This paper aims to give a general description of the Gran Chaco, with emphasis on areas that lie in Argentina and Paraguay, and with special emphasis on agriculture. Its varied environment and climatic zones, its land use and its patterns of crop and livestock production have to be seen against a background of other uses of the Chaco, notably forestry and as a wildlife habitat. The main differences in climatic and edaphic conditions have been partially tackled by several authors; here an overall synthesis is provided. The agricultural potential, in the widest sense, is discussed; ongoing research programmes are described; and opportunities for further research and development work noted.

It is hoped that this synthesis of information will provide a clear picture of the present state of development in the Gran Chaco, as well as prospects for the twenty-first century, and thereby lead to a better understanding of its problems and potential by both technicians and planners. Major opportunities and problems have been pointed out in the hope that some of the recommendations will be taken up by the local governments and regional and international organizations, including aid agencies.

The Gran Chaco, located approximately between latitude 17° and 33° South and between longitude 65° and 60° West, is a vast plain that extends through northern Argentina, southeastern Bolivia, northwestern Paraguay and into a small area of southwestern Brazil. It stretches for about 1 500 km from north to south, and about 700 km from east to west, without any important physical barriers intervening (Figures 1a & b).

Figure 1a. Biogeographical provinces of South America (from Cabrera and Willink, 1973)

Figure 1b. Simplified vegetation map of South America, excluding the Andes, based on Figure 1a, highlighting the corridor of xeric vegetation extending from the caatinga (1) in NE Brazil to the chaco (2) in Argentina, via the cerrado (3) savannahs.

The approximate area, according to Hueck and Seibert (1972), is about 850 000 km², with the following distribution: Argentina - 520 000 km²; Paraguay - 230 000 km²; Bolivia - 90 000 km²; and Brazil - 9 000 km². The Chaco is one of the major wooded grassland areas in Central South America, and therefore could be of major economic importance, although this resource is being overutilized and suffers from intense degradation through unrestricted forest and bush clearing, overgrazing and continuous monoculture. It is important to stress that there is no desert zone within the Gran Chaco and that it extends into both tropical and temperate zones. This vast region is an almost flat plain sloping gradually eastwards, varying from 100 - 500 m above sea level, except for the Sierras in Cordoba, Argentina, which reach 2 800 m; some hills in Paraguay are over 700 m.

The region forms part of the River Plate basin and is traversed by the Pilcomayo, Bermejo, Juramento and Salado rivers. The area is also characterized by the smaller rivers and tributaries which dry-up periodically. The border areas between Paraguay and Bolivia comprise long tracts of sand dunes caused by high winds.

2.1. Climate

The climate is typical of the wet-dry seasonal type common at such latitudes; it varies, as would be expected over so vast an area, but does not involve sudden changes since there are no great natural barriers. According to Fatecha (1988), the climate corresponds to semi-arid and arid. Temperatures rise from south to north and rainfall from west to east (Figures 2 and 3). Environmental gradients are shown in Table 1, adapted from Bucher (1982).

Figure 2. Annual mean temperature (1951-1980; °C) across Paraguay, decreasing from north to south.

Figure 3. The main sub-divisions of the Gran Chaco, based on annual precipitation

Annual rainfall is characterized by a decrease from east to west, with highest values in the Paraguay River, decreasing gradually by between 1 and 1.5 mm/km for 400 to 500 km. There is, however, some increase due to the proximity of the Andean ranges in the west. Highest rainfall is found along the Paraguay River, with certain variations, with a maximum of 1 300 mm in Asunción and Formosa, while it diminishes towards the north (Bahia Negra) and to the south (Santa Fe), where annual rainfall is approximately 1 000 mm. The 500 mm isohyet goes through the border between Paraguay and Bolivia in the west. Table 2 shows the temperature and rainfall gradients of the Chaco.

The months with highest temperature coincide with those of maximum rainfall, according to Savaria Toledo (1993). This fact has favoured the evolution of herbaceous forage species of the C₄ type, which can be efficient in their photosynthetic activity at high temperature. The Chaco is principally a sub-tropical zone, with winter frosts occurring less frequently in the northeast and with higher frequency going towards the southwest. Because of its continental climate there are large variations in temperatures between summer and winter, as maxima of 40° C can be reached when the sun is high, whereas in winter some frost does occur. Annual mean temperatures range from 24° C to 25.5° C. The main rainfall, as a result of isolated storms, is from October to March; the driest months are July and August. The average rainfall varies from 450 to 850 mm. During the rainy season, 400 to 600 mm fall. The water balance has negative values for 10 to 12 months of the year.

Soils

Soils are formed principally from fluvial deposits based on transported material from the rivers Bermejo and Pilcomayo, which have had many courses over the centuries due to their shallow gradient. In addition to the predominantly flat topography, the scarcity of rain results in an arid zone that experiences frequent dust storms and extremely high temperatures. The soil evolved over deep sedimentary deposits that are either neutral or slightly alkaline, with a high level of base saturation (90-100%). There are marked differences between the soils of the eastern and western parts of the region: the eastern soils are predominantly clay with impeded drainage, frequently solodized with a shallow water table. The western soils are more loamy, comprising sandy loams in the far western areas. They have relatively good drainage and vary from low acidic or neutral to slightly alkaline in the surface horizons. The watertable is deep and may have high salinity. Because of the low rainfall, primary minerals and salts are abundant and result in areas of saline soils (FAO/UNESCO, 1971). Typical values for the surface layer (0-20 cm) of Paraguyan Chaco soils are shown in Table 3.

The soils are suitable for forestry and pastures and for cultivation in rotations, while perennial crops are grown under irrigation. Fertility in general is high, but availability of water is a limiting factor, as is the resulting salinization from poorly managed irrigation. There are no major restrictions to the use of machinery, necessary for all types of crop production.

As is usual in areas of warm climate with a long dry season, there is a strong flush of available nitrogen with the onset of the rains due to mineralization of the soil organic matter. Investigations in the Argentinean Chaco (Hang et al., 1995) showed that, on land where Prosopis flexuosa is the dominant tree, mineralized, available and immobilized N were higher under trees than in open spaces between trees, and were higher in selective thinnings (trees plus spaces) than in natural woodlands.

Vegetation types

According to Bucher (1982), the predominant vegetation of the Chaco is open dry woodland with cacti and bromeliads, stretching continuously over large areas, with a grass ground cover. Schinopsis sp. (Anacardiaceae) is the dominant tree. Other typical vegetation types are palm savannahs, savannah parkland, low tree and shrub savannah, with halophytic shrubs on saline patches. Detailed descriptions provided in the following sections clearly show its wealth of plant genetic resources and biodiversity.

The Paraguayan Chaco has mesophytic vegetation in its eastern (oriental) part where rainfall is higher, varying from 1 000 to 1 300 mm/yr, but changes to xerophytic in the extreme west, where rainfall falls to a maximum of 600 mm/yr. The intermediate central region is described as mesoxerophytic with annual rainfall between 600 and 1 000 mm. The vegetation and climate of the Chaco has been classified by various authors (Adamoli et al., 1990; Morello and Adamoli, 1968; Boelke, 1964; Hueck, 1966; Ragonese and Castiglioni, 1970; Galera and Ramella, 1997; Karlin et al., 1994; Vargas Gil, 1988; etc.).

The vegetation types of an area proposed as a national park in the Bolivian Chaco in a zone with between 500 mm and 900 mm of precipitation are described by Kempff Saucedo (1995). Dry Chaco woodland is the dominant formation and covers large tracts to the west, south and southwest of Las Bañados del Izozog. It is low woodland with a sparse tree layer, which is almost entirely caha (Aspidosperma quebracho-blanco) in the form of scattered trees, and candelabra cacti (Stetsonia coryne and Cereus forbesiii). Its main characteristic is a nearly impenetrable shrub layer, 4 to 6 m high, of chorquetta (Ruprechtia triflora) which also occurs in the forests of the sub-humid Chaco. Gallery forests are characteristic in the immediate vicinity of watercourses, where the soil is perpetually moist; they are dominated by Cathormion polyanthum and Geoffroea striata; the composition of the shrub layer is very variable. Palm groves occur on land that is flooded for most of the year with shallow, non-flowing water. Copernicia australis is the only palm; Prosopis alba, P. nigra, Parkinsonia aculeata, Tabebuia sp. and Geoffroea striata are common, with a ground cover of Paspalum sp., Cyperus sp. and Rhynchospora sp.

Most early settlement in this part of Latin America was in a narrow strip along the coast, with little penetration inland, except along major waterways such as the Paraná River. Early settlers followed a European style of agriculture, with grapes, wheat, maize and cattle; subsequently, cash crops such as cotton (Gossypium spp.) and sugar cane (Saccharum officinarum) became more widespread. Before settlement, the undisturbed vegetation of the Chaco was made up of a variety of forest types, characterized by different tree densities with accompanying smaller trees and shrubs and ground cover of grasses of diverse value for grazing animals.

As noted by Adamoli et al. (1990), some of the older cities of Argentina, founded by the Spanish over four centuries years ago, were on the edge of the Chaco. For a long period, settlements were near the mountain highlands or Sierras, as it was easier there to obtain water for domestic use and to irrigate small orchards. Gradually, with the construction of the railways, the flat land or plains were settled, and henceforward the impact of man on the overall Chaco ecosystem began to be felt. In 1876, it was shown that refrigerated ships could transport meat from the Argentine to Europe, and this important advance in technology led to the development of a livestock industry in the interior of Argentina, including the Chaco, to supply the new meat-packing plants. Large tracts of land were colonized, the indigenous population - who had lived in harmony with the environment - were displaced, and the exploitation of the Chaco began in earnest.

Thereafter, the construction of the railway to the interior of the Chaco established human settlement firmly within the region (Morello and Savaria Toledo, 1959). Railways were built, trees felled for railway sleepers and charcoal, and by 1912 over 32 000 km of railway lines carried livestock from the Chaco and Pampas to Buenos Aires. Most of the development was financed by European capital. Today, only a few railway lines still function, but hardly any intensive beef operations for export remain in the Chaco. The wealth of the region has declined seriously, but even today overgrazing, excessive monoculture and unrestricted clearing continue to cause environmental damage.

Once the export beef industry had been developed, local demand for agricultural and forestry products was stimulated by the rise of mining. It was only at the end of the nineteenth century that human settlements begin to increase as a result of political events in Europe. At that time, with the Industrial Revolution in Europe a century old and urban communities greatly expanded, trade in agricultural products was ever-increasing to meet the demand for cheap food.

At the end of the nineteenth century, settlements grew up along the banks of the Paraguay River, associated with the tannin industry, based on wild stands of quebracho (quebracho colorado [Schinopsis lorentzii]andwhite quebracho [Aspidosperma quebracho-blanco]). The size of these settlements has varied according to the price of tannin and overexploitation of the forest. Between 1910 and 1920, the southeast of the Paraguayan Chaco became an area for extensive cattle production and sugar cane growing. These areas were not far from Asunción, so labour was available for their development.

The Mennonite community has had a strong influence on the development of the Chaco, especially in Paraguay; the first settlers came from Canada in the 1920s and more followed from the Soviet Union in the1930s; another large contingent from the USSR migrated after the Second World War. This Mennonite settlement of the Central Chaco in Paraguay, which began in 1926, led to the development of a major agricultural research and extension infrastructure, covering most aspects of crop, grassland and livestock production in this area, which is almost totally settled by Mennonites. The research has had a major impact in the country’s present and future development plans. Since the late 1960s, the trans-Chaco highway has speeded overall development.

For a long time the emphasis on cash crops has had a deleterious effect on the environment and natural resources. It is in particular the need to export goods that has led to an intensification of agriculture through heavy investment in machinery, fertilizers, agrochemicals, etc., which is not sustainable. This has been particularly true in the case of the Chaco, with the encroachment of agriculture into grazing-cum-forestry areas for the purpose of developing crops such as soybean and cotton; much of such investment is foreign. Nevertheless, constant pressure from various sectors of the population has created an awareness of the danger to natural resources as a result of the drive to exploit agricultural resources for a fast economic return. New legislation is being developed to combat such damaging processes, but it is still in its infancy.

Settlement led to environmental degradation: grazing was totally unmanaged, there was uncontrolled burning, and over-felling of the forest led to its replacement by undesirable thorny vegetation. Some authors claim that before the arrival of European settlers, the Chaco ecosystem was in balance. This may be so, due largely to the low population density and the absence of domestic livestock. The absence of an overall land settlement policy has been one of the major factors in the overexploitation and degradation of the vast resources of the Chaco, and applies equally in all countries that share the Gran Chaco.

Cattle numbers in the Argentinean Chaco rose steadily until 1940; production typically was based on unfenced ranches of 5 000 ha. Increase in livestock numbers was through expansion of the settled area, with primitive management and low levels of technology. As stated by Adamoli et al. (1990), the stocking rate in 1940, at one beast per five hectares, was already too high and has led to the present stage of overall degradation.

With the present situation of land tenure, a considerable proportion of land is held in communal ownership, and often land has been invaded by squatters. In other instances, very large tracts of land without demarcation has again led to excessive exploitation of the resource. The severity of overgrazing, coupled with monoculture and felling of the forest, makes it difficult to conceive plans for redistribution or even to attempt to reduce the present rate of extraction. There is no way to ascertain who is responsible for the use of the crop land, the grazings or the forests (Riveros, 1993). Table 4 shows the distribution of land in the Argentinean Chaco (Calella, 1989), and Table 5 that in the Paraguayan Chaco.

The Chaco is not like areas with an old agricultural tradition, where the distribution of cultivated plants or native plants protected by man can define precisely the environment in which they grow. In Spain, for example, the olive (Olea europaea) and carob (Ceratonia siliqua), protected or cultivated, define with considerable precision soil types and bioclimatic conditions. In the Chaco, however, crops are not yet correlated precisely with environmental conditions. A cotton plantation does not define a place ecologically except in very general terms. Crops have been chosen according to short-term economic speculation, without considering their optimum ecological and environmental requirements. The predominance of horticultural crops, tobacco or sugar (under irrigation) in a region are not necessarily an indication of their suitability for that area, so it is easy to understand the lack of sustainability of many of the crops being grown in the Chaco.

In principle, it is not difficult to gauge bioclimatic conditions from the undisturbed vegetation of a site, using vegetation as the instrument to identify, define, describe and delineate those areas that can be considered similar in terms of resources and their problems. Nowadays, remote sensing using satellite imagery can be used to make comprehensive and detailed studies of natural resources and their utilization. However, the Chaco remains wrongly exploited, with little, if any, concern for sustainability and for future generations. Improved technologies have been developed by research institutes, such as the National Institute for Agricultural Technology (INTA) in Argentina, and applied successfully by a few groups, but what really controls farming methods is short-term economic gain.

4.1 Vegetation types

The vegetation types are presented based on Boelke (1964), within a framework of three agro-ecological zones, and Figure 4 provides information on subregions and vegetation in Argentina.

Figure 4. The Argentinean Chaco - Sub-regions and vegetation (after Morello 1968)

This is basically a parkland formation, where patches of Quebracho Colorado Chaqueño (Schinopsis spp.) forest alternate with open grassland. There are also areas dominated by the Copernica alba palm, usually under swamp conditions with accumulation of salts. On the border of watercourses there is dense bush vegetation, and flooded areas are covered by aquatic plants. The absence of any significant relief, coupled with heavy seasonal rainfall, favours the formation of numerous shallow lagoons, which are covered by bogs known regionally as esteros.

The common native forage plant associations indicate the wealth of plant genetic resources.

Wet areas (esteros): summer - autumn tufted perennial grasses, such as Paspalum intermedium and Oryza latifolia, with annuals such as Echinochloa crus-pavonis.

Cane brakes (canutillales): summer - autumn tufted perennial grasses, such as Diplachne universaria and Eragrostis lugens, with rhizomatous grasses such as Echinochloa helodes and Luziola peruviana, and stoloniferous grasses (e.g. Hemarthria altissima).

Palm groves (palmares): summer - autumn tufted perennial grasses, including Sporobolus pyramidalis, Diplachne universaria, Panicum milioides and Paspalum intermedium, with rhizomatous grasses such as Echinochloa belodes.

High grass pastures (campos altos): summer - autumn tufted perennial grasses, including Paspalum dilitatum, P. almum, P. urvillei, P. simplex and Digitaria insularis; with rhizomatous grasses (such as Paspalum notatum, Axonopus compressus and Cynodon dactylon) and annuals (e.g. Digitaria sanguinalis).

Quebrachales: summer - autumn tufted perennial grasses, including Leptochloa virgata, Paspalum simplex, Setaria argentina and Chloris canterae, and trees and shrubs such as Schinopsis balansae, Caesalpinia paraguariensis and Geoffroea decorticus.

Algarobales: trees and shrubs, such as Prosopis alba, P. nigra and Acacia aroma.

Forest clearings (abras de bosque): summer - autumn tufted perennial grasses, including Paspalum plicatulum, Bothriochloa barbinodis, Chloris canterae, C. distichophylla, Leptochloa chloridiformis, Setaria fibrigii and S. argentina; and stoloniferous grasses such as Paspalum alcalinum, P. proliferum and Chloris polydactyla.

There are also some important legumes in these vegetation types, including Desmodium canum, Vigna luteola, Phaseolus lathyroides, P. adenantus, Rhyncosia minima, Eriosema sp. and Desmanthus sp.

This is present in several provinces of the Argentinean Chaco, in the east of Bolivia and the western part of Paraguay. There is a small area in the south west of Brazil. Basically it is a huge area of flat land, becoming increasingly arid from east to west. Open grasslands derived from forest through bush clearing and fire occupy a significantly smaller area than is found in the humid and sub humid zones. Forests are dominated by xerophytes and are less dense than in the eastern Chaco. Cacti, including tree-cacti are common among trees and shrubs. Pastoral resources include a vast number of trees and shrubs, but only in man-made clearings can useful numbers of forage species be found:

Quebrachales: trees and shrubs, including Schinopsis lorentzii, Caesalpinia paraguariensis, Acacia aroma, A. praecox, A. macracantha, A. tucumanensis, Prosopis alba, P. nigra, P. torguata, P. uscifolia, Ziziphus mistol, Mimosa detinens, Cercidium praecox, Geoffroea decorticans, Ruprechtia triflora, R. corylifolia, Porlieria microphylla, Mayanthus spinosa, M. vitis-idaea, Celtis spinosa, Castela coccinea, Achatocarpus nigricans, etc.; cacti, such as Cereus coryne, C. validus, Opuntia quimilo and Harrisia pomanensis. Grasses, as below.

Clearings and open habitats: summer - autumn tufted perennial grasses, such as Setaria argentina, S. cordobensis, S. geniculata, Trichloris crinita, T. pluriflora, Chloris polydactyla, Gouinia latifolia, G. paraguayensis, Pennisetum frutescens, Elionurus tripsacoides, Leptochloa virgata, L. dominguensis.

Soils with high salinity: Sporobolus pyramidalis, S. phleoides and Atriplex sp., plus cacti as in the quebrechales.

Esteros: summer - autumn tufted perennial grasses, such as Panicum prionitis and Elionurus tripsacoides,plus floating species such as Panicum elephantipes, and other herbs, such as Eichhornia crassipes, Typha domingensis and Scirpus californicus.

Northwest savannahs: summer - autumn tufted perennial grasses, including Paspalum urvillei, P. commune, Digitaria californica, Setaria spp., Panicum spp., and rhizomatous grasses, such as Paspalum notatum, and stoloniferous grasses, such as Axonopus compressus.

This zone is mostly in Argentina, but is also extends into Bolivia and to a small extent into Paraguay. The landscape is broken by hill formations, which have a higher rainfall than the lowlands. The hillsides also collect moist air coming from the east (Atlantic). The forest vegetation contains many species found in the lower Chaco, and some trees of higher rainfall areas, such as Alnus jorulensis, Schinopsis haenkeana, Acacia caven, A. furcatispina, Prosopis chilensis, P. nigra, Geoffroea decorticans, Caesalpinia mimosifolia and C. paraguayensis. The grass cover is very limited.

The description of agricultural production systems in the Argentinean Chaco is based on a regional approach, grouping provinces according to similarities in their major climatic parameters and patterns of agriculture production.

CLICK HERE FOR A MAP OF ARGENTINIAN PROVINCES

Catamarca and La Rioja

The provinces of Catamarca and La Rioja cover approximately 18 000 km² and 15 000 km², with about 30% and 55% in the Chaco, respectively. Both provinces are in the arid Chaco, with rainfall under 400 mm/yr. They present a variety of environments, according to local topography. The Chaco is in general a flat plain with savannah type vegetation, although there are significant areas of natural forest, but those forest areas are being rapidly damaged by indiscriminate logging. The major limiting factor to the expansion of sustainable crop production is water. There are currently 290 km² under irrigation in the province of Catamarca and 170 km² in La Rioja. There is an urgent need to improve irrigation methods so as to make better use of the available water. In this respect, the search for more efficient crops has a high priority. In both provinces, cereals, vegetables, fruit trees and citrus are of major importance to the economy. Land tenure problems, as in the rest of the Chaco, are common and have still to be addressed. Livestock rearing is widespread among small farmers, and includes cattle, large flocks of goats and sheep, and some camelids in the mountains. Stock rearing systems are mostly extensive, and there is ample room both for improvement of herd management and for development of fodder and grazing management.

INTA, at its regional centre in La Rioja, has several programmes dealing with important aspects of crop and livestock production. Pasture improvement, based on oversowing buffel grass (Cenchrus ciliaris) into natural grassland at low cost, is providing good incentives to graziers. Several lines of indigenous grasses are also being studied. Work on lucerne (Medicago sativa) is important, since up to eight cuts can be obtained per year. In the horticulture and fruit production sector, work on olives and figs is proceeding successfully, and considerable effort is dedicated to onions and capsicums. Efforts are also underway to improve the quality of grapes. To sum up, the small area under irrigation, the inefficient irrigation methods being used and the lack of efficient extension, all lead to unsustainable crop production and major erosion problems. Land tenure problems are widespread and serious, and encourage exploitation patterns that lead to rapid loss of soil fertility and to forest resource degradation.

Cordoba

The province of Cordoba has an area of 160 000 km², including 35 000 km² in its northwestern part considered to be part of the Chaco. Approximately 95% of the area is used for cattle rearing, mixed with goats on natural pastures and scrub, with very low annual productivity (18 kg/meat/ha). Crops are unimportant, except in some areas with irrigated fruit and vegetables. Most of the agricultural enterprises in this area are not viable. Erosion is widespread as a result of fires on the grasslands and scrublands, monoculture in the arable land, and indiscriminate felling of forests. The most common vegetation types of an area west of Cordoba are shown in Figure 5.

The INTA Regional Centre concentrates its efforts on crops for the areas of higher agricultural potential, especially sorghum (Sorghum bicolor), groundnut (Arachis hypogaea) and soybean (Glycine max). The Faculty of Agriculture of the University of Cordoba has been studying the productivity of sylvipastoral systems in the Chaco, with promising results based on maintaining a good proportion of indigenous trees and adjusting the stocking rate to avoid excessive overgrazing. Detailed research on Prosopis spp. demonstrates their potential and value in maintaining soil fertility. Nevertheless, large tracts are still being cleared for monoculture of cereals.

The potential for sub-regional cooperation on the use and management of Prosopis spp. has been recognized for some time. In Ayzera et al (in FAO, 1988b), the potential offered by Prosopis in the Dry Argentine Chaco is discussed, with especial emphasis on furthering regional development through the establishment of sylvipastoral production systems.

Figure 5. Typical successive vegetation forms in an area west of Cordoba (after Marcelo Cabido, 1994)

Northwestern Chaco

Salta and Jujuy

The provinces of Salta and Jujuy, in the northwest, cover 60 000 km² and 18 000 km², respectively, and share many contrasting environments (80% of Salta and 30% of Jujuy are in the Chaco), varying in altitude, topography, rainfall and temperature. Eleven different ecological zones have been identified for Salta and seven for Jujuy. Both provinces have temperate areas due to altitude. Sub-tropical crops are grown under irrigation, including sugar cane, tobacco and intensive horticultural crops. The expansion of agriculture in the north of Salta has increased areas under beans, soybean and maize. In the south, crop and livestock production (beef) are becoming commoner.

There is irrigation in central Salta and in parts of Jujuy. Soybean is the main crop, but, more recently, cotton, grapefruit (Citrus paridisi) and peaches (Prunus persica) are becoming more important; rice (Oryza sativa) and chickpea (Cicer arietinum) are recent introductions. Animal production concentrates on breeding, and a large feed-lot operation has started to winter-fatten beef cattle. Livestock is the main product of the sylvipastoral areas of the west, with 500 mm/yr rainfall, which necessitates a very extensive pastoral system. The potential in Salta province for expansion of animal production is particularly good, as only 27% of the suitable area is currently used.

In the higher foothills of the west, where forests have mostly been cleared, there is good natural pasture, and livestock operations are more intensive. Within the inter-montane valleys, the low rainfall of 400 mm/yr suits extensive goat production, with fewer cattle. Holding size largely determines the farming system, coupled with the availability of irrigation. Horticultural crops, tubers, pulses and citrus are mostly produced on units of less than 10 ha. Tobacco and orchard crops are common on holdings of 10 to 100 ha, while cereals and oilseeds dominate, with citrus and bananas, on holdings larger than 100 ha.

As in many other regions in Argentina, irrational exploitation of the forest has led to serious erosion, and complete loss of the more valuable species (precise information would be of value for future planning of forest operations). Erosion has been aggravated by the intensive cultivation of field crops such as beans, maize and sorghum. Common beans and soybean are sown at the peak of the rains, causing devastating erosion, as the soil has no protecting cover. Because of low yields and poor stubbles, wind erosion is serious in the dry season. Irrigated perennial crops (sugar cane and fruit trees) do not cause erosion, although over-watering without suitable drainage is causing serious salinization. The major problems in the medium term in Salta province are the phasing out of tobacco, raising productivity of livestock, and overcoming the generalized income deficit of small-scale farmers.

Small farms are often very intensively exploited and show signs of soil degradation. Large farms, often owned by non-resident farmers, are extensively managed and consequently productivity is low. The INTA Regional Research Centre in Salta, in conjunction with other regional centres, has been developing sustainable crop systems, with beans, garlic (Allium sativum), and improved curing of tobacco to increase returns. Similarly, in the fruit sector, a network is testing new cultivars of avocado (Persea gratissima) and mango (Mangifera indica). Methods of improved soil preparation for small-scale farms are being studied, with minimum tillage and direct sowing of both beans and maize.

Central eastern Chaco

Santa Fe

The province of Santa Fé has approximately 60 000 km² in the Chaco, extending through its eastern and central parts, which are almost entirely given over to cattle rearing and fattening. Crop production is of little importance, with grain sorghum as the principal crop, with some sunflower (Helianthus annuus) and cotton. There remains room for development. Typical vegetation types of an area north of Santa Fé are presented in Fig. 6

Figure 6. Typical woody vegetation groups in the sub-humid Chaco in the area of Norte de Santa Fe

The most serious problem, as in other areas where there is crop production, is the monoculture of cereals and the resultant propensity for soil erosion. The integration of livestock and pastures into the farming system is being studied as a means to achieve sustainable production. It is now recognized that lack of an integrated approach that rotates fodder with crops has led to the present state of degradation of the arable areas. The present shortage of beef in Argentina, with concomitant high prices, could bring about a change in attitudes, which would benefit both the crop and livestock industries.

The INTA Regional Centre in Santa Fe is concentrating its efforts on the higher potential areas. Nevertheless, efforts to improve forage conservation in the northwest, in order to reduce the problems caused by shortage of feed in the winter, have shown excellent results, using mainly maize (Zea mays) silage.

The province of Santiago del Estero, in the northwestern central part of the Chaco, covers approximately 120 000 km². It borders Tucuman, which covers 40 000 km² and has broken topography and relatively high hills or Sierras. Santiago del Estero is predominantly typical flat Chaco, though with low hills with the common serrano vegetation and a strip of sub-humid Chaco parallel to the Paraguay River. Vegetation types of the alluvial forest plains in the south of Santiago del Estero are shown in Fig. 7. Agriculture in Tucuman is based on sugar (about 50%), soybean (23%), maize (7%) and wheat (4%). The rest is tobacco and potatoes, with only 1% under cotton. In Tucuman, a lack of diversification means that 50% of the land is under very inefficient irrigated sugar. Moreover, the extensive nature of livestock production, sometimes with a high proportion of sheep and goats, is causing irreversible damage to the natural vegetation. There is also shortage of extension and information systems for farmers.

Figure 7. Typical vegetation types of the alluvial forest plains in the south of Santiago del Estero

In contrast Santiago del Estero has large areas of forest in the north that contribute substantially to the income of the province, and large areas of native grazing used for extensive beef production, with some sheep and goats. The area sown to crops totals 6 000 km², with cereals (34%), oilseeds (19%), and industrial crops (11%, mainly cotton), and smaller areas under horticulture, forage and orchards. Common beans (Phaseolus vulgaris), which are grown by smallholders, traditionally occupied vast areas in the northwest; this land is now totally degraded and little if any are now grown. Excessive subdivision of holdings in the southeast is also resulting in intensive erosion and overall degradation. However, an area in the southeast with medium to large intensive dairy farms, using a high degree of modern technology, shows that dairying is well adapted to this environment and demonstrates that there is definitely potential to improve traditional systems.

The problems affecting both provinces are to a large extent similar to those of the entire Chaco. Degradation of the native forest is widespread, and only about 20% of the area that was under forest in 1914 remains; this has led to severe soil erosion compounded by monoculture, overgrazing and poor irrigation methods. Obsolete irrigation systems and poor management exacerbates the overall problem of sustainable crop production. Lack of adequate drainage is leading to increasing salinization.

Strengthening of extension and technology transfer is given high priority. The work of the INTA Regional Centre in Santiago del Estero is beginning to show encouraging results. The result of screening exotic pasture grasses (Brachiaria spp., Setaria spp. and Panicum maximum cv Gatton) indicates new ways for intensifying beef production. At the same time, methods of improved natural pasture management by strategic use of fire and better animal husbandry are in progress. Development of cotton technology, with mechanized harvesting and rational control of pests and diseases, is giving good results. In the horticulture sector, strawberry growing is attracting small farmers, and research into drip irrigation is underway. In the forestry sector, an inventory and detailed evaluation of native vegetation is being prepared, alongside field trials on planting native trees and shrubs of economic value.

Northeastern Chaco

In the northwest, the Provinces of Formosa and Chaco cover over 170 000 km², approximately 32% of the Argentinean Chaco. Both provinces have large areas of crops. The cultivated area is about 7 100 km², of which 3 150 ha are under cotton, 1 700 ha under sunflower and 1 400 ha under sorghum, the rest being horticulture and forage. Cotton here represents 70% of the total area of the crop in Argentina. Wheat and maize have gradually diminished in view of marginal climatic suitability and reduction of subsidies. In contrast, soybean has increased substantially, from 600 km² in 1990-1991 to 2 000 km² in 1996-1997. Other crops, such as rice, sugar cane, tobacco, linseed, fruit and vegetables, account for less than 3% of the area, although they are locally important.

Overall yields of cotton, sunflower, grain, and cultivated pastures are low due to a combination of mediocre husbandry, periodic drought, or heavy rains with resultant flooding due to the lack of opportunities for run-off. Small producers usually grow cotton in monoculture, with low input levels. Cotton marketing is poorly organized, but there are some alternative production systems available that could increase present incomes. Another serious problem is the shortage of labour for cotton harvesting: mechanization is gradually improving the situation, but there are still farmers who harvest late and lose quality through late rain. The loss to cotton producers in Chaco province in 1997 was estimated at US$ 31 000 000.

There has been rapid deterioration in soils through excessive mechanization, introduced without clearly defining soil management and conservation techniques, in particular in water erosion, loss of organic matter and reduced retention of moisture and soil nutrients. There are no proper crop husbandry models to follow and pest control is inadequate.

Livestock is important in mixed farming systems within the cropping areas, particularly in the central region of both provinces, where 22.4% of the producers cultivate 66.7% of the agricultural land and keep 28.2% of livestock. Livestock are about 3 000 000 head, mostly in breeding herds, holding heifers and steers for a year prior to their going for fattening in the south. Some livestock are overwintered before being sold for finishing in the more fertile grasslands of the south. Low winter temperatures substantially reduce the availability of forage for livestock. There is little adoption of improved technology among graziers, and animal health and hygiene are also poor.

Most livestock are on natural pasture, with large winter weight losses. Carrying capacity is 2 to 4 ha per livestock unit; heifers take 3 to 4 years before service; and steers take 4 to 5 years before finishing. Pregnancy percentage is 50 to 55%, and seasonal calving is not practised, so calves are often severely affected by climatic conditions, particularly as poor feed in winter provides little milk. Livestock handling facilities are minimal.

The division of land use in these provinces is as shown in Table 6.

Activity in forestry has been gradually reduced due to the depletion of the resource, although forest still covers 110 000 km², which represents 25% of the native forest of Argentina. Reforestation is minimal, although there is some interest for the future. Forestry is entirely extractive, with little management to encourage regeneration of valuable species. Charcoal follows logging and leaves the ground bare and liable to severe erosion and degradation.

From the above, it can be seen that the economy is highly dependent on cotton, and to a lesser extent on forestry and livestock.

The Regional Centre of INTA is looking carefully at improvement of cotton quality, and yield and resistance to diseases. In the case of livestock, supplements during winter are providing excellent results. Work on evaluation of grasses and legumes is proceeding with success, particularly for areas with poor drainage, where the grasses Bothriochloa insculpta and Paspalum atratum are used together with the legumes Desmanthus virgatus (collected in Paraguay) and Aeschynomene americana, from Florida. Work on swine production is of major interest to small-scale farmers.

In Paraguay, the Chaco is referred to as the western or occidental region. Like the rest of the Gran Chaco, it is an almost flat plain, with some 32 000 km² suitable for crops (Meza, 1988). However, only a very small area is under cultivation. The area used for livestock covers 124 000 km², mostly on natural grassland.

The Pilcomayo river basin is a dominating feature of the Chaco; two characteristics of the basin govern its behaviour there:

• an extraordinary capacity to produce sediments, some 60 000 000 m³ annually on average, from erosive processes in its upper catchment, with concentrations of 50 to 100 kg/m³ in the river. The catchment and sources of sediments are in the Andes, so the problems of the river are due to natural forces operating outside the Chaco; and
• a great variation in flow, with peaks of 2 000 m³/second and lows of only 3 m³/second.

The most serious effects of the vast load of sediments and the great variations in flow are:

• the retrocession of its course; this is the most alarming phenomenon among the problems of the Pilcomayo. The river crosses a great plain, about 320 km as the crow flies; since 1944, the course of the river has moved laterally at a rate of about seven kilometres annually, because of the great quantities of sediment it deposits in its bed.
• flooding caused by overflowing; these floods are beneficial since they feed the great wetlands of the Bajo Chaco and support an ecosystem unique in the world. Their negative side is that the floods are wholly unpredictable in volume and as to which side of the river they issue from; and
• the siltation, blockage and retrogression of the stream bed is closely related to the process of sedimentation. During peak flows, there is an enormous concentration of sediment in the water, which cannot be contained within the main bed and spills over on to the floodplain, depositing silt and the finer sandy particles. Much sediment, however, remains in the river bed; this is accentuated by the massive presence of a shrub, Tessaria integrifolia (Compositeae), with a superficial root system; floods uproot the shrubs and deposit them along the banks where they form a live dike which retains the sediments; this process raises the river bed above the level of the surrounding countryside and the old bed becomes a raised bank between the waters.

5.1 Vegetation Types (based on Ramirez and Laneri, 1989)

The vegetation of the Paraguayan Chaco is mesophytic in the east, where rainfall varies from 1 000 to 1 300 mm per annum, changing gradually to xerophytic in the extreme west, where rainfall is below 600 mm/yr.

Two main vegetation groups are recognized for mapping purposes in the Chaco Boreal of Paraguay: xeromorphic and mesomorphic. The xeromorphic group dominates the landscape. Mattoral (scrubland) is the main formation in all the centre, north and west of the Chaco Boreal; the dominant species vary with soil texture. On the lighter sediments and sandy soils of the northeast, typical species are Pithecellobium chacoense, Mimosa castanoclada, Cnidoscolus vitifolius var. cnicodendron, Jatropha excisa and Opuntia quimilo. In the taller layer, Schinopsis quebracho-colorado, S. heterophylla and S. cornuta are the dominant plants par excellence, but Aspidosperma quebracho-blanco is present to a lesser extent. On the crests of dunes, the main plants are Jacaranda mimosifolia, Aspidosperma pyrifolium, Schinopsis balansae, S. cornuta and Pterogyne nitens. Where soils are better structured and richer in clay, the main plants are Ruprechtia triflora, Ziziphus mistol, Sideroxylon obtusifolia, Cordia bordasii, Quiabentia pflanzii, Capparis retusa, C. tweediana, C. salicifolia, C. speciosa, Prosopis kuntzei, P. elata and Acanthosyris falcata. Soils with a high salt content have a characteristic vegetation. The mesomorphic vegetation that dominates towards the south and centre-east of the region develops on heavier, better structured soils different from those of the central Chaco. This area is covered by a mosaic of vegetation with alternating forest of Schinopsis balinese, Caesalpinia paraguariensis and Phyllostylon rhamnoides; palm-savannahs of Copernica alba; and marshes that contain, among others, Eichhornia crassipes, Pistia stratoides, Salvinia auriculata, Eleocharis elegans, Canna glauca and Rhynchospora corymbosa.

The representative tree species are: quebracho colorado (Schinopsis balansae), quebracho blanco (Aspidosperma quebracho-blanco), trebol (Amburana cearensis), urunday (Astronium balansae), guayaibi (Patagonula americana), yuqueri (Pisonia zapallo), and guayacan (Caesalpinia paraguariensis). In the southern part, timbo blanco (Enterolobium guaraniticum) and timbo colorado (Enterolobium contortisiliqua) are characteristic, with a number of shrubs as undergrowth.

Some areas are colonized by Prosopis spp., which are widespread in the Argentinean Chaco, such as algarrobo negr (Prosopis nigra), algarrobo blanco (Prosopis alba), algarrobo paraguayo (Prosopis hassleri) and vinal (Prosopis ruscifolia), and aromito (Acacia aroma) and several other Acacia spp, also occur. A characteristic plant throughout the Gran Chaco’s more poorly drained sites is palma caranday (Copernica alba), which has a variety of uses.

According to Ramirez and Laneri (1989), there are four types of native grasslands in the Chaco.

• Lowland grasslands: Located towards the south as a strip parallel to the Paraguay River with two distinct types:
  Swamps: these grasslands are covered most of the time with deep water. They can be highly productive in the upper layer, and comprise Cyperus gigantaeus, Scirpus validus, Typha latifolia, Thalia geniculata and Thalia multiflora, while the lower layers contain some good quality forage species such as Leersia hexandra, Panicum elephantipes, Hymenachne amplexicaulis, Diplachne uninervia and Oryza subulata. The palm Copernica alba is also present.
  Marshes: these are in lowlands where the water is not as deep as in the swamps. There are dry periods alternating with floods. The vegetation is made up of grasses of high nutritive value, such as Hemarthria altissima, which is severely overgrazed and being replaced by Paspalum spp., together with Eriochloa punctuate, Leersia hexandra and Cynodon dactylon. Also found are the palm Copernica alba, with Acacia spp. covering the drier sectors. The most important forage legumes are Aeschynomene montevidensis, Zornia diphylla, Galactea spp. and Discolobium leptophyllus.
• Palm groves: these takes their name from the caranday palm (Copernica alba), which grows in the lowlands, above the marshes, in broad strips. The understorey of the palm groves consists of grasses with weeds and some shrubs. Grazing animals eat the fruits and spread the seeds of the palm, which pass undigested into the faeces. The groves are on footslopes between the higher ground and the swamps. Espartillo-type grassland occupies the higher areas. The main grasses are Sorghastrum agrostoides, Paspalum panciliatum and Paspalum plicatulum. Among the legumes are Desmanthus virgatus, Desmodium incanum, Galactea sp. and Phaseolus lathyroides, all of them well known forages that have been the basis for several commercially-grown forage cultivars in Australia. The tree cover includes Prosopis campestris, P. nigra, P. algarrobillo and Cathormion polyanthum.
• Espartillo grasslands: as noted above, these occupy higher ground and are not flooded regularly. The soils are hydromorphic over clay subsoil, though the external drainage is good. The soils are well supplied with phosphorus, calcium and potassium. The following grasses are found: Elionorus latiflorus, Aristida circinalis, Tridens brasiliensis, Paspalum plicatulum, Spartina argentinensis, and Trichloris pluriflora. There are very few herbaceous legumes; Desmanthus virgatus is found occasionally. Trees include Prosopis campestris, P. nigra, P. algarrobilla and Copernica alba.
• Thorn forest: this is found in the central and eastern plain, where trees and shrubs dominate over grasses. Soils are mainly alluvial, saline and alkaline saline. Drainage is fair and allows growth of woody perennials such as quebracho colorado (Schinopsis lorentzii), quebracho blanco (Aspidosperma sp.) and palo santo (Bulnesia sarmiento). Also found are vinal (Prosopis ruscifolia) and algarrobilla (P. algarrobilla). Herbaceous forage legumes are rare, e.g. Desmanthus virgatus and Phaseolus lathyroides. The grass cover is made up mainly of Elyonurus latiflorus, Aristida adscensionis and Heteropogon contortus. Trichloris trinita is found in low-lying, more humid, areas but does not contribute greatly as livestock feed. The high density of Prosopis makes it difficult for animals to enter the grazing areas. As Prosopis pods are eaten by livestock, undigested seeds are dispersed and add to the spread and invasiveness of the trees.

5.2 Agricultural production

Crops

Agricultural land in the Chaco is only 2.7% of Paraguay’s crop area of 13 244 km². Thus only 357 km² of the Chaco are cropped, 90% of which is in the Department of Boqueron. Most of the cultivation began after 1943, and the crop area doubled between 1956 and 1981. The main crops in the Department of Boqueron are sorghum, cotton, groundnuts and cassava. Since 1956, groundnut has increased from 2 500 ha to 15 000 ha; farmer’s yields are almost 20% below those obtained on research stations, with much higher shortfalls (60%) on a national basis.

The last census showed that the area of cotton was decreasing; in the Chaco, with only 7 780 ha under cotton, compared to 102 917 ha in the rest of the country. Sorghum, with 12 396 ha in the Chaco, compared to 3 586 ha in the rest of Paraguay, dominates crop production in the Chaco. Table 7 gives the various crops and their areas in the Chaco and in the rest of the country from the 1996-1997 census; it shows that the only crops of real importance to the Chaco economy are grain sorghum, cotton and groundnut. New developments in grain sorghum by the International Centre for Research in the Semi-arid Tropics (ICRISAT) and other institutions show the possibility of large increases in yield by using hybrids specifically adapted to tropical or sub-tropical conditions.

In recent years, the Experimental Station for the Central Chaco, at Cruce Loma Plata, has had an extensive programme on crop agronomy, and soil management and conservation. Considerable attention is being given to the use of green manures and tillage systems to reduce erosion and conserve moisture. It is still too early to make final recommendations, but cultivation in strips has reduced wind erosion substantially; equally advantageous is the use of the chisel plough to reducing erosion caused by heavy rains after the sorghum harvest. Work on subsistence cropping systems, including maize, cassava, sweet potato, groundnut, beans, watermelon and pumpkin, have not yet yielded the expected results, except that they show that mechanization does not provide a significant increase. In general, mixed cropping gave lower yields than when crops were grown pure and in strips. More attention must be paid to the cultivars used and in particular that the crops are suitable for growing in association, rather than competing for the same environment (light, soil nutrients, water and humidity).

Crop research at the Experimental Station for the Central Chaco of the Ministry of Agriculture and Livestock in cooperation with Gesellschaft für Technische Zusammenarbeit (GTZ), at Cruce Loma Plata is providing good, alternative and new crop and grassland production systems. Some experiments are described briefly below, but further details can be obtained from the Annual Report 1993/1994 of the Station. Crop rotations with a ley farming component including forages such as pangola (Digitaria decumbens [now D. eriantha Steud]) mixed with Lotononis bainesii and Stylosanthes hamata cv. Verano. Arachis followed by green manure, grain sorghum - cotton, and in the second year, Arachis (groundnut), sorghum and cotton, etc., have shown varying results with transfer of nitrogen from Stylosanthes to the following crop. Water stress can be negative on the crops when forage legumes continue to grow during the dry season.

A programme on green manuring tested many legumes, including Stylosanthes hamata cvs Verano and Amiga, S. scabra cvs Seca and Siram, S. guianensis var. intermedia cv. Oxley, Centrosema pubescens, C. pascuorum cv. Cavalcade, Lotononis bainesii, Cassia rotundifolia cv. Wynn, C. sturtii, Leucaena leucocephala cv. Cunningham, Desmanthus virgatus cv. Filadelphia, and Clitoria ternatea. Although preliminary results show that frost eliminated all but Stylosanthes guianensis cv. Oxley, Lotononis, Desmanthus and Leucaena, it should be pointed out that Leucaena does persist there, but not further south in the Argentine Chaco (Santiago del Estero); this fodder cannot be used in winter-cool areas of the Chaco.

The residual effect of tillage and green manure in strip cropping on the yield of grain sorghum and groundnut are under study. Chisel ploughing combined with green manure (Lablab purpureus) shows significant advantages over traditional systems. Replacing disc with chisel ploughs reduces wind erosion substantially. Several experiments on green manuring of cotton, where water stress is not a limiting factor, show that green manuring increases yields; residual-effect data were not available at the time of writing.

Trials on planted pasture have been established to determine optimum stocking rates for various grasses, such as Cenchrus ciliaris, Cynodon plectostachyus [although this name is probably erroneous, as C. plectostachyus is a grass of volcanic sites in the East African Rift Valley but is not widely domesticated; however, in the earlier literature, much of the non-rhizomatous African "giant star grass" was tested under this name, but belongs to C. nlemfuensis and C. aethiopicus], C. dactylon, Digitaria decumbens, Brachiaria decumbens, Chloris gayana cv. Callide, Panicum maximum cv. Gatton, and P. coloratum var. makarikariensis cv. Bambatsi. This work will help determine optimum management and provide valuable information on species persistence and agronomy.

A programme of experiments on mixed cropping for subsistence agriculture show promising results, including maize, cassava, sweet potato, groundnut, cowpea, watermelon and Cucurbita pepo. In general, crops, when associated, show considerable competition and because of their very low productivity and yield it is recommended that they be sown in strips. Mechanized tillage gave slightly higher yields than manual tillage. Preliminary results show an urgent need to study indigenous systems of mixed cropping, as well as to evaluate local landraces and their suitability for mixed cropping.

Eleven million hectares, 60% of the Paraguayan Chaco, are grazing lands. The carrying capacity (see Table 8) varies from 3 to 15 hectares per head (Ramirez and Laneri, 1989). Livestock production (Table 9) is extensive and, as elsewhere in the Chaco, involves producing young stock to be fattened elsewhere. Some milk is produced by the Mennonite colonies, mainly to supply Asunción with fresh milk. According to Ramirez and Laneri (1989), the productivity of the natural grasslands of the Paraguayan Chaco have limitations due to the encroachment of woody perennials, the drastic seasonal changes between dry and flood conditions, the low nutritive value of the dominant species and the generally poor standard of animal husbandry (with serious animal health problems, including external and internal parasites). Nevertheless, it is important to consider the possibilities of improvement through better grassland and stock management, with, on favourable areas, the testing of better adapted forages.

Improved and cultivated pastures

The role of cultivated pastures in cropping systems in the Paraguayan Chaco has been highlighted by Ramirez and Laneri (1989) (Table 10). It is well known that forage or pasture crops, if adapted to local conditions and adequately managed with the necessary inputs, can support sustainable livestock production systems. However, inadequate management in the Chaco leads to invasion by undesirable thorny and unpalatable weeds. This is caused by loss of soil fertility, which must be kept at high levels to assure survival of the introduced forages. Proper adjustment of carrying capacity is also essential. Natural grassland is better than degraded, weed-infested, "improved pasture." Sown pastures may have an important role in the Paraguayan Chaco compared to the rest of the Gran Chaco, because they face less competition from crops and a short-term outlook.

Table 11 shows that most sown pastures are in Boqueron and President Hayes, areas that have more favourable conditions. The 3 290 km² is 36% of the total area of cultivated pastures in Paraguay according to Ramirez and Laneri (1989). However, the Ministry of Agriculture 1996/1997 Statistics Bulletin lists indicates 18 840 km² of improved or cultivated pastures in the Chaco. Different systems of collecting or interpreting data are involved, and a thorough analysis of the available data would clarify discrepancies that currently cloud discussions on future potential. The main problem of fodder supply in the Paraguayan Chaco is its seasonality of production, as noted earlier: shortages occur in autumn and winter (dry and cool season), with production concentrated in spring and summer.

Solutions for the problems of maintenance of soil fertility may be possible through the use of herbaceous and woody legumes. Pasture improvement by the use of introduced forages has a long history compared to other regions of the Gran Chaco. The area settled by the Mennonites in the 1930s was one of the first, in 1950, to plant pasture for dairy stock. In 1930, beef production per hectare was 5-10 kg and dairy cows produced 3-5 litres of milk; today beef production is of the order of 80 to 100 kg/ha/year and lactations over 5 000 litres per cow. Initially the most successful forage was Salinas grass (Pennisetum ciliare), and more recently, Panicum maximum ‘Gatton Panic’.

Improved pastures have a place in the Paraguayan Chaco, where environmental and economic conditions are suitable, with its fertile soils and higher rainfall. However, success depends on persistence and sustainability, which is wholly dependent on the management of the pasture and its integration into cropping systems that provide weed control and cover most of the costs of the establishment of an improved pasture. This type of strategy should be used as a model in other parts of the Gran Chaco. Secondary shrub invasion is a major problem in "improved" grasslands in the Paraguayan Chaco. Glatzle et al (1996) describe work on land which had been re-seeded with buffel grass many years previously, and had an initial shrub density of 1570/ha, predominantly Acacia aroma. A range of different combinations of mechanical and chemical control measures were applied; herbage biomass was estimated to be increased by at least three-fold by any treatment compared to the untreated control. It was concluded that repeated low-cost intervention was needed to maintain herbage production and keep shrub competition low; however, the efficiency of most of the treatments in reducing shrub density was low. Chopping with a blade-roller was by far the cheapest treatment.

On the same subject, Glatzle (1995) proposes a method of land clearing that avoids the use of fire. Currently approximately 50 000 ha/yr year of natural vegetation in the Chaco is cleared to sow pasture. Through burning, most of the organic matter is lost. Glatzle’s system uses the traditional thorn crusher, but selectively. Selective clearing produces good timber and fence posts, which have good value and can leave islands with native forest and accompanying vegetation, which are invaluable to maintain biodiversity and wildlife, in particular birds. Advantages of clearing without fire include less denuded soil, more organic matter in the soil, and a balanced biotope with predators and prey. Also, if burning is avoided, it is possible to leave many clumps of forest, which maintain biodiversity, provide shade for livestock and wildlife habitat, act as windbreaks, reduce the risk of salinization by maintaining a lower watertable, provide a source of feed in the dry season and create a pleasant, parkland landscape. Any method that maintains areas of forest while allowing good pasture establishment should be encouraged. The economics of mechanized selective clearing require further elucidation, however, and it is probable that, for extensive, low-output land, manipulation of grazing pressure and controlled burning may be the only economic option.