Man has used and abused soil throughout the history of agriculture by producing food crops, most of the time without realizing he was destroying it by using inadequate tillage methods. The proliferation of machinery and the excessive tillage operations during land preparation have also played an important role in soil degradation. As a result, desert areas are increasing rapidly. At world level, erosion has already destroyed 430 million hectares, equivalent to 30% of the current farmed area (FAO, 1992). India is one of the countries under extreme situation, having more than 60% of the land heavily eroded. African and Latin American countries are approaching this figure. 

There is growing concern about the current rate of soil degradation and its negative effects on agriculture productivity and the environment all over the world. Tropical and subtropical soils present more problems than temperate ones. Nevertheless, much more research has been done to manage soils in temperate than in tropical regions. This is despite the fact that tropical soils are particularly fragile and heavily degraded due to excessive cultivation and heavy rains; in addition where farming is practiced in hilly areas by small holders. 

The tropical and sub-tropical regions of the world are characterized by low maize yields: from 1.0 t/ha in Central Africa to 1.9 t/ha in the Central America and the Caribbean Region. It must be kept in mind that the current world population of 5,800 millions will increase to 9,000 millions by the year 2030, and that most of this increase will occur in tropical and sub-tropical areas of the world. This problem is becoming worse considering that almost 40% of the world population lives in the tropical and sub-tropical regions, traditionally struck by malnutrition. Two out of five people are currently undernourished in some areas of these regions. Consequently, efforts to increase total production through yield improvement per unit area should be reinforced, since there is no more agricultural land for expansion. For instance, N. Borlaug estimated that grain yields will need to increase by 80% by the 2025. Until now yield increases have come largely from more use of fertilizes, herbicides and insecticides and genetic improvement of the species grown. The challenge now is for soil and water conservation technologies to be applied, being the no-tillage system the most practical and effective one. 

* Agronomist (PhD), Consultant. ** Agronomist (PhD), Consultant, Professor of University of Chile. *** Wayne Haag, Country Director of Mozambique, Sasakawa Global 2000.
 
 
  Agricultural Constraints for Increasing Crop Production.

The task of increasing crop production is difficult since there are several constraints: 

• There is no more land for crop expansion.
• Most of the land is in the hands of traditional, small-subsistence farmers, with little purchasing power to buy inputs, particularly fertilizers.
• Most crops are grown under rainfed conditions constantly struck either by severe droughts or monsoon type rains.
• In low rainfall areas, crop stubble is used for animal feed and little residues are left to protect the bare soil from water and soil losses.
• Weeds grow so fast that farmers cannot control them on time manually.
• Appropriate technology, necessary to economically improve yields, does not reach the farmers and/or does not exist.

Soil Conservation Situation in Tropical Africa and Latin America.

Tropical Africa and Latin America are having large land pressure in their efforts to produce enough food for the increasing population to satisfy their basic needs. Deforestation, intensive farming (excessive tillage) and over grassing is common in most of these countries. This situation deteriorates due to the destructive effect on heavy tropical rains on unprotected plowed steep soils in which a hundred tons per hectares of top soil can be washed away in a few hours. These are some of the reasons why current subsistence agriculture is not sustainable. 

Time consuming and high cost technologies have been used for reducing soil erosion and water utilization, among them: 

• Land leveling.
• Terracing.
• Contour planting.
• Cross tide ridges.

Still these technologies have not solved the problem. Some of them produce high soil losses initially, since large amount of soil are required to be moved to prepare certain structures. Fortunately the zero tillage or the no-till planting system was developed which, as proven, is the best way for saving soil, fossil energy, water, labor and time. Soil disturbance of any sort (ploughing, harrowing, ridging, and cultivating) must be discouraged by all means to avoid erosion, destruction of soil structure, burning of organic matter, moisture losses and reduction of microorganism and meso-fauna activities. The no-till system, developed in advanced countries, is based on ancient planting methods still practiced by the most traditional farmers. Nevertheless, the technology involves the use of expensive and sophisticated equipment. But in developing countries the system is much simpler that could be practice by small holder farmers, as it is described ahead. 

In many regions of America, Asia and Africa, traditional farmers continue to use the hoe and/or cutlass as the only method for preparing the soil for planting. In Mexico and Meso-America, they simply cut the vegetation at ground level and place it in somewhat parallel rows about one meter apart. Then they plant the seeds with a wooden stick through the mulch, or they scatter these residues and plant with the stick through this layer of mulch. This is a truly no-tillage system. 

At present, on small farms, modern no-till procedures involve slashing weeds of previous crop residues with cutlasses and spraying herbicides by means of knapsack pumps for weed control. Mulch by itself, in adequate amounts, prevents the germination and growth of annual weeds due to its shading effect. Then, sowing is done directly through the mulch by opening planting holes (5-10 cm deep) with a stick or a narrow spade, and covering the seed with loose soil, without packing it. All crop residues are retained, and fertilizer and amendments are broadcast on the soil surface. Crop residue mulch and other organic wastes, beside being produced in situ, could also be brought in from an adjacent area or from outside the farm (Lal, R., 1993). Mechanization makes the major difference between no-tillage on small and large farms. 

Spreading of the No-Tillage System in the World.

The no-tillage system has spread rapidly among large mechanized farms, covering millions of hectares in America, Europe and Australia (Table1). Controversially, the adoption of this system in developing countries is going rather slow. 

Table 1. Evolution of No-Till in the World (Adapted from Croveto, 1996)
 

Maize under No-Tillage.

Maize is one of the most suitable crops for cultivation under no-tillage. Besides producing short term favorable effects on the maize crop, in the long-term conserves or improves the soil’s physical and chemical conditions. This is further enhanced when maize is planted in rotation with legumes. No-tillage optimizes production and productivity under the existing soil, climatic, and agro-economic circumstances in which the maize crop is produced. It can be said then, that no-tillage is a sustainable system for maize production, which can be even improved by a judicious use of fertilizers and agrochemicals for weed and insect and disease controls. 
 
 
Traditional Tillage Versus No-Tillage for Crop Production.

Is really conventional tillage essential for successful crop production? 

Since the invention of the plow, manufacturers, farmers and agricultural professionals, have justified soil preparation on the basis of reasons not quite scientifically proven. Some of the alleged justifications for soil preparation with manual or mechanical implements are (Faulkner, 1943): 

• Efficient weed control.
• Handling or incorporation of plant residues.
• Improvement in soil aeration.
• Seedbed preparation.
• Disease and/or insect control.
• Improvement of physical conditions of the soil (structure).
• Disagregation or pulverization of soil.
• Improvement of water infiltration and storage.
• Fertilizer and amendment incorporation.
• Plowpan elimination.
• Improvement of root development.
• Utilization of hand labour, animal traction and machinery.

However, today it is easy to refute all these arguments for tilling the soil in favour to the no-till system by the following well known facts listed by Violic (1998): 

• Weeds are controlled with herbicides, cover crops (e.g. mucuna) and even with mulch alone.
• It is better to leave crop residues on the soil surface than to incorporate them, since crop residues are very efficient in controlling soil erosion and increasing water infiltration.
• By lowering soil temperature in tropical areas, mulch protects the soil against excessive water losses through evaporation. Additionally, residues maintain soil moisture close to the surface and prevent crusting. This favours normal coleoptile growth.
• Mulch dissipates kinetic energy of raindrops that upon impact with bare soil would otherwise loosen soil particles and disperse its aggregates, and finally, would cause crusting. Mulch also reduces water run-off.
• Soil aeration does not constitute a problem in untilled soils, except in cases of excess soil moisture. For example, the root system of a maize crop consists of more than 120,000 km of roots per hectare which upon withering, leave sufficient channels for aeration, unless they are destroyed by mechanized tilling.
• A hole made with a stick, or a cut made by the disk of a maize planter is more than sufficient to prepare a seedbed in which to bury the seed. Moving more than 7,000 t/ha of soil per hectare during seedbed preparation cannot be justified for the sole purpose of providing a place to deposit the seeds.
• Although soil preparation has been found to have coadjutant effects in maize disease and insect control in temperate regions, the situation in the tropics is controversial. An adequate solution for these problems should be faced through an integrated insect and diseases control, and crop rotation to break the cycle of these organisms.
• No-tillage have the effect of increasing water infiltration and storage due to the soil coverage (mulch) and the undisturbed soil structure in relation to the conventional tillage. In addition, mulch reduces water evaporation, keeps moisture on the soil surface, favours biological activity and makes water available to the root system.
• Even though it is said that soils are prepared to improve their physical structure, paradoxically the more the soil is worked, the more the pore structure is destroyed. Plowpan and soil compaction are direct consequences of the use of plows and harrows.
• Adverse effects of compact soil on plant growth has been recognized for centuries. Limited water and nutrients availability to plants due to compaction are major constraints to plant growth and yield in many soils. But, compaction also limits plant growth and yields by affecting water infiltration, aeration, plant diseases and yield quality.
• Farmers believe that if handlabour, animal traction and machinery are available, they have to make good use of them. But not always the rationality behind this is correct. Therefore, if tillage operations are not required, why doing them? For instance if animals (oxen) are in good conditions and the farmer have time, he goes ahead and ploughs his fields. Sometime he does not have even a technical explanation for doing so. Perhaps, because all the farmers are ploughing and this is a tradition. On the other hand, farmers in USA that have adopted the no-till planting consider that farmers that still are practicing conventional tillage are doing this for recreation ("recreation tillage) because they do not have any better thing to do.

Climate and Soil Types.

Climate and soil typesare the two factors that help identify the most appropriate systems for land preparation. Due to the fact that most of the cultivated area in the tropics was formed under Savannah or jungle, its incorporation under cultivation represents a drastic change for a rather fragile system. If these soils are poorly managed with excessive tillage they rapidly lose their production potential.Therefore, to practice soil conservation through the no-till system would effectively protect these soils. 
 
 
  No-Tillage as a Viable Alternative.

No-tillage should be chosen and utilized as a viable alternative, especially in the following cases: 

• When the soil is susceptible to wind and/or water erosion, and loss of organic matter.
• When soils have suffered severe degradation, particularly structural deterioration, under intensive farming systems, manifested by surface crusting and a hard, highly compacted layer in the subsoil, resulting in poor water infiltration rate.
• When sloppy land must be brought into production, without endangering the loss of the arable layer from erosion.
• When timing of operation is difficult.
• When the cost of soil preparation is high due to the cost of machinery, fuel, and/or manual labor.
• When there are serious concern about the sustainability of natural resources, especially soil and water needed. These are needed to feed the ever-growing future generations, which means to sustain continuous growth in agriculture productivity while maintaining the resources devoted to agriculture.

Most, if not all, of these problems are present in tropical and subtropical soils. In such cases, there is no doubt that the most conservative and suitable system for maize production under these fragile condition, is no-tillage. This is the only system that secures a sustainable crop production without disturbing the soil, and provides a protective mulch cover against soil and water losses. 
 
 
  Benefits of No-till Planting.

The no-till system present several benefits in relation to the conventional tillage as follows: 

• Reduction on the cost of production.
• Requirement of less time for field operations.
• Planting can be done on time.
• Less utilization of machinery.
• Less hand labour required.
• More moisture is conserved and made available to the plants.
• Higher water infiltration and storage.
• Reduction of soil erosion.
• The soil structure is protected.
• Less artificial formation of hardpans by heavy equipment.
• Progressive reduction of weeds (particularly perennial).
• Progressive higher yields.
• Higher profits.

Areas in Which No-Tillage Definitely Presents Clear Advantages.

Tillage, Soil Erosion and Runoff Control. 

Soil erosion is a major environmental problem worldwide, causing loss of topsoil, nutrient reserves and water, and is the most widespread and serious form of land degradation. Traditional tillage adds a heavy weight to this problem. Reduction in crop yield from erosion may be in the range of 20 to 30% for every inch of soil loss for shallow soils of the tropics (Lal et al., 1990). Erosion is more severe on land cleared mechanically than manually. Table 2 shows an example of average soil losses in maize production according to tillage methods in relation of the average soil formation (information from University of Cornell, USA). 

Table 2. Example of Annual Soil Losses (t/ha) in Maize Production According to Tillage Methods in Relation to the Average Soil Formation. 
 

* Net soil benefit in reference to the no-tillage system 

Although erosion has occurred throughout the history of agriculture, it has intensified in recent years. About 75 billion metric tons of soil per year are removed from the land by wind and water erosion, most coming from agricultural land, and rendering it unproductive (Pimentel et al., 1995). About 12 million hectares of arable land are destroyed and abandoned annually because of non-sustainable farming practices. The higher the land slope the higher the soil losses (Table 3). The extent of soil degradation in some tropical regions is alarming (Table 4). 

Table 3. Examples of Annual Soil Losses in Relation to the Slope. 
 

Table 4. Extent of Soil Degradation in Some Tropical Regions in Million Hectares (Cited by Lal, 1993). 
 

Crop Residues for Soil Protection and Increasing Organic Matter 

Crop residues are needed for surface soil cover and to replenish soil organic matter. Croveto (1996) mentioned that "the grain is for the farmer and the residues is for the land". He reported organic matter and cation exchange capacity (CEC) increases of 650% and 250% respectively at the soil surface (0-5 cm depth), leaving the mulch over the soil from continuous no-tillage for 17 years (Table 5 and 6). He also reported that raising the level of soil organic matter with continuous no-tillage has the effect of increasing the total count of different bacteria (e.g. nitrate-oxidizing bacteria) and various types of insects, particularly earthworms, in relation to traditional tillage. Unfortunately, many farmers, especially the ones at the subsistence level, harvest crop residues for animal feed, or allow animals to overgraze the plant stubble and weeds from their fields. Although some of these materials are returned to the fields as animal wastes, the potential for influencing crop and soil sustainability could be much greater if all residues were left as mulch. The common practice, in most African countries, of releasing all animals after the crop harvests are finished, should change. The use of electric fences (solar power), widely used in other latitudes should be encouraged among villagers to confine livestock in particular areas. On the other hand, especial fields should be selected for producing forage and rotate them with crop fields. The present type of farming where crops and animal are mixed together is not sustainable. 

Table 5. Increments of Soil Organic Matter under No-tillage for 17 Years at Chequén Farm, Chile (Croveto, 1996). 
 

Table 6. Increments of Cation Exchange Capacity (CEC) under No-tillage for 17 Years at Chequén Farm, Chile (Croveto, 1996). 
 

Plant residue removal affects soil water conservation and storage, and depletes plant nutrients and soil organic matter, and change soil physical properties. 

Clear examples of crop residues removal are offered by China and Bangladesh where 60% and 90% are removed and burned for fuel each year respectively. In this areas fuel is scarce, seven roots of grasses and shrubs are collected for this purpose. 

Traditional Tillage Increases Soil Erosion 

Soil preparation with traditional tillage, more than contributing to erosion control, has generally worsened it. It is estimated that the moldboard plow contributed greatly to the loss of up to half of the arable layer of soils in the USA during the past century, although it helped pioneers conquer the West and transform it into the breadbasket of the world. However by the 40’s the traditional tillage system was responsible for severe wind erosion that affected most of the middle west agriculture land, creating the so call dust bowl. 

On the average African and Latin-American soils loose 7 t/ha/yr., whereas in contrast, the loss of European soils is only about 0.8 t/ha/yr., being overgrazing and forest destruction the main causes, followed by nomadic agriculture. Erosion attributable to migratory agriculture occurs due to demographic pressures. Farmers are forced to plant their crop on steep slops and reduce the fallow period to a minimum, tilling the land and leaving the soil exposed in some period to erosion. The magnitude of erosion in Africa, probably the highest recorded in world tropical regions, may be attributable to the shifting from migratory to permanent agriculture, in highly erodible soils. 

Erosion can also result in the loss of organic matter and the sealing of soil surface with the consequent loss water infiltration capacity, and thus, in the increase of runoff. Runoff occurs when rainfall intensity exceeds the infiltration capacity of the soil, which is a measure of the ability of the soil to absorb and transmit rainwater. It results in washing away crops and fertilizer inputs, loss of soil moisture and recharge capacity, and in frequent drought stress in crop production. Runoff is an extremely important water management problem on rainfed croplands, not only because of the loss of a potential water resource, but also due to the subsequent damage in terms of soil erosion. 

No-tillage is the most efficient and economic method for controlling and conserving runoff. Runoff is controlled by keeping the soil surface continuously covered and protected from raindrop impact. In terms of water conservation, crop residue mulch with no-tillage has proven to be effective in Humid and Sub-Humid tropics (Lal, 1986). 

Aina (1993), compared the effects of mulch vs. bare soil on water runoff and soil erosion from studies conducted in 3 West African countries (Ghana, Cote d’Ivoire and Nigeria). On the average, 45.2 % of the rainfall was lost from runoff in bare soils as compared to only 1.2 % in mulched fields. As for erosion, bare soils lost 245,5 t/ha comparable with 2.6 t/ha in mulched soils. Soza et al (1995) also compared the effect of mulch in Ghana, especially in the forest and transition areas, where traditional farmers under shifting cultivation practice a non-conservationist no-tillage system. They slash and burn the vegetation, and establish the maize crop by making small holes to place the maize seed without disturbing the soil surface. As an alternative to burning, they chop the vegetation with cutlass, use handhoes and herbicides. They found that in the plots in which the slashed mulch was burnt, maize yield averaged 4,394 kg/ha, whereas in the unburned mulch plots, maize yieded 7,668 kg/ha.

No-Tillage and Water Availability. 

No-tillage increases the water availability for the crop. Maize requires 550 to 650 mm per unit area for good growth. To reduce weed competition for moisture is important throughout the growing cycle. If the soil profile is at field capacity at the time of planting and there is 350-400 mm of well distributed rains throughout the growing cycle, there will be sufficient moisture to produce a good crop. As mechanical tillage is intensified, available water in the soil, for the crops is reduced. 

Semiarid and Arid regions in Sub-Saharan Africa are more suited for maize production than the humid regions (Lal, 1993). The vast majority of tropical cropping areas are rainfed, which makes imperative to capture and store as much water as possible in the soil profile. 

Water sources for the crops come from moisture stored in the soil before planting, from rainfall during the crop season, from irrigation and, in small amount, from dew. Surface evaporation wastes a large amount of water that can otherwise be efficiently used by the crop, especially in regions of high temperature, frequent but not intensive rains, and poor soil protection. Rainfall and its distribution throughout the year is adequate in many tropical regions, for more than one maize crop per year. But in other areas with relatively high annual rainfall, frequent drought spells add an extra insecurity to the crop. For these reasons, water conservation is a must, especially considering that the production of every gram of maize grain requires 600 liters of water. 

Mulch reduces evaporation by insulating the soil from radiation and by producing a high relative humidity between the soil surface and the mulch cover. As a consequence, the amount of water stored in the soil profile is significantly higher. It also protects the seeds from excessive high soil temperature that may interfere with germination. Mulch has proven to be very effective in the humid and sub-humid tropics for conserving moisture. 

Although other technological solutions to face drought problems have been proposed, ranging from genetic solutions to management technologies aiming at improving water collection and storage, such as borders, contour furrows and terraces, just to mention some, no-tillage, or its combination with drought tolerant varieties, is the one presenting more advantages. One of the alternatives to mulch, is irrigation , which although expensive should, in general, be very important in the tropics, since if water is secured, higher input levels can be used by farmers at a reduced risk, especially fertilizers, which become very effective when moisture is not a problem. But for most farmers, either because or economic reasons or lack of water sources, irrigation is not feasible. 

Complementary management technologies, such as the adjustment of population densities, the use of animal manure, planting with the onset of the first good rains or perhaps even dry planting, control of weeds early in plant development, etc. may play a synergistic role. They present less shortcomings for African small holders than other solutions. 

No-Tillage and Weeds. 

Weeds is an important factor that can result in total loss of crop production. In the majority of cases, yield losses due to weeds surpass those caused by the combination of insects and diseases. However, weeds can be managed successfully in crop production under no-tillage. 

Weeds compete with crops for nutrients, light and water, and some weed species (Rottboelia exaltata, Cyperus rotundus, Imperata cylindrica, and other spp.) exert allelopatic effects to cultivated plants. Nevertheless, estimating weed damage is more difficult, because the effects are observed very late in the development cycle, when weeds have already competed for light, and especially for water and nutrients during critical growth stages, substantially reducing yields. Several studies have demonstrated a clear and negative correlation between weed dry weight and maize yield, with actual grain weight reduction of up to 95 %. Hand weeding may utilize 35-70 % of the total agricultural labor in Africa, where most weeding is done by hand (Ransom, 1990). 

In terms of competition for nutrients, some weeds absorb up to two times more nitrogen and phosphorous and up to three times more potash than maize, on a plant dry matter basis. Fertilizer stimulates weed growth to the extent that crop loss increases due to competition with vigorous weed plants. This simply means that the feasibility of overcoming weed competition by adding more fertilizers is questionable. Positive weeding x nitrogen interaction are commonly observed in maize, whereby timely weed control optimizes the use of N and other nutrients, with the corresponding savings in fertilizers. As regarding to light, some weeds grow faster and taller than maize, during the first growing stages of the crop, depriving the crop of adequate light supply for photosynthesis. 

Where water is a limiting factor, weeds exacerbate the water stress during the critical growth period between two weeks before and two weeks after flowering, to which the crop will respond with a lower yield. However, at times, weeds may provide some benefits, such as helping to control erosion by wind and water, mainly on hillsides. Well-managed weeds may be as important as mulch in conservation tillage systems. 

Mulching is a very efficient and sustainable weed control method. In this case, previous crop stubble and weeds (which may be sprayed well in advance with a contact (i.e. Paraquat or Diquat) or a non-selective (i.e. Glyphosate) herbicide, and chopped with a cutlass, covers the soil in such a way as to exclude light from growing weeds, preventing their photosynthesis and growth. Other materials could serve the same purpose, such as manure, saw dust, straw, etc. The problem arises in those cases in which farmers must use weeds and stubble, in general, to feed their cattle. Soza et al, (1995), found that in Ghana, an effective system for controlling weeds is by using cover crops such as Mucuna sp. They reported results from on-farm research, indicating that Mucuna not only controlled weeds but also produced large amounts of mulch and contributed with about 100 kg/ha of nitrogen equivalent for the following maize crop planted under no-till system. 

No-Tillage and Insect Pests. 

Depending on the environment, crops are attacked and affected by a complex of insects and disease organisms that significantly reduce both the quality and quantity of production. Under a context of sustainability of crop production, pest and disease management can be enhanced by rotating crop species, especially against insects and pathogens which show specific host ranges. Added to this, the rotation of chemicals may help reduce the probability of developing pest resistance. 

As far as insect pests is concerned, it is estimated that 60 % of the 55 million hectares planted to maize in tropical and subtropical countries are seriously affected by insect attack. Depending on the environment, maize is attacked and affected by a complex of insects and disease organisms that significantly reduce both the quality and quantity of production. 

For most maize farmers in developing countries, the lack of capital, or credit, and the high risk of loosing their crops to drought and pests, prevent the use of control measures beyond minimal cultural practices (Mihm, 1944). For this reason, Integrated Pest Management, or the utilization of means and measures to reduce pest populations and damage to a minimum, or below the economic injury level in a crop or cropping system should be used, principally based in the use of host plant resistant cultivars, and cultural practices. 

With the advent of no-tillage it was thought, at first, that pests such as Diabrotica, Colapsis, Phyllophaga and wireworms, to mention a few, would constitute limiting factors under this system. Fortunately, the accumulated experience of a few years of no-tillage experimentation in the tropics revealed that this has not happened (Ortega, 1991). For instance, Diabrotica population was not affected by tillage systems and, on the other hand, insects such as fall army worms tended to be less important in conservation tillage systems that in conventional tillage. It is speculated that the presence of mulch affects the visual and olfactory perception of moths. 

As for other insects, such as borers that survive between crop cycles in plant residues, although some experiments indicate that residues should be incorporated, it was found that in most cases the differences between conventional and conservation tillage do not justify the burning or incorporation of the mulch. 

Even though evidence is scarce, trends indicate that beneficial flora and fauna (Enthomophagous fungi and nematodes, Carabids, Staphylinids, virus, bacteria, Parasitic wasps, Orius, Nabis and Gebloris) increase in conservation tillage systems, in contrast with conventional tillage. The above, partially explains how population densities of some harmful insects become similar in both tillage systems. Also, the availability of abundant organic matter in no-tillage, a food source for some insects, such as the lesser cornstalk borer (Elasmopalpus lignosellus), eliminate them from the category of pests (Ortega, 1991). 

Time of planting has a strong bearing in control strategies of several pests, as demonstrated in several species. In Pakistan, for example, maize planted in early May suffers lower infestation by Chilo partellus than planted in late May or June, which may sustain over 70 % infestation. Similar damage patterns have been reported in Kenya. 

No-Tillage and Diseases. 

In terms of diseases, in spite of the fact that in many regions located in the tropical and subtropical regions maize is cultivated the whole year around, in such a manner that there are, almost always, maize fields scattered in the area, many times at different growth stages, the frequency and intensity of the diseases are not as high as expected. This, by no means suggests that there are no devastating diseases such as downy mildew in Asia and Africa, streak virus in Africa or corn stunt disease in Central America that may, sometimes, annihilate the crop. It simply means that the problem could be even worse, although it is estimated that fungi, bacteria and viruses occurring in maize, are responsible for yield losses of 9 % (Cassini et al., 1979). As in conventional tillage systems, resistant varieties and rotations should be considered when facing diseases. 

Crop Yields under No-Tillage 

Crop yields under the no-tillage system could be equal or higher than the conventional tillage if production technologies are equivalent in terms of inputs and practices. Several authors have reported that crop yields under continuous no-till planting resulted in progressive increments. Croveto (1996) recorded continuous increments of grain yields of maize and wheat rotation under no-till for 16 years at Chequén Farm, Chile (Table 7). 

Table 7. Grain Yields of Maize and Wheat Rotation under No-Till System. 
 

CIMMYT Experience

Starting in 1973, and for more than 25 years, the CIMMYT Maize Production/ Research and Training Program in Mexico, conducted hundreds of Station and On farm Research no-till maize planting experiments. These were managed with direct participation of thousands of trainees and visiting scientists from over a hundred developing nations. Research involved the relations and interactions of no-till systems with factors that affect maize production, such as weeds, water, nutrients, insect populations, and cost efficiency of the system in relation with conventional planting methods. The new system spread rapidly in some areas, such as of Mexico, Central American and Caribbean countries, by CIMMYT alumni after resuming their research and extension programs in their respective countries. A follow up was done by CIMMYT staff to assist the professionals to conduct practical (hands-on) training courses in order to accelerate the technology transfer process. 

IITA Experience

IITA has also conducted intensive research on no-till planting in Nigeria, particularly by R. Lal (several citations from his research work are mentioned in this paper). No-till was adopted at Ibadan and other research stations of IITA. Thousand of professionals, particularly from African countries, several seminars and international symposiums were conducted on the subject at Ibadan which helped the dissemination of the system. 

Ghana Experience

The Ghana experience in soil conservation has been taken as a case study for this paper. The Crops Research Institute (CRI), the Grains and Legumes Development Board (GLDB), large private seed growers and many farmers adopted the no-till system. This was as a result of the implementation of the Ghana Grains Development Program (GGDP), sponsored by CIDA, and implemented by CRI, the Ministry of Agriculture, CIMMYT and IITA. Sasakawa Global 2000 (SG 2000) also played an important role, supporting training activities for technical staff and farmers. 

Planting without ploughing was the traditional farming practice in pre-colonial times in Ghana, but this was always associated with burning. Farmers still continue this practice, particularly in the forest area, where they slash the existing vegetation, burn it when dry and plant crops when the rains are established by making a hole for the seed without ploughing. In other areas, farmers prepare the soil by using handhoes or by poughing with tractors, causing serious soil erosion. Since 1991, the GGDP, after 10 years of research, adopted the tillage/mulching/no-burning system in five main research stations for planting maize and grain legumes. The most common weed control program used has been the application of glyphosate two weeks before planting, atrazine plus alachlor after planting, and paraquat (band or spot application) or hand weeding during the first 45 days, when required. Equal or higher yields, reduction of tractor operations, savings of time and money and drastic reduction in soil erosion have been achieved. The system has also been tested with or without herbicide in the on-farm research program. SG 2000 has assisted farmers’ groups, and particularly seed growers to use the system extensively. Extension staff have been trained to encourage adoption among Ghanaian farmers. 

No Tillage Approach 

The no-tillage approach consists of a protective soil cover, especially before the crop canopy closes, which effectively conserves soil and moisture. Mulch is readily available, particularly in the humid tropical areas of Ghana. In a low-input farming system, the use of mulch becomes particularly important for the increase of crop yields. Mulch improves water storage, moderates high soil temperatures, protects the soil surface against raindrops impact reducing erosion, increases soil infiltration, reduces weeds growth, increases soil organic matter and fertility, and improves soil structure. 

Farmers’ Fields 

In order to improve the traditional system, burning is being discouraged among farmers, so as to obtain the full benefit of stubble mulch which would result in crop yield increases (Table 8). The recommendation is to replace the burning practice by manual operations using cutlasses and handhoes for early and effective weeding and/or the use of herbicides. Weeds must be slashed in such a way that they do not reach post flowering stage to set seeds or to multiply the roots system. 

Table 8. Maize Grain Yield as Affected by Stubble Mulch Management under No-Tillage Conditions. 
 

An effective way to suppress weeds resulted in the use of cover crops such a Mucuna sp. Results from controlled weeds, produced large amount of mulch and made a contribution of 100 kg/ha of nitrogen equivalent for the following maize crop planted with no-till. Good crop yields are obtained under this system even without the use of fertilizers, when the natural soil fertility is maintained with adequate crop rotation (legumes and cereals) or planting on fallow land. Then, with the application of recommended level of fertilizers, yields increase drastically. For instance, research results showed that maize yields could vary from 0.7 to 1.6 t/ha when conditions are not adequate, such as late weeding, continuous maize without crop rotation and no fertilizer application, regardless of the land preparation system used (tillage vs. no-tillage). When crop management is improved (crop rotation, good weed control, no-tillage and mulching) maize yields could go as high as 3 to 4 t/ha and, by adding fertilizer, 5 to 6 t/ha could be easily obtained (Table 9). 

Table 9. Maize Yields Associated with Farming Practices. 
 

No-tillage with herbicides represents additional benefits for farmers because of the drastic reduction in hand labour required, if compared with conventional tillage (handhoe or/and animal traction). This becomes especially relevant when considering the needs for increasing food production as well as improving farmers’ living conditions. For instance, the requirement of hand labor to produce one hectare of maize (from planting to harvesting) could practically be reduced from 120 to 20 man working days (Table 10), since most of the labor is required for tillage and weeding operations. 

Table 10. Man-Working Days, Maximun Number of Cultivated Hectares and Yields Associated with Tillage Systems in Ghana. 
 

• Total working man-days required to produce 1 ha of maize considering all operations, from soil preparation to crop harvest.

When the no-tillage + herbicides system is used, the potential area under cultivation and expected yields increase drastically, particularly if fertilizer is applied. 

The introduction of herbicides, particularly Glyphosate which is safe for the environment and human beings, is an effective solution especially for smallholder farmers for increasing the area planted to maize and grain yields. The technology for the use of herbicides is relatively simple and can be handled by these type of farmers. More and more farmers in Ghana can afford the use of herbicides, especially if they are aware that their yields substantially increase without weed competition. If farmers are using fertilizers, they should also use herbicides to make sure that the nutrients are utilized by the plants and not by the weeds. 

Research Stations in Ghana 

The GGDP adopted the no-tillage system in the mayor five CRI research stations in 1991, after more than 10 years of research in maize and grain legumes. Its adoption was encouraged by the successful results obtained in other countries, particularly in the USA, where the system is being rapidly used by farmers. CIMMYT’s no-tillage experience in on-farm research, conducted by the Maize Training Program, was very importance since many of GGDP staff participated in training courses in Mexico. The intensive research on no-tillage conducted by IITA and the adoption of the system in its research stations in Nigeria, for more than 10 years, was also an excellent training ground, particularly due to the equivalence of the agro-ecological conditions in both IITA and CRI research stations. 

Table 11 compares maize and legumes crops management systems used at CIR stations before 1991 with the no-tillage approach used thereafter. No-tillage provides important advantages for research stations and farmers in the reduction of time and use of machinery, which produces large savings. 

Table 11. Comparison of Conventional and No-Tillage Operations at CRI Research Stations in Ghana. 
 

Results 

In five years of continuous maize and grain legumes (cowpea, soybean and groundnut) planting with the no-tillage system, approximately equal or higher yields have been obtained in relation to the conventional soil preparation (Table 12 and 13). In the case of maize variety trials, average grain yields increased by 18.4% between 1991 to 1995 as compared with previous years. Rainfall data indicated that the amount and distribution of moisture during the growing season, largely influenced crop yields, regardless of the land preparation systems utilized. With the no-tillage system, important saving in time and money, and drastic reduction in tractor tear and soil erosion was achieved. The 9 to 12 operations dropped to only 4 to 6 using no-tillage (Table 11). The incidence of weeds has been significantly reduced. In some fields where Cyperussp was a serious problem, this weed has now almost disappeared. Consequently, the doses of herbicides have been gradually reduced, with the associated reduction in production cost. 

Table 12. Average Yields of Maize Variety Trials Under Conventional and No- Tillage at Five CRI Research Stations in Ghana. 
 

Table 13. Average Yields of Cowpea Variety Trials under Conventional and No-Tillage at Five CRI Research Stations in Ghana. 
 

An additional benefit in terms of operations opportunity was attained, since the crops can now be established in a shorter time, thus coinciding in the most favorable planting period. Initially, the application of the no-tillage system tended to make things apparently more difficult, so a period of learning and adjustment was needed. But, after the staff gained confidence, the execution of operations became easier and with less pressure in time. 

Training and Propagation of the No-Tillage System. 

Research and extension staff was trained on the no-tillage system to encourage the propagation of the system among research stations and farmers. Several training courses were conducted by GGDP during five years. CRI planted 12 ha of commercial maize under no-tillage and improved crop management in 1993, obtaining double yields compared to previous years with the conventional tillage method. SG 2000 and GGDP assisted a farmers’ group (Inyanase, Cape Coast) in planting about 36 ha of maize using a no-till planter (provided by GGDP), herbicide (glyphosate) and fertilizers. The average maize yield was double in relation to what farmers normally used to obtain with their traditional tillage system and cropping practices. The Grains and Legumes Development Board (GLDB), in-charge of foundation seed production, started using the no-tillage system in 1995. Successful results promoted the adoption of the system in all the GLDB seed farms throughout the country. 

Concluding Remarks. 

Crop research in the tropics should emphasize on soil and water management studies for their efficient use and conservation for the generations to come. Efficient and sustained increase in yield in tropical crops will be the resultant of applying methodologies which besides checking erosion, improve soils nutrient status and ensure the capture and storage of water from precipitation to fulfill the crop needs. 

Maize and other crops have been produced under no-tillage for about 5,000 years. With the domestication of animals, a large variety of ploughs, harrows and other animal traction implements were developed. Later on, heavier equipment was manufactured to match the high power of tractors. As a result of soil disturbance through excessive tillage for centuries, the world is actually phasing severe soil degradation affecting vast regions, particularly in the tropics. 

There is a need of turning into more conservatory crop production systems. Soil disturbance (poughing, harrowing, ridging, and cultivating) of any sort must be discouraged to avoid erosion, destruction of soil structure, losses of moisture and organic matter. In many countries, top soil has been lost in more that 60%. This fact is endangering the future of large regions in terms of crop production sustainability. 

Francis (1993), citing research ideas described by Lal, Brown and Thomas, and Sanchez and Benitez, makes reference, among others, to the following research priorities for the tropics: 

• The study of alternative primary tillage operations to asses their energy use and efficiency, the effect on soil losses, the potential to maintain residues in surface, and the efficiency to control weeds.
• The investigation of reduced and no-tillage alternatives and their adaptability to tropical cropping systems, climates and soils.
• The study of alternative methods for water trapping and harvesting and storage within the soil profile for use by subsequent crops
• The selection of crops and cultivars that are well adapted to reduced tillage systems in the tropics
• The study of intensive tropical crop rotations, intercrops and agroforestry patterns that can reduce primary tillage costs and increase efficiency in water use.

Farmers, crop production research scientists and extension workers are facing a major technological and socio-economic challenge in the tropics. They must change the ancestral practices which have led to the impoverishment of the typical small holders. The modernization of production operations is a must, and cannot and should not be postponed. While improving the production efficiency, the agricultural assets must be protected and saved for future generations. 

The augmentation of per capita and area production and productivity are imperative. The changes must consider schemes of sustainable agriculture emphasizing soil conservation and, whenever possible, rescue as much of the traditional systems, in order to make the technological transition more agreeable by smallholder farmers and their families. 

Improved technologies could take from 5 to 20 years to be transferred and utilized by farmers in advanced countries. This process is slower in developing countries. Soil conservation through no-tillage is an example of delayed technology to reach farmers. Researchers, extension workers and policy makers cannot wait too many years, witnessing severe soil destruction, before effective and dynamic actions on soil conservation are taken. It is recommended to conduct practical training courses on no-till planting for research and extension staff in most of African countries following the experience of Central America and Ghana. 

There is no doubt that this symposium will open a new window to the agricultural future, and significantly contribute to generate the necessary ideas and promote actions which will help to meet the new challenges. 

References