australia indicates that direct drilling (no tillage prior to seeding) is not a practical alternative to conventional methods of seedbed preparation on much of our cropping land because of limitations to crop production associated with various soil, weed control, crop growth and cost factors. There is however considerable scope for the continued development of reduced tillage systems, where the use of tillage, herbicides, trash retention, stock and other management practices is integrated to best advantage for weed control, crop production and the conservation of our fuel and land resources.

It can be expected that these systems will vary considerably, both in time and space, as farmers adapt the systems to best meet their individual requirements. It is important that any toxic effects of herbicides (and other chemicals) on the current and future crops, the operator, and the overall environment be considered as well as their beneficial effects on crop production.

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