5. CONSERVATION AGRICULTURE

Improving soil conditions

Concerns about soil erosion affecting soil productivity in rainfed areas have resulted in an emphasis on trying to stop negative effects on crop yields attributed to erosion and runoff. This has been attempted by putting cross-slope barriers in fields designed to catch or divert soil and water moving downslope. This approach has not been particularly successful either in halting the problems or in raising yields, resulting in disillusionment among farmers. Money has been spent to little effect and damage to land has not been stopped.

However, if the emphasis is shifted towards the soil as a habitat for roots and if soil loss and runoff are recognized as consequences of prior damage to soil porosity, a different perception emerges. This is based on more positive thinking which considers first the soil conditions that allow plant roots to function optimally, and then the improvements necessary to bring any current inadequate state of the root habitat to that desired condition. Land uses would ideally be cross-matched with variations in land suitability with respect to erosion hazard - i.e. the most protective forms of land use would be allocated to places with the highest hazards of erosion. However, especially for farmers with few resources and small farms, low yields of subsistence crops may dictate that they be planted on all land units irrespective of erosion hazard. In both situations however, improving soil conditions to meet the needs of plant roots will often greatly reduce problems of soil loss and runoff.

Key goals of improving and maintaining excellent soil conditions for and with roots include:

• increasing the reliability of plant production in the face of unpredictable variations in the weather and other hazards of the environment;

• reducing production costs and raising net returns to producers;

• increasing the quality of the land and its resilience to extreme weather conditions.

Residue-based zero tillage in Brazil and Paraguay

On increasingly large areas of Latin America there has been a revolution in agricultural practice over the past 30 years. The adoption of zero tillage methods of crop production by large numbers of farmers provides convincing validation of the value of such conservation-effective forms of agriculture, in agronomic, environmental, economic and social terms. This is being achieved on farms whose sizes range from less than twenty hectares to thousands of hectares and in a wide range of ecological zones.

PLATE 63. A forest litter of leaves and twigs - even of Eucalyptus, as here - which affords a protective cover to the surface, food for soil organisms and ultimately is a source of soil organic matter within the profile (with plant roots themselves)

[T.F. Shaxson]

PLATE 64. Disk tillage not only buries much of the crop residues but also can pulverize the soil and induce serious compaction immediately beneath the tilled layer - Cerrado, Brazil

[T.F. Shaxson]

PLATE 65. Both surface soil and seeds have been moved by runoff from an earlier storm and deposited in the channel of a broad-based conservation bank

[T.F. Shaxson]

PLATE 66. Broad-based conservation banks are supposed to control runoff and soil erosion - Tabatinga, Brazil

[T.F. Shaxson]

Conservation agriculture (CA), as defined during the First World Congress on Conservation Agriculture (1-5 October 2001)^[6] promotes the infiltration of rainwater where it falls and its retention in the soil, as well as a more efficient use of soil water and nutrients leading to higher, more sustainable productivity. It also contributes positively to environmental conservation. In many environments conservation agriculture can be considered the ultimate soil and crop management system. Conservation agriculture has been successfully implemented in both small-scale (Sorrenson et al., 2001) and large-scale (FAO, 2000e) farming, where it has given economic benefits as well as improved water resources.

History

Zero tillage has been successfully practised in the United States for several decades, with regular annual growth in the total area. In Latin America there has been an impressive rate of adoption and accelerating growth over the past two decades.

Brazil and Paraguay suffer erosive rainstorms of very high intensities during the southern summer, which result in severe damage year after year. On almost all cultivated land, soil tillage for crop production, often with heavy disk ploughs followed by disk harrowing, resulted in many problems. These included:

• loss of the porous organic covering of the forest floor where land had been cleared (Plate 63);

• pulverization of surface soil together with compaction of the subtillage layer (Plate 64);

• loss of organic matter from the upper soil layers by rapid oxidation from the exposed surface;

• loss of potential soil moisture as runoff;

• reduction in soil depth by erosion of topsoil, resulting in losses of seeds and fertilizers, and causing additional replanting costs (Plate 65);

• declining flow and drying-up of streams and rivers during dry seasons.

Downstream there were problems with eroded sediments clogging urban water purification plants, sedimentation in stream valleys and reservoirs, damage to bridges and roads. A common response was to construct conservation banks on the contour, such as broad- and narrow-based bunds to control runoff and soil erosion (Plate 66). However, they did not stop erosion occurring on uncovered soil. Infiltration of runoff was impeded by the severe compaction along the channel where it collects. The channel beds are probably the most compact lines in the entire field.

As time passed and the runoff and erosion problems continued, larger and larger banks were built, but without conspicuous success in halting the problem. Declining productivity and profitability on family farms resulted in collapsing net farm incomes, falling land prices and families leaving their farms for some other livelihoods.

In 1972 there were 500 hectares under residue-based zero tillage on one farm in southern Brazil. The technique spread slowly at first, because of scepticism and insufficient knowledge. There was a lack of appropriate equipment, suitable cover crops and weed control techniques. As the economic and technical advantages of residue-based zero tillage became apparent the rate of spread accelerated, largely as a result of farmer-to-farmer contacts. By 2001 in Brazil, there were more than 13 million hectares managed in this way (Figure 23).

FIGURE 23. The growth of residue-based zero tillage in Brazil 1972 - 1999 (after Landers, 1998 and FEBRAPDP, 2002)

In the State of Santa Catarina^[7], Brazil, residue-based zero tillage has been adopted on 400 000 ha by 1998-1999 within a programme to promote these systems. As a result, some or all of the improved practices were spontaneously adopted on a further 480 000 ha outside the formal remit of the project, from a base of 120 000 ha in 1993-1994 (World Bank, 2000). The State's small farmers have been ingenious in devising their own equipment and methodologies to fit zero tillage to their individual circumstances, together with governmental and non-governmental arrangements for their technical and institutional support (FAO, 2000b).

In Paraguay, zero tillage was first used in the late 1970s but was not widely adopted on mechanized medium and large farms until 1990. It had expanded to 20 000 ha by 1993, to 250 000 ha by 1995 - 1996 (FAO, 1997) and to 480 000 ha by 1997, which represents 51 percent of the total cultivated area of Paraguay (Sorrenson et al., 1998).

Implementing conservation agriculture

Planting a crop in the residues of the previous crop, which is the essence of conservation agriculture, is fast becoming a successful and sustainable cropping practice, especially in the subhumid tropics. Implicit in this practice is the absence or limitation of tillage practices that incorporate surface residues or disrupts soil porosity.

The quantity of crop residues produced is clearly very important and varies greatly with crop type, variety and yield. Invariably there are residues of weeds associated with crop residues that also contribute to soil cover, especially during the initiation of no-till. Large quantities of crop residues are usually obtained from sorghum, maize, rice, cotton and sunflower, whereas soybean, wheat and beans generally produce small quantities (Barber, 1994). Traditional varieties often yield greater quantities of residues than improved varieties, especially those of short stature and high harvest index. Most information on the optimum quantity of crop residues to be left on the soil surface is based on the amounts needed to reduce soil losses to acceptable levels on different slope gradients, rather than the amounts needed to maximize rainwater infiltration. Data exist showing that cover is less effective in reducing runoff than soil losses (Barber and Thomas, 1981; Lal, 1976), but there is little information on the influence of cover on infiltration and runoff, especially on 20 to 50 percent slopes, which are commonly cultivated by small-scale farmers. Usually, a minimum value of 70 percent surface cover - equivalent to 4-6 t/ha of maize straw for example - should be adopted.

The quantity of residues remaining during the cropping season is also influenced by the rate of residue decomposition. Nitrogen-rich legume residues, such as those from beans and soybean, decompose much more rapidly than nitrogen-poor cereal straw and other residues with high C/N ratios. On the other hand, legumes used as a cover crop can provide a weed-smothering cover, protection from raindrop impact, and important additions to organic matter (Plate 67). Harvesting procedures can drastically affect the quantity of residues remaining in the field.

PLATE 67. Oilpalm underswon with a creeping legume - Anki Mabela, Fiji

[Natural Resources Institute]

The widely acclaimed success of conservation agriculture is mainly attributed to improved surface porosity (Plate 68) that results in increased infiltration and reduced runoff, and a greater water availability to crops. As additional benefits, conservation agriculture also lessens evaporation losses, reduces erosion, enhances earthworm activity and soil structure, improves soil fertility and lowers labour, machinery and fuel costs. With time, yields increase substantially provided crop rotations are well designed and include leguminous crops or cover crops. When compared with only applying a soil cover (mulches, crop or cover crop residues) in a conventional system, no additional time is required for land preparation in CA (apart from herbicide application in some cases), which allows earlier sowing and all the advantages that this confers. Consequently returns to labour are substantially increased.

PLATE 68. Soil conditions in a no-till system - Paraguay

[T.F. Shaxson]

There is evidence that the yield of a crop is significantly higher when sown directly into the residues of a previous crop than when it is sown in a previously tilled soil to which the same quantity of crop residues are applied as a mulch. This is attributed to the benefits of little soil disturbance: the soil structure created by the root channels from the previous crops as well as by the biological activity of earthworms and other soil fauna facilitate deeper rooting and enhance the infiltration and percolation of rainwater.

Conservation agriculture principles are implemented optimizing the soil as a dynamic habitat for roots as follows:

• Residues of crops and of cover crops are distributed evenly and left on the soil surface.

• Once the soil has been initially brought into good porous condition, no implements are used to turn over the soil, to cultivate it or to incorporate crop residues.

• Weeds and cover crops are controlled by slashing with a knife roller or by preplanting application of a non-polluting desiccant herbicide.

• A specialized planter or drill cuts through the desiccated cover, slotting seed (and fertilizer) into the soil with minimum disturbance.

• Crop rotation is fundamental to zero tillage. It promotes adequate biomass levels for permanent residue cover and assists in control of weeds, pests and diseases. Rotations also ameliorate soil physical conditions, recycle nutrients and can fix atmospheric nitrogen. In semiarid conditions, appropriate crop rotations involving deep-rooting crops can also make still better use of residual soil moisture.

• As a result, soil erosion is reduced by about 90 percent and soil biological diversity maximized (adapted from FAO, 2000e)

In such systems soil damage is reduced and recuperation of soil architecture is much more quickly achieved than by unimproved fallow systems. Appropriate crop rotations are as important as the soil cover and no-tillage practices (Plate 69). Grasses, in particular, increase the aggregation and stability of soil particles which provide a range of small voids resulting in increased porosity (Plate 70).

PLATE 69. A dense growth of nitrogen-fixing vetch within a zero tillage rotation

[T.F. Shaxson]

PLATE 70. Development of porous soil architecture beneath a grass crop in rotation

[T.F. Shaxson]

PLATE 71. Scarifying the soil with tines to a depth of about 30 cm to break-up a subsurface compacted layer and let in more of the rainwater - Apucaraná, Brazil

[T.F. Shaxson]

PLATE 72. Interplanting maize in furrows drawn through a young cover of a low-growing vetch; in the foreground is soil which has been scarified - the earlier alternative. Caxambú, Brazil

[T.F. Shaxson]

Residue-based zero tillage is implemented gradually on structurally damaged soils. At the start, tillage with tined equipment (scarification) can be used to break up the underlying pan and let more rainwater back into the soil, while leaving some of the plant remains on the surface (Plate 71). In this way the soil is opened up and the previous crop's residues are incorporated. It may necessary to start renovating the soil by enabling more rainfall to become soil moisture, but too frequent scarification can also damage soil architecture because of the shattering effect on soil structural units.

Following the break-up of the underlying pan, strip cropping with a legume between rows of the main crop (e.g. maize) could be carried out (Plate 72). Finally a complete cover of crop residues without further soil disturbance by tillage could be established (Plate 73). The residues change overtime from being a protective cover to becoming an integral component of the soil (Plate 74). In the process, the worms and other soil mesofauna burrow within the soil seeking food and thereby provide channels and biopores through which air and water can move easily.

Effects of conservation agriculture

Effects on crop yields

Farmers' own experiences confirm what was anticipated by results of two six-year experiments with wheat and soybean between 1978 and 1984, comparing effects of conventional tillage, minimum tillage/scarification and zero tillage (Table 14).

TABLE 14
Yields of wheat and soybean, averaged across rotations, under three different soil preparation methods in Londrina, Brazil (Derpsch et al., 1991)

Effects on soil moisture

It might be expected that zero tillage would be no better than scarification (opening large spaces in the soil and leaving a rough surface) in increasing moisture in the soil, but this is not the case, as shown in Figure 24. This shows changes in levels of soil moisture under wheat, at three depths, under conventional soil preparation, scarification (minimum tillage) and zero tillage, during the crop's vegetative stage in the 1981 growing season. Plant-available moisture was greater and water stress, due to drought, shorter under zero tillage than under the other methods.

PLATE 73. Zero-till maize planted in a narrow slot cut through the residues of the previous crop of wheat by a pair of sharp disks - Mauá, Brazil

[T.F. Shaxson]

PLATE 74. In the same field, note the dark decomposing wheat residue materials (left-hand side of photo) beneath the light-coloured surface straw - Mauá, Brazil

[T.F. Shaxson]

FIGURE 24. Soil moistre available to plants at different depths during the vegetative phase of wheat growth, under three methods of soil preparation (Derpsch et al., 1991)

Plate 75 shows the differences in the soil physical conditions from a residue-based zero tillage system and conventional tillage system on the same soil type. Other experimental work showed that where the cover of residues was similar, the percentage of rainfall which infiltrated into scarified and zero-tilled soil differed by only 2-3 percent (Derpsch et al., 1991). Nevertheless, Figure 24 shows a disproportionate benefit to zero tillage in terms of amount of soil moisture and duration of its availability to the plants. This reflects differences in pore space distribution within the soil architecture between scarification and zero tillage.

PLATE 75. This farmer has studied the comparative effects of zero tillage - on the left - vs. conventional tillage - on the right - on the same soil type since 1978 (Ponta Grossa, Brazil)

[T.F. Shaxson]

The fact that differences in the three-dimensional arrangement of the root habitat contribute to differences in root growth and function, even though soil moisture conditions may be almost the same, has profound implications. The best conditions for root growth and function appear to be where there has been no disturbance by tillage implements and where soil organisms are doing the work of burrowing, transforming and aggregating soil constituents. It may also be that differences in soil moisture inferred from runoff measurements under different tillage treatments may be insufficient to explain differences in root measurements and in final yield.

A pioneer farmer in Paraná, Brazil, whose soil conditions have been monitored from 1978 to the present, has kept detailed yield records. These show that under residue-based zero tillage, yields of both maize and soybean have been rising and have become less variable from year to year (Figure 25). Annex 8 provides information on similar experiences of a large-scale farmer in Chile.

FIGURE 25. Frank' Anna farm's production graphs 1978-2000 in Paraná, Brazil (after Dijkstra, 2000)

Effects on some other soil health indicators

The impacts of zero tillage (ZT) and conventional tillage^[8] (CT) on soil health are shown by comparing some soil indicators for both systems:

• diameter and stability of soil aggregates (Table 15)

• soil organic matter content at 20 cm depth (Table 16)

• number of earthworms (Table 17)

TABLE 15
Changes in mean diameter and stability of soil aggregates after 7 years of rotation under residue-based zero tillage (ZT) and conventional tillage (CT) in Paraná, Brazil (FAO, 2001c)

TABLE 16
Buildup of soil organic matter under ZT compared with conventional cultivation (FAO, 2001c)

* Discounting the crop residue mulch layer above

TABLE 17
Influence of different methods of soil preparation on population of earthworms in Paraná, Brazil (FAO, 2001c)

Saturnino and Landers (1997) measured the number of maize roots in each 10 cm layer of soil to 1 m depth after 15 years of constant treatment (zero tillage and conventional tillage). The results in Table 18 show marked differences. Zero tillage and crop rotation favour recycling of nutrients and better soil structure, resulting in better root development and higher production.

TABLE 18
Number of maize roots to depth of 1 m after 15 years of zero tillage (ZT) and conventional tillage (CT) in Paraná, Brazil (after Saturnino and Landers, 1997)

A research report from 1983 showed similar differences in root distribution of soybeans. While the total number of roots was the same to 1 m depth, they were more evenly distributed down the profile with zero tillage than with conventional tillage (Derpsch et al., 1991).

Effects on erosion and runoff

Conservation agriculture compared with conventional tillage results in markedly reduced soil erosion and runoff, as shown in results from Brazil and Paraguay. This effect is attributed to the increased soil porosity beneath residues due to biological activity. Note the saving of 441 mm water by reduction of runoff in southern Brazil and 186 mm in central Brazil (Table 19).

TABLE 19
Losses of soil and water under conventional tillage (CT) and residue-based zero tillage (ZT) (Saturnino and Landers, 1997)

In the Municipio of Tupanssi in Paraná it was reported that, following adoption of residue-based zero tillage, the turbidity of the river water has fallen from an index of 8 000 to 80 (author's field notes). A group of farm families, whose houses were on the slopes of cultivated fields recently transformed by zero tillage, said they were pleased that the runoff water and sediment no longer rushed down the hillsides into their houses, damaging the rugs and carpets on the floors (author's field notes).

If runoff and erosion are symptoms of soil misuse, the major reduction in both occurrences signifies that their causes must have been significantly reduced.

Effects on catchment hydrology

An example of positive changes in catchment hydrology is provided by a representative catchment near Toledo in Paraná, Brazil. Soon after the adoption of zero tillage on rolling wheat lands, farm families observed that a pond which formerly had been dry for much of the year filled with water and hydrophytic vegetation took hold again (Plate 76). Further down the catchment, the river, which had ceased to flow in the dry season began to flow again throughout the year, so that a small farmer on its banks was able to improve his livelihood by investing in irrigation equipment and in excavating fishponds. This farmer now keeps fishponds full of water through the year and charges people to come fishing for fun at the weekends (Plate 77).

PLATE 76. Improved management of the soil upslope resulted in this pond reappearing and persisting through the dry season (Toledo, Brazil)

[T.F. Shaxson]

PLATE 77. Further down the same catchment, the adoption of zero tillage crop production above showed its effect in much-extended river flow, with considerable income improvements for this small farmer (Toledo, Brazil)

[T.F. Shaxson]

Effects of zero tillage systems on farm economics

Farmers have responded to the economic benefits of zero tillage. Yield increases of 20 percent or more, coupled with reduction of production costs by a similar percentage, have had positive effects on farm income. Savings of time and labour have contributed to improvements in farm families' livelihoods.

For instance in Paraguay, on farms using conventional tillage systems, severe losses of soil, nutrients and organic matter were seen as a root cause of declining yields of a range of crops. Some farms had adopted zero tillage, others not. Farm records over 10 years were used to construct economic models and indicators of differences. On representative mechanized 135 ha farms growing rotations including oats, soybean, sunflower, maize, wheat, crotalaria, vetch with zero tillage (ZT) farm incomes rose while those using conventional tillage (CT) for rotations with soybean, oats, wheat, maize fell. The returns on capital increased on farms using zero tillage, but declined on those using conventional tillage. Reduction of tractor-hours, reduced use of fuel and lower costs of repairs, etc. contributed to the economic benefits of zero tillage on these farms (Table 20).

TABLE 20
Comparative short- and long-term economic results on typical 135 ha farms with tractor power, from conventional tillage (CT) and residue-based zero tillage (ZT) in San Pedro and Itapua regions, Paraguay (FAO, 1997)

In another study in Paraguay, the economics of zero tillage on seven smaller farms (20 ha or less) without tractors were studied. Five out of the seven farmers had both conventional and zero tillage areas on their properties (Table 21).

TABLE 21
Summary of farming system results on small farms with cotton, soybeans, tobacco, maize (Sorrenson et al., 1998)

The small-farm study illustrates that zero tillage is not only financially attractive to small farmers but also has high economic pay-off for the nation. In Paraguay it has been estimated that for 1997 the national economic benefit due to the adoption of zero-tillage systems reached US$941 million. These included the saving in nutrients lost from soil from erosion, plus the costs saved in reduced tractor hours, less fuel and fertilizer.

The 1980 agricultural census of Paraná State, Brazil showed that there were over 6 million ha of annual crops. A 1989 report indicated the annual benefits if residue-based zero tillage systems were to be applied to the full 6 million ha (Box 8).

Observations about residue-based zero tillage systems in Latin America

From the full application of both the concepts and integrated techniques of residue-based zero tillage (also called Conservation Agriculture), farmers have achieved many direct and indirect benefits, often recorded together on individual farms (Instituto CEPA/SC, 1999; FAO, 2001b).

On-farm benefits included:

• marked and rapid increase of organic matter content in upper layers of soil and increased biodiversity, number and activity (of earthworms, fungi, bacteria, etc.) in the soil;

• better soil structure and stability of soil aggregates; significantly higher infiltration rates; soil loss reduced by over 80 percent, runoff by 50 percent or more; more intensive but safe use of sloping areas made possible;

• increase in nutrients stored, greater availability of P, K, Ca, Mg in the root zone; less fertilizer needed for same result;

• better germination and development of plants, better root development and to much greater depth; better resilience of crops in rainless periods due to increased water holding capacity;

• yields often higher, typically + 20 percent for maize, + 37 percent for beans, + 27 percent for soybean, + 26 percent for onions; with less year-to-year yield variation;

• reduced variations of soil temperature during the day, with positive effects on plants' absorption of water and nutrients;

• less investment and reduced use of machinery and animals in crop production; reduced costs for labour, fuel and machinery-hours perceptible within 2 years. Operational net margins per ha rose by between + 58 percent and + 164 percent, because of combination of lower cost of production and increase in yields, which provides greater resilience against falling market prices and bad weather;

• greater flexibility in farm operations especially over optimum dates for planting; increasing possibilities for diversification into livestock, high-value and different crops, vertical integration into product processing and other activities; improved quality of life.

Off-farm benefits widely noted by rural agency staff and others, included:

• flooding risks reduced by 30-60 percent due to greater rainfall infiltration and delays to overland flows. Extending the time of concentration; better recharge of underground aquifers, improving groundwater reserves and dry season flow in springs and streams;

• less herbicide use after first years; less pesticide use, more recycling of animal wastes; reduction of pollution and eutrophication of surface waters by agricultural chemicals carried in surface runoff and eroded soil; less sedimentation and infrastructure damage, e.g. silting of waterways, large dams. A conservative estimate for the Cerrado region was given as US$33 million per year;

• reduced water treatment costs (ca. 50 percent) due to less sediment, less bacterial and chemical contamination;

• savings of up to 50 percent in costs of maintenance and erosion avoidance on rural roads;

• reductions in fuel consumption of 50-70 percent or more and proportional reduction in greenhouse gas emissions;

• reduced pressure on the agricultural frontier and reduced deforestation by high-yielding, sustainable conservation agriculture and increased pasture carrying capacity through rotation with annual crops;

• enhanced diversity and activity of soil biota;

• reduced carbon emissions through less fuel use and enhanced carbon sequestration by not destroying crop residues and increasing, rather than losing, soil organic matter (FAO, 2001a).

The zero tillage systems of Latin America thus are not only a great improvement on former tillage-based systems, but also have major off-site and national benefits, to which improvements in soil moisture management make a large contribution. The effects are illustrated by the colour of the water going over the Iguassu Falls in southern Brazil (Plates 78 and 79). By chance these two Plates were taken from the same viewpoint 7 years apart, one in the wet season when high runoff also transported much eroded soil, the other in the dry season when water that had seeped down through the soil to the groundwater provided the dry-season flow.

PLATES 78 AND 79. River flow in two seasons before and after the improvements in the catchments wrought by widespread conservation agriculture in the form of residue-based zero tillage (Foz do Iguassu, Brazil). People who recently visited the site during the rains say that the water even in the wet season is now as clear as it is in the dry season (Benites, pers. comm.)

PLATES 78

PLATES 79

[T.F. Shaxson]

Constraints of conservation agriculture and some approaches to overcome them

Conservation agriculture has been successfully employed in subhumid as well as humid climates, but there are still some constraints in semiarid environments that may hinder its immediate application. Typical of these constraints are:

• shortage of water limiting crop and residue production;

• insufficient residues produced by the economically or socially important crops and lack of knowledge of suitable cover crops;

• sale or preferential use of crop residues for fodder, fuel and building materials;

• inability to control livestock grazing, especially in areas where communal grazing is traditional (tenant farmers are often obliged to allow the landowner's cattle to graze the residues after harvest);

• inability to control residue consumption by termites;

• insufficient money or credit to purchase appropriate equipment and supplies;

• lack of knowledge of conservation agriculture by extension and research staff.

A number of approaches have been explored and are being tested to overcome these constraints. In situations where crop residues are preferentially used as fodder, additional new sources of fodder may be produced, provided they can be protected from grazing by, for example, live fences (León, 1994). Hay or silage may be produced as additional dry-season fodder from improved pasture species, or from forage trees or crops of high biomass grown specifically for this purpose (Barber, 1998). Forage trees can be established as live fences along farm and field boundaries, and forage grasses may be produced as live barriers, on bunds, and along field boundaries and roadways. In Bahir Dar, Ethiopia, farmers are increasing fodder production by undersowing forage legumes in other crops, establishing forage strips between arable crops, and by oversowing mixtures of legume seeds on grazing areas (Lemlem, 1998).

Certain crop sequences are less suited to direct sowing into crop residues because of the likelihood that weed, pest or disease problems will become intensified by being transmitted from one crop to the next. Examples of less suitable crop sequences and their specific problems encountered in eastern Bolivia (Barber, 1994) are:

• wheat every year - disease problems;
• soybean every year - pest and disease problems;
• soybean-sunflower sequences - disease problems;
• maize-sorghum or sorghum-black oats - weed and pest problems;
• sunflower-cotton - the problem of volunteer sunflower weeds;
• bean-soybean sequences - pest and disease problems.

Weed problems may also be caused by volunteer germination of the previous crop; for example, sunflower volunteers can be particularly difficult to eradicate. To avoid such problems, appropriate crop rotations, acceptable to the farmers, must be selected.

In environments where there are many constraints to the introduction of conservation agriculture, a pragmatic, phased approach may be the most feasible, in which individual constraints are progressively overcome until an appropriate system of conservation agriculture can be fully implemented. This may require the planned introduction of measures such as improved grass species and fodder trees, hay and silage production, live fences, stall-fed livestock, improved crop rotations with cover crops, formation of farmers' associations, credit supply and local or international training visits for farmers, extension and research staff (FAO, 2001b).

The introduction of conservation agriculture is unlikely to be immediately successful on seriously degraded soils with surface crusts, compacted layers, low fertility or severe weed infestations unless these problems are first overcome by appropriate remedial actions. Hardsetting soils may not be immediately suitable for conservation agriculture because of the difficulties of overcoming soil compaction problems and maintaining good soil porosity within the topsoil and subsoil. Consequently crop rooting is frequently restricted to shallow depths. In this case, deep tillage followed by the establishment of cover crops prior to introducing conservation agriculture, and then the adoption of crop rotations that produce large quantities of residues, will progressively improve the physical condition of these soils and make conservation agriculture possible.

Conservation agriculture is less likely to be successful in poorly drained soils because the added residues will intensify anaerobic conditions, in which toxic substances harmful to crop growth may be produced.

The cost of no-till planters and seed drills needed for direct sowing may be a major constraint for mechanized farmers, unless it is possible to modify their existing seed drills and planters. For small farmers, hand tools and animal-drawn equipment exist and local blacksmiths can often adapt them, provided they have access to information and samples.