Brazil - part II
by
Paulo César de Faccio Carvalho
Aiming to present the Brazilian pasture resource in a clearer and more organized way, and since Brazil has continental dimensions, the description will be made according to different officially recognized Brazilian biomes (see Plate 6).
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Plate 6. Brazilian biomes (IBGE, 2005) |
Amazônia Forage Profile
In the nineteen-sixties livestock production systems occupied 150,000
km2 of native pastures, composed mainly of grasses, legumes
and Cyperaceae in upland areas (cerrados and savana well drained
from Amapá and Roraima) and flooded areas ("Ilha de Marajó"
and the Amazon river). Since then, up until the nineties, the government
encouraged the establishment of cultivated pastures, mainly Panicum
maximum, Hyparrhenia rufa, Brachiaria decumbens (see Plate 7) and
Brachiaria humidicola. The process of pasture improvement consisted
of clearing and burning the forest followed by seeding manually or by
plane. In the last decade it was estimated that 62% of deforestation was
due to cattle enterprises, where 25,000,000 ha of cultivated pastures
were established. Burning pastures at the beginning of the growing season
is a common practice. These enterprises are very extensive and beef oriented.
Apart from 19,000,000 cattle, the region contains more than a million
buffalo.
Amazonian soils are characteristically very acid, with extremely low P levels and low CEC, besides other mineral deficiencies (Peixoto et al., 1986). The high P fixing capacity of those soils contributes to reducing opportunities for pasture development. Guineagrass and Hyparrhenia rufa are more responsive to P than Brachiaria humidicola, and tropical legumes may be more tolerant than the grasses to lower levels of P. Lowland grasslands are inundated periodically; species of Echinochloa, Hymenachne, Oryza, Leersia, Luziola and Paspalum, cover poorer soils over huge areas. The problem of these areas is the absence of adjacent land to graze the animals during the floods, so animals lose weight due to nutritional and health constraints.
The upland grasslands, which represent around 60% of the region, display a similarity in botanical composition, where Andropogon spp., Axonopus spp., Trachypogon spp. and Paspalum spp. set the productivity and forage quality. Also important are the legumes of Pueraria spp., Centrosema spp. and Dolichos spp. Arachis is recently increasing in importance in some areas. This ample substrate produces poorer quality forage than the lowland grasslands. Within those grasslands nutrient cycling is the driving force for their sustainability. Burning and the high grazing pressure are critical to attain it.
After clearing sections of the tropical rain forest, pasture development brought significant ecological changes to the environment. Initially there was an increase in soil fertility due to the ashes. The rapid establishment of guinea grass, Brachiaria humidicola and Andropogon gayanus pastures encouraged intensive grazing and within three years signs of degradation were evident. But more leniently grazed pastures could be maintained for more than ten years. The P levels of these soils imposed limitations on pasture productivity, although some regional authors (e.g., Dias Filho & Andrade, 2005) showed five to six fold (up to 25 - 36 tons DM ha -1) increases in pasture response of the upland areas when fertilized and sown to cultivated species.
The level of phosphorous, and its decrease following establishment, and overgrazing are the most important factors in pasture degradation, a problem verified in 61.5% of the Amazonian Occidental cultivated pastures; according to local authors (e.g., Dias Filho & Andrade, 2005) an area estimated at around 12,500,000 ha is affected. Spittle-bug and disease are also important in contributing to pasture degradation, as well as the indiscriminate use of fire.
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In site-specific Amazonian areas, there are success stories with the use of grass-legume mixtures. Pueraria, the most impressive, accounts for more than 450,000 ha, being present in more than 30% of pastures in the State of Acre.
Since 1995 the cultivated pasture area increased almost 70%, being estimated at almost 57 million ha by 2003. Cattle stocks increased from 19.18 to 33.93 millions in the same period, illustrating the concerns about ecosystem conservation. As this region presents currently the highest Brazilian livestock expansion, and at the same time is characterized by low research investment (financial and human resources), it is of great concern for the future (Dias-Filho & Andrade, 2005).
Cerrado Forage Profile
Natural pastures comprise 30,000,000 ha of Cerrados (see Figure 6). The
main native grasses in this region are Echinolaena inflexa
and others such as Paniceae, species of Aristida, Arthropogon,
Axonopus, Paspalum, Schizachyrium, Andropogon, Trachypogon
etc. The legumes are represented by Arachis, Centrosema, Desmodium,
Stylosanthes, Macroptilium, Rhynchosia, Aeschynomene and others, which
in combination with grasses and other species make up the natural pastures
of the region. Strongly supported at the beginning of the nineteen-eighties
by government action, cultivated pastures, which accounted for 11,000,000
ha, increased up to 29,000,000 ha in the nineteen-eighties, and the area
is nowadays estimated to be around 60,000,000 ha, reaching the ecological
limit established for this ecosystem. After an increase of 25% in the
last 10 years (2001-2005), the expansion rate of cultivated pastures is
attaining a plateau, with a current tendency to stabilize. Environmental
requirements and the increasing utilization of integrated crop-livestock
systems are contributing to this. From the total Brazilian herd, Cerrados
has 41%, or 72.3 million cattle.
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In the last 25 years the green revolution was initiated and most of the "Cerrados" vegetation has been replaced by agriculture and after one or two years of growing crops, the land was turned into sown tropical pastures. Stocking rates were increased several fold, from 0.3 to 1.0 head/ha (Macedo, 1997). Brachiaria spp is the most widespread genus, covering more than 50,000,000 ha (see Plate 8) of the tropical savannah, representing 85% of cultivated pastures. B. decumbens (15 million ha) and B. brizantha (30 million ha) are the most cultivated pastures (Macedo, 2005). Panicum (see Plate 9) is the second most important genus, representing 12% of cultivated pastures or 7.2 million ha. Pasture establishment and/or regeneration being integrated with crops is now well adopted.
Low forage availability and quality in the dry season are the main limiting factors, lengthening the productive cycle in cattle raising. Burning is common. Deferment is sometimes used and constitutes one of the rare technologies employed in the most extensive areas. No more than 1-2% of cultivated pastures are legume-based, the genus Stylosanthes being the most important. For most of the area, there was no technological support to ranchers who stay away from agricultural administration, and livestock were practically raised by nature.
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Pasture degradation is considered the most important phenomenon facing the sustainability of livestock production in Cerrados, with overstocking and the lack of maintenance fertilization considered the main problems.
Mata Atlântica Forage Profile
The eastern portion comprises only 12% of Brazil but is responsible for
almost 50% of all milk produced and nearly 22.8% of the total Brazilian
herd (36,000,000). Dairy enterprises are predominant (Assis, 1997). This
is Brazil’s richest region and the most important agricultural and
industrial centre, containing 70% of the population.
Most of the dairy production is based on pastures developed on cleared pasture-land, where Melinis minutiflora predominated on the poorer soils of steep slopes, whose forage mass was of acceptable nutritive value, but had a low carrying capacity. Pennisetum purpureum is another important forage in dairy areas. Pastures based on Hyparrhenia rufa were persistent, but their productivity was also low. On some of the remaining fertile soils, Panicum maximum had thriven and is still the main beef pasture for the region. In the last 20 years the Brachiaria sp. took over and Brachiaria brizantha cv. Marandu is being strongly recommended due to its resistance to spittle-bug disease (Deois flavopicta and Manarva sp.). Cultivated pastures can provide conditions for high levels of animal productivity (25 to 30,000 kg milk/ha per year and 1,000 to 1,600 kg LWG/ha per year in well fertilized soils, producing more than 30,000 kg DM during the growing season. Digitaria decumbens, Brachiaria decumbens and Brachiaria humidicola are still important locally.
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Like the "Cerrados", eastern Brazil makes low usage of fertilizers for introduced pastures that are grazed at high stocking rates, so the high grazing pressure causes a weakening, and these pastures soon degenerate. Pasture degradation is an important limiting factor for farmers. Tropical legumes are scarcely used, and the highly seasonal dry matter production does not balance forage quality with animal needs. There is a feed shortage during the winter dry season (April/September), when supplementation is sometimes used, particularly in dairy systems. Intensive management of highly productive cultivated pastures is the most significant technology, but is still weakly adopted.
Caatinga Forage Profile
The semi-arid region of NE Brazil with a dry season of 8-9 month, and
an average rainfall of 400 - 600 mm per year uses mixed livestock and
involves the use of the natural resources. These areas have been used
in an extractive way since the eighteenth century and the natural vegetation
of mostly forage species is being replaced by annual and ephemeral ones.
High grazing intensity and periodic droughts are responsible for the desertification
started in some areas. The drought and the irregularity of precipitation
are considered the basic problems of this region, which has the highest
concentration of Brazilian small ruminant stock (8.8 million goats and
8.01 million sheep) besides the 23.9 million cattle. Livestock operations
represent food security, as in drought years the productivity of agricultural
products can decrease by more than 70%, whilst livestock no more than
20%. The common farming system is agrosilvipastoral, where animals have
an important role in the distribution of nutrients (Cândido et al.,
2005).
The main genus for the region are Mimosa, Caesalpinia, Dalbergia, Paspalum, Setaria, Cenchrus, Aristida, Elionorus, Zornia, Stylosanthes, Centrosema and others. In the short rainy season the herbaceous vegetation and green leaves of trees compose the forage mass. The Caatinga phytomass annual mean production is around 4.0 t/ha and dicotyledon herbaceous and grasses make up 80% of the ruminant diet, but in the dry period the importance of woody species increases. With the onset of the long dry season the leaves of the trees become dry and fall to the ground and are eaten by livestock. By the middle of the dry season 62% of their diet is composed of dead leaves of woody vegetation and up to 28% is from the standing herbaceous vegetation. Early in the rainy season, green leaves of trees comprise 65% of the diet and the herbaceous vegetation the other 35%. These prolonged droughts have a greater impact on the cattle population than on goats and sheep, since they are better adapted to adverse conditions.
Due to the importance of trees in the diets of grazing animals, manipulation of the vegetation is very important, and it is a common practice to pollard (cut the old branches of the trees and the top of the trunk) to develop new sprouts and branches from where the goats get most of their feed. Thinning of the stand is also done, and gradually they get trunk heights of less than 0.50 m from the ground, when all leaves are within reach of the animals, increasing the foraging substrate. Under natural conditions of the "Caatinga" vegetation, mixed grazing of cattle, sheep and goats is more productive. By thinning the vegetation, cattle and goats are favoured. But when that canopy is manipulated, and the trunks are cut close to the ground for new branching, cattle alone or cattle and sheep make better use of these natural resources. In areas of higher rainfall the main cultivated grasses are Cenchrus ciliaris and Brachiaria spp, Andropogon being less important. Important considered plants to face the semi-arid conditions of Caatinga and feed animals are “palma forrageira” (Opuntia spp.), sugar cane, sorghum and manioc. During prolonged drought periods, cattle are first supplemented, then sheep. Goats are supplemented only at very critical conditions. Young and non productive animals are free-ranging the Caatinga to find the remaining forages.
Pantanal Forage Profile
The Pantanal (see Plate 10) is a huge plain ranging from 16 to 210
S and 55 to 580 W covering 139,111 km2 (Allem and
Valls, 1987). Its vegetational heterogeneity is known as the "Complex
of Pantanal". Most farms are extensive cow-calf operations, with
stocking rates as high as 3.6 ha/head. Considering flooded areas this
stocking rate can decrease by 50%. The total cattle herd is estimated
at around 3,800,000. This conservative stocking rate, obligatory by nature,
accounts for ecosystem sustainability. There are no large wild ungulates
in the Pantanal, which has been called a sort of "herbivore emptiness"
by local authors. Commercial livestock at conservative stocking rates
are believed to be positive for local diversity. However this means a
very low productivity (18 kg LW/ha/year), which in turn determines farm
scale (3.5 percent of farms correspond to 57% of the Pantanal’s
area). Natural vegetation is a mosaic depending on soil flood characteristics
(level, duration, origin, etc.). Communities are known by the dominant
vegetation (Canjiqueiral - Byrsonima intermedia ; Carandazal
- Copernicia alba ; Paratudal - Tabebuia caraiba
; Gravatal - Bromelia balansae ; Caronal - Elyonurus
muticus and Pirizal - Cyperus giganteus and Scirpus validus).
Other important species are: Axonopus purpusii, Paspalum pantanalis,
Paspalum plicatulum, Paspalum hydrophilum, Paspalum carinatum, Paspalum
lineare, Paspalum repens, Panicum stenodes, Panicum laxum, Leersia hexandra,
Aeschynomene fluminensis, Aeschynomene sensitiva, Camptosema paraguariense,
Indigofera lespedeziodes and others. Native pastures are predominant
(99%), grasses being the most frequent group (240 species) followed by
legumes (212 species). Savannah and Cerrado are also important formations.
Brachiaria decumbens is one of the most important cultivated pastures
and its use can be strategic during flooded periods. Native pastures are
used basically in two ways: cows remain all year in the same area where
flooding depth is not greater than 1 m, or graze flooded areas during
the dry period. Paddocks usually have huge surfaces (1,000 ha on average)
making difficult the grazing management. The space-temporal variability
of herbage growth and the low carrying capacity of these pastures are
considered the main constraints to animal production. Eco-tourism is becoming
important in this region, which diversifies the economic activity.
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Pampa (Campos) Forage Profile
Around 33o S up to 26oS, in southern Brazil, the
low soil fertility, low soil pH, below critical P levels and shallow soils
account for the presence of only a few individuals of different species
of legumes such as Adesmia, Vicia, Lathyrus, Trifolium,
Medicago, Desmodium, Rhynchosia, Aeschynomene,
Arachis and Vigna. This region holds an enormous plant diversity
(about 400 grass and 150 leguminous species), Among the grasses
those in the genus Paspalum, Axonopus, Andropogon, Panicum, Setaria,
Digitaria, Schizachyrium, Bromus, Stipa are the most important,
The whole of the region enjoys the same thermal effects of the climate,
encompassing a wide range of soil types and elevations, and where the
moisture is abundant the dominant tall grasses, such as Andropogon,
Schizachyrium, Setaria, Bothriochloa, Paspalum, Stipa, Aristida,
and Axonopus restrain legume growth. As a consequence, the
massive dry matter (DM) production in the long warm season is of low quality
(< 60% digestibility), the species diversification and selective grazing
throughout the seasons of the year makes that growth accumulate, senesce
and lose quality even more in winter and it often has to be burned before
the following spring season. This accumulation shades cool season grasses
and prevents their growth, causing scarcity of forage and quality feed
in winter (see Plate 11). A common average daily growth rate for non fertilized
natural pasture is between 0-5 kg DM/ha in winter to 25-35 kg DM/ha in
spring/summer, and total annual forage production between 2,500 and 4,000
kg DM/ha.
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The most important cultivated forages are annual winter grasses like Avena strigosa and Lolium multiflorum (Nabinger et al., 2000), as well as legumes of the genera Trifolium, Lotus, Medicago and others. Tropical pasture species are mainly annuals, such as Pennisetum americanum, and Sorghum spp., but perennials are becoming important (Panicum, Cynodon, Digitaria, Paspalum, other Pennisetum, etc.). To a lesser extent, some perennial winter grasses are cultivated (Festuca, Phalaris, Dactylis, etc.). Cultivated pastures comprise around 7,000,000 ha, while native pastures attain 13,700,000 ha. So native pastures feed most of the 26,200,000 cattle and 6,000,000 sheep. Cattle and sheep are mainly raised in mixed-grazing on the native pasture areas. In Southern Brazil this natural ecosystem is under threat, decreasing at a rate of 135 000 ha per year (Nabinger et al., 2000), being replaced mainly by cash crops and reforestation. It is the basic habitat for 3 000 vascular plants, 385 species of birds and 90 terrestrial mammals. More than 50 forage species have been classified recently as in danger of being extinguished by mismanagement of natural resources. Overgrazing and lack of nutrient replacement are mainly responsible for the weak sustainability of the system.
Traditional pastoralism predominates in the use of these pastures, and feed is used mostly for maintenance purposes. This region is the second most productive in Brazil in terms of volume of milk. Integrated crop-animal production systems are becoming an interesting option mainly in summer crop areas with soybean, maize or rice.
6. OPPORTUNITIES FOR IMPROVEMENT OF FODDER RESOURCES
(also see Maraschin, 2001 for this topic)
Greater Use of Legumes
Tropical America is the centre of diversity for many important tropical
forage legumes, but the evaluations were limited to a few species, marginally
adapted to the edaphic conditions prevailing in major livestock areas
of the region. Perhaps this is one of the reasons for the low use of forage
legumes in Brazilian pastures. Other factors, which certainly contribute,
are the critical stocking rate and the lack of fertilization.
In many regions of Brazil there are ample opportunities to use forage legumes, since they are endemic to the area. Tropical legumes are also important in restoring lands degraded by imprudent cropping or grazing. These pastures, although contributing, are often unstable, and the management necessary to maintain an adequate proportion of legume is still little understood. The efforts devoted to selecting grass and legume germplasm adapted to the acid soils found in important ecosystems such as savannahs and humid tropical forests, revealed that S. capitata and S. guyanensis are suitable for the savannahs and the humid tropical rain forest, and Arachis pintoi is compatible with aggressive and stoloniferous grasses and is very persistent under heavy grazing. Pueraria phaseoloides and Stylosanthes capitata which animals select in the dry season also show potential for the tropical rain forest environment. Development of technologies for local seed production to supply legume based forage systems was important. Evaluations under grazing yielded results from 200 - 400 kg LW ha -1 in locations with dry-season stress, and up to 500 - 600 kg LW ha -1 in areas with no dry-season stress.
For other marginal areas, in the two storey vegetation of herbaceous plants and Acacia caven, the deciduous foliage of the trees replenished the soil minerals removed by the grazed herbaceous plants. Even with the lush growth of the trees, the herbs under that shading were luxuriant relative to those under full sunlight. The trees got most of the water they need from deeper soil layers, allowing more for the herbaceous vegetation that explores the upper layers of the soil. Lots of similarities are also found in the northeast within the "Caatinga" vegetation.
The spectacular increase in individual animal performance when grazing legume-based cultivated pastures compared with native savannah grassland deserves mention. It is hoped that legumes will contribute with low-cost nitrogen to the associated grass. This characteristic and the tolerance to low fertility and acid soils are always major themes of research and the first objectives pursued. The problem is that Brazilian grazing history with large herbivores is short and these legumes have, in general, very limited adaptations to tolerate heavy grazing, having more specialised escape mechanisms; this deserves more attention by forage breeders.
Managing Grazing Intensities
Grazing at optimum carrying capacity is a compromise between optimising
intake and individual animal production with animal production per hectare,
since intake decreases when available herbage per animal decreases. When
herbage allowance does not limit animal intake, animals with high yield
potential can express their growth to near maximum, without supplementation,
thus lowering production costs.
It has long been known that lower grazing pressure, or high herbage allowance, allows very high levels of animal performance from either natural or cultivated tropical pastures. Values range for natural pastures: 0.69 - 1.0 kg an-1 day-1 and for cultivated tropical pastures: 1.5 - 1.27 kg an-1 day-1. This should encourage grazing management practices that use lighter grazing intensities on native or tropical cultivated pastures, but unfortunately this is not the case in commercial situations.
Managing Grazing Intensities on Native Pastures
The natural grasslands of southern Brazil are the main feed resource for
an equivalent of 15,000,000 AU. Despite the large area occupied by natural
grasslands and its importance to animal production in Brazil, there is
a paucity of research about the ecosystem and its dynamics. Vegetation
in these areas is highly complex and characterized by a huge diversity,
with differences in structure, quality and metabolic pathway (C3 and C4).
In such an environment, tufted grasses increase their abundance according
to grazing pressure and can comprise more than 50% of ground cover at
low grazing intensities, and less than 5% at high grazing intensities.
Inter-tussock vegetation cover decreases from 100 to 67% when grazing
pressure increases from 8.8 to 25 kg LW/kg DM per day, although the actual
grazing intensity at the inter-tussock vegetation remains similar (Silva
& Carvalho, 2005).
The ranching philosophy of pasture utilization predominates in the use of natural pastures despite the strong support from research results. Grazing efficiencies greater than 50% occur at daily herbage allowance levels from 13 to 18 kg DM 100 kg LW-1 with peak efficiencies occurring from 6 to 9 kg DM 100 kg LW-1. Daily herbage allowance values below 20 kg AU -1 allow for 4.4 kg DM per 100 kg LW and restricted the intake of a 454 kg mature cow.
It has also been suggested that deferring grazing to recover or rejuvenate these native pastures and to increase the biomass productivity and the frequency of the desirable species for the site, would be beneficial. Reduction in stocking rate and sowing cultivated species is an alternative. By reducing stocking rate one slows down the rate of degradation but not the trend toward degradation, since the grazing animals will continue to overgraze the preferred species selectively within plant communities. This is an ever present phenomenon, and can be a critical one, where pasture biodiversity is not counterbalanced by different species of grazing animals. On the other hand, the introduction of cultivated species is feasible once there are provisions against the limiting factors that might jeopardise the new pasture, especially fertilization.
The distinct pasture canopies can range from prostrate forms of growth under heavier grazing pressures, to rank vegetation under more lenient management. Tufted grasses may represent a forage resource depending on the availability of preferred species, and so the concept of “forage” itself may be quite variable, and represents an additional challenge to the characterization of the grazing environment and to pasture utilization. An accumulation rate up to 16.3 kg DM ha -1 day -1 through the season for natural pastures maintained at an optimum stocking rate was determined to be at a herbage allowance of 13.5% LW per day, which was equivalent to maintaining the herbage mass at a level of 1,400-1,500 kg DM ha -1. The conversion efficiency of this ecosystem in capturing radiation energy from sunlight to transform it into primary production ranges from 0.20% for high grazing pressures to 0.36% at appropriate ones, which represented an 80% increase in system efficiency. This demonstrates the important improvements that can be made simply by managing this pasture ecosystem at the correct grazing intensity. These values and relationships mean that the productivity of these grasslands can be 100% increased at a cost of harvesting forage by the grazing animal, with no other energy input than thought.
Carvalho (unpublished) summarises the main current knowledge about natural pasture management in southern Brazil, as follows:
i) natural pastures have enormous plant diversity (400 grasses and 150 legumes);
ii) C4 grasses predominate and are responsible for the strong seasonal variations in productivity and forage quality;
iii) Moderate grazing intensities allow light interception and yield to reach their potential (herbage mass about 14000 kg dry matter/ha);
iv) Moderate grazing intensities allow secondary production to reach its potential (daily herbage allowance of 12 to 13 kg dry matter/ 100 kg LW);
v) Compared to the mean current productivity of the natural pastures in Southern Brazil, the use of adequate grazing intensities may triplicate animal production. It may reach 700 kg LW/day when adequate grazing intensities are associated with fertilization;
vi) Plant diversity is lower in the extremes of grazing intensities, therefore, moderate intensities promote biodiversity;
vii) This ecosystem is highly resilient;
viii) Higher grazing intensities lead to vegetation with prostrate types of plants, with a predominance of escape mechanisms to grazing. On moderate grazing intensities we can see a mixture of bushes, tussock grasses and prostrate species with a diversity of escape and tolerance mechanisms;
xi) High grazing intensities decrease organic matter, reduce the water infiltration rate, reduce soil cover, and reduce the availability of nutrients;
x) Native pastures respond positively to the removal of limiting factors, especially to fertilization. Moreover under such conditions C3 grasses and legumes may be introduced by overseeding on the natural pasture, to fill the forage gap in winter;
xi) Deferment is necessary for correct management;
xii) Use of fire reduces the system productivity and compromises the chemical and physical conditions of the soil;
xiii) The grazing method has little effect on animal production. Grazing intensity is the determinant variable;
Managing Grazing Intensities on Cultivated Pastures
In all environments it is necessary to find the best compromise in combining
three factors: plant growth, plant utilisation and animal performance,
to reach the maximum efficiency taking into account the production costs.
It is recognised that improved pastures have been shown to greatly increase rates of animal production. For example, very interesting was the assessment concerning Cenchrus ciliaris, introduced into the native pastures of the dry-NE Brazil, promoting a two fold increase in the LWG ha -1, but with no changes in the average daily gain per animal (ADG an -1).
When dealing with productivity the entire system has to be considered, with both seasonal and annual variations included. There is evidence that an increase in ADG an-1 under grazing is the best way to reduce animal production costs, showing that animals per hectare express the rate of stocking that allows for the optimum individual animal performance.
The results from central Brazil and the "Cerrados" indicate that persistency of cultivated pastures depends on initial soil fertility and lenient grazing management. Lenient grazing is important, but not enough for pasture sustainability. The decline in soil fertility status is the starting point of pasture degradation. The first symptom observed is the reduction in carrying capacity under equivalent forage allowance; the pasture regrowth does not acquire its previous status after resting; and the reduction in forage mass and quality reduces ADG an-1. Bare spots become visible in the pasture, weeds invade and some native species return. So, judicious monitoring of carrying capacity would indicate the beginning of the degradation process. When detected early it may cost 100US$ ha-1 to recover the area, while later detection raises costs to 200US$ ha-1. Reducing the stocking rate helps to maintain animal performance but does not overcome the trend in pasture degradation.
Another choice that is acquiring importance is the integration of crop and grassland agriculture within a complete system of land use and farm productivity. The annual crops will generate financial resources for improving the soil fertility level, thus reducing the costs of pasture establishment. Consistent results are showing new pastures with increased pasture potential for the region, where fertilized Panicum pastures are producing 740 kg LW ha -1 year -1, while the Brachiaria sp. reach a ceiling yield at 600 kg LW ha -1 year -1. These differences come from the higher ADG an -1 and higher number of steers carried by the Panicum, compared to Brachiaria pastures. For enterprises adopting low levels of technology, Brachiaria is the option, while for those adopting the existing high level of technology, Panicum pastures are recommended.
New experimental results with Brachiaria cultivars showed different rates of leaf growth for the wet and the dry seasons. Some ecotypes displayed higher leaf DM yield in the dry season while others performed better for the wet season, and were recommended to be tested for animal gains. They have already shown high pasture potential. Hopes rest on the animal potential of those new varieties.
The promotion of grazing based on the green leaf lamina dry matter (GLLDM), contributes to our knowledge and understanding of what happens in the pasture profile, to evaluate pasture dynamics and to watching the shaping up of the silhouette of a steer gaining weight and being finished on pasture under grazing. At low levels of GLLDM the animals graze more frequently, promote tiller density as well as invasion of other plant species, and adversely affect root mass, plant diameter, internode length, tiller weight, rate of accumulation of GLLDM and total GLLDM yield. It seems that heavy grazing does not help the pasture.
With clearly defined levels of pasture GLLDM management being applied to Mott dwarf elephantgrass, studies reveal that a sustainable optimum ADG of 1.043 kg an-1 at a herbage allowance of 10.5% LW day -1 of GLLDM yielding 1,188 kg LWG ha -1 can finish a slaughter steer within 210 days of grazing, under continuous stocking.
Based on the acquired knowledge in conducting grazing experiments to evaluate herbage allowance and animal and pasture responses, it is suggested that at least 1,500 - 2,000 kg ha-1 of live green leaf lamina DM of the forage mass is required from which the grazing animals will get their diet.
Sward targets: a recent orientation for grassland management (see Silva & Carvalho, 2005)
The tropical/sub-tropical environment is unique, requiring creative and site-specific solutions to overcome production constraints in order to realise its potential. The range of plant species, their varying size, morphology and physiology highlight the need to review some of the concepts and general views relating to animal performance from these pastures. Recent experimental work on pasture ecophysiology and grazing ecology in Brazil has been conceived under the conviction that control, monitoring and manipulation of sward state is an important feature of grazing management. This is very different from the traditional and simplistic view of production, in which control of the grazing process is made by means of fixed stocking rates, herbage allowances, grazing intervals and grazing method and allows for significant variation in sward state.
New work is in progress in Brazil for the main forage resources (e.g., Brachiaria, Panicum, Cynodon, Pennisetum, etc.) about how sward structure is built and how animals capture forage from these structures. As a consequence of this new knowledge, and aiming to optimize pasture production (i.e., light interception) and animal intake (i.e., animal performance), some very recent sward guidelines are being recommended, some of which are illustrated below:
i) In continuous stocking, B. brizantha should be managed between 20-40 cm height during the growing period, and around 15 cm just before and during winter. When using rotational grazing, a pre-grazing sward height of 25 cm and residual herbage height about 10-15 cm are desirable, according to different categories and objectives;
ii) P. maximum cv. Mombaça, in rotational grazing should be managed with a pre-grazing sward height of 90 cm while for P. maximum cv. Tanzânia the target is 70 cm for pre-grazing sward height. For both cultivars the recommended residual herbage height is 30-50 cm, according to different categories and objectives;
iii) Cynodon cultivars when used in continuous stocking, should be managed between 10 and 20 cm height;
iv) Pennisetum glaucum when used in continuous stocking, should be managed around 30 cm height;
v) Annual temperate pastures, such as Lolium multiflorum and Avena strigosa when used in continuous stocking, should be managed around 15 and 25 cm height;
vi) Natural pastures used in continuous stocking maximize animal intake when inter-tussok areas are managed around 13 cm height;
Recent evidence generated under these conditions demonstrates that well fertilized and managed tropical/sub-tropical forage species can produce herbage of sufficiently high quality to ensure satisfactory animal performance throughout the year. Low quality herbage during the winter period can be a result of inefficient harvest during previous favourable growing seasons and is not necessarily an intrinsic characteristic of the herbage produced. Additionally, non-nutritional factors (e.g., sward structure) have a greater relative importance than nutritional factors regulating herbage intake of grazing animals.
Animal production systems for tropical/sub-tropical pastures have an additional and significant constraint, i.e. the pronounced seasonality of herbage production. This generates variation in the feed supply-demand balance of the system between and within seasons of the year, which must be managed if sward control is to be achieved effectively. Management practices have direct and indirect impacts on sward control, structure and animal performance that need to be known in order to allow for the correct planning and decision making process on a farm scale. Consequently, the current research and the management orientation for tropical/sub-tropical grasslands are being based on the careful control and planning of sward state.
Pasture Fertilization
It is well documented and recognized that the levels of available P in
the soils are extremely low. The use of fertilizers to increase DM and
quality of native pasture production has taken too long to be accepted
by those involved with livestock production; cultivated pastures were
fertilized at establishment, but not for maintenance.
In a classical experiment done in southern Brazil, phosphorous was applied broadcast in a natural pasture (160 kg ha-1), and continuous versus rotational grazing was under evaluation. After 11 years of grazing, there was a 10% increase in livestock production from rotational grazing compared with continuous stocking. However, the outstanding response was to P fertilization, which showed an increase in productivity of around 4.95 kg LWG ha-1 kg-1 of applied P. After 11 years the fertilized natural pasture was producing 70% more than the unfertilized natural pasture. No doubt, there was a very lasting and contributing effect of the P application.
To illustrate the possibility of changing botanical composition by fertilizers and altering the quality status of pastures, a trial has shown that Desmodium incanum increases from 3.3% up to 24.4% in a pasture as a result of high phosphorus applications in southern Brazil. With regard to nitrogen, when a natural grass such as Paspalum notatum is fertilized and water doesn’t limit growth, pasture production reaches 12.0 tons DM ha -1, and places doubt on the necessity for cultivated pastures. This information has had a considerable impact on the southern research programmes resulting in the promotion of native pasture fertilization at medium levels for pasture sustainability.
Maintenance of soil nutrient status is a key variable in pasture sustainability. There are sharp residual effects of P fertilization on forage DM production of subtropical pasture mixtures, with increased legume contribution as the P levels are increased. However, the P levels in the soil are gradually being reduced due to extraction by plants when there is no P replacement, and after five years phosphorous can be reduced to 1/5 of what it was at the beginning.
Annual fertilization of a mixture of guinea grass, Siratro, Glycine wightii and Stylosanthes guianensis on the other hand, kept that pasture productive for more than ten years. The annual application of 20 kg ha-1 of P helped in maintaining pasture production while the application of 40 kg ha-1 of P increased the pasture production by 30%, whereas under no fertilization production dropped at a rate of 15% annually. Panicum spp. were also identified as responsive to fertile soils, where high levels of P were applied, while Stylosanthes capitata, Stylosanthes guianensis and Zornia spp. with Brachiaria humidicola, Hyparrhenia rufa and Andropogon gayanus were identified as species with good performance on soils with low P supply. Researchers thus emphasized the development of a philosophy of pasture productivity to take advantage of fertilizer use and applied knowledge in pasture management all over the country to benefit from the improvement brought about by the fertilizer applications.
In fact, farmers do not frequently adopt fertilizer use. A 1997 survey revealed that only 663,000 tons of NPK fertilizers were annually applied to 90,000,000 ha of introduced pastures in Brazil, i.e. about 7.4 kg of NPK fertilizer/ha of pasture per year. Results from grazing trials conducted in eastern and central Brazil, in the "Cerrados" revealed that fertilization increased yield of cultivated pastures from 150 to 400 kg ha-1, irrespective of the final animal product. The search for cheap sources of nitrogen is suggested to be of paramount importance to supply pasture ecosystems and obtain livestock production at low cost.
The efficiency of P fertilization is 4.6 kg LW ha-1 kg-1 of applied P, while for N fertilization it is 1.6 to 2.0 kg LWG ha-1 kg-1 of applied N (reported for tropical pastures in central Brazil). The magnitude of responses is associated with the stocking rate, experiencing sharp declines in LWG ha-1 as the stocking rate is increased.
Supplements ( see Corsi et al., 2001)
The high herbage accumulation rate in tropical grasses favours stockpiling
practices. On the other hand, the use of stockpiling practice limits animal
productivity for quantity and quality reasons. Long regrowth periods without
grazing (or cutting) are prone to excessive herbage losses and in extreme
cases the forage damps-off. It is also well established that long regrowth
periods are deleterious to forage quality given the reductions on the
potential pasture intake due to the nutritive value of the forage.
To better utilize the standing low quality forage some farmers are feeding protein supplements (0.1% of animal live weight) containing ionophores to correct nutritional deficiencies in winter (dry and cold). While some nutrients are indeed corrected, results seem to indicate live weight gain is primarily additive rather than the supplement.
Grazing animals can also benefit from feeding supplements during the summer/autumn period. Supplementation at that time of the year aims to improve individual performance as well as the output of animal products per unit area. In dairy enterprises this strategy has been widely used and it seems that a similar average response of 1.4 kg of milk/kg of concentrate supplementation might be expected for lactating cows on temperate and tropical grasses.
For beef cattle better responses to concentrate (and possibly degradable fibre) supplement feeding seem to occur in late summer/autumn compared to the beginning of the grazing season. At this time supplemented animals grazing tropical grasses are expected to show live weight gains above 1 kg/head/day, though feed conversion is a function of the supplement type. Information regarding the synchronism between carbohydrate and protein fractions in the rumen and, consequently, the substitution of pasture DM for concentrate DM, and the high concentrate prices in tropical regions seem to be the most limiting factors restraining the adoption of supplementation programmes on the best-managed farms.
During the (wet and warm) summer, tropical pastures can support high stocking rates, peaking up to 15 AU/ha. This carrying capacity is much lower during winter, averaging 10 to 40% of summer values. Therefore the intensification of tropical pasture-based systems in summer should consider the use of conserved forages or by-products in winter to guarantee the balance between food supply and demand during the whole year. Options such as silage (maize, sorghum or perennial tropical grasses), hay (perennial tropical grasses), and several by-products (sugarcane, citrus, brewer’s grain, etc.) are available.
Feedlots during the winter are also a possibility to allow high carrying capacities on tropical grass pastures in summer. However, feedlots are economically questionable when practised on a small scale, and with high grain prices and low availability of by-products.
Recently, irrigated tropical pastures have been used to enhance carrying capacity for both beef and dairy enterprises. Production costs have been reported of about US$ 0.9/kg carcass weight in irrigated tropical pastures at the same time as selling prices were US$ 1.36/kg carcass weight. Cost analysis studies indicate that adoption of irrigated tropical pasture depends on production costs in the dry pasture system, overall production costs and selling price and the increase in productivity in the irrigated system.
Below are presented different views from research and extension, which give an idea of future possible scenarios for cattle production systems and their potential in Brazil. The following conclusion were drawn by Corsi et al. (2001):
1. Animal production from tropical pasture-based systems has a high plant and animal productivity potential when soil fertility is adequate (e.g., fertilizers are used) and more than 25,000 kg milk/ha/year and 900 kg liveweight gain/ha/year can be obtained.
2. Technology transfer in these systems is constrained because animals and land are used as a capital reserve and animal products (milk and meat) in these situations are by-products rather than products. Economic policies toward cereal/legumes crops instead of high-cost cattle enterprises also constrain uptake of technology. This frequently makes smallholder livestock farmers unable to adopt the necessary technology to efficiently and intensively manage beef and dairy enterprises. On the other hand, capitalized farmers and companies having already reasonable economic outputs in extensive systems have no interest to adopt more intensive ones.
3. Stockpiling practices and grass-legume mixtures are pasture management options when stocking rates are low, generally up to 1.2 - 2.0 AU/ha/year. Stocking rates above this limit are necessary to efficiently use the high herbage growth rate potential of tropical pastures. Higher stocking rates during the summer should consider the use of rotational grazing and the need to plan winter feeding, e.g. the utilization of conserved forages, feedlots, or irrigated tropical pastures.
4. Tropical grass management (mainly for tussock-forming grasses) seems to have different characteristics compared to temperate pasture management. As herbage accumulation rates in tropical species increase above 100 days, tropical pasture management should be oriented toward forage quality and/or pasture structure.
5. The supplementation of grazing animals in summer/autumn periods in tropical environments can be a valuable tool to increase the individual animal performance and the overall pasture output.
6. The use of fertilizers seems to be a pre-requisite to sustain high pasture productivity. Practices that increase the pasture productivity while maintaining the environment quality should be emphasized.
With regard to pastures, according to Zimmer and Euclides (1997) it is necessary to develop better quality forages, which can provide a better animal performance in favourable growth periods as well as in drought. Special emphasis should be given to increase production in the rainy season when the conditions are better, improving animal performance in this way, where the available tropical pasture at present does not fulfil the genetic potential for animal gain. In dry periods the nutritional needs of some animal categories should be met using alternatives for feed supplementation, since in this period forages do not meet animal needs. It is essential that genetic improvement of forages focus on the search of legumes for animal diet improvement and, particularly, for biological nitrogen fixation. Biological N fixation is important for sustainability and for the reduction of possible environmental damage.
According to the National Agriculture Confederation [see CNA] the main demands from the beef cattle enterprise are:
Technological demands
(a) Genetics: selection for desirable characteristics, like precocity and biological efficiency, particularly concerning live weight and carcass; studies about volume and quantity of meat produced; production of steers with monounsaturated fat; obtaining animals with greater live weight gain in less time.(b) Nutritional management: production costs reduction by low-cost ratios; reduce problems concerning loss of live weight on drought periods.
(c) Production of younger steers: comparative studies (younger animals/ordinary animals, carcass output, flavour, tenderness, juiciness and acceptability by consumers.
(d) Characterisation of buffalo meat "in natura": sensorial analyses by sex and animal age, concerning tenderness, flavour, visual quality, etc., and its acceptability by consumers; studies on meat stability during storage.
(e) Behaviour of diseases at farm level and necessity for its effective control: research efforts based on actual data.
Non-technological demands
(a) Evaluation of generated products: market research to better know the final consumers.(b) Definition of consumers’ health objectives.
(c) Production of younger steers: quantitative analysis to quantify market and classical economic analysis.
(d) Animal husbandry: diagnostics of reproduction.
(e) Re-dimensioning of taxes in all components of the meat chain.
(f) Improvement of the laboratories of quality control: inputs and products.
(g) Increase relationship and co-ordination along the chain.
(h) Increase efficiency of international affairs to negotiate better agreements to the meat chain.
7. RESEARCH AND DEVELOPMENT ORGANIZATIONS AND PERSONNEL
Several organizations, mainly public, are involved in agricultural research and development. On a national scale, Federal Universities, EMBRAPA and EMATER are important institutions. At State level, we should consider the efforts of public Universities and/or Research Institutes financed by each state. Private Universities are not consolidated concerning research. Investment in scientific and technological research arising directly from Government or from research foundations can be assumed to amount to some 80 percent of the total.
The main overall research efforts are being made in the following areas: plant and animal breeding with the main focus on productivity and capacity to cope with harsh environments; technologies to rehabilitate degraded areas; grazing management focusing on intensive systems, animal nutrition focusing on feed-lot systems.
Private enterprises have recently made efforts, notably on animal genetics and plant breeding. Some of them do development work and assist farmers as a market strategy.
Some key persons and their research areas are presented (this list is not intended to be exhaustive nor definitive and it should be viewed only as a way to start contacts in the following areas):
| Topics |
Names |
e-mail address |
| Grassland management |
Moacir Corsi |
|
| Rangeland management |
Gerzy E. Maraschin |
|
| Tree-pasture association systems |
João Carlos de Saibro |
|
| Temperate forage breeding |
Miguel Dall’Agnol |
|
| Tropical forage breeding |
Cacilda Borges do Valle |
|
| Forage collection and characterization |
José F. M. Valls |
|
| Forage cytogenetics |
Maria T. Schifino-Wittmann |
|
| Native forage species flora |
|
|
| Pasture reclamation |
Moacyr B. Dias Filho |
|
| Forage conservation |
Clóves C. Jobim |
|
| Pasture utilization |
Sila C. da Silva |
|
| Ecophysiology of grasslands |
Carlos Nabinger |
|
| Pasture fertilization |
Francisco A. Monteiro |
|
| Forage quality |
Harold O. Patiño |
|
| Tropical pasture seed production |
Ronaldo P. de Andrade |
|
| Temperate pasture seed production |
Lúcia B. Franke |
|
| Forage seedbanks |
Renato B. de Medeiros |
|
| Forages in cropping systems (Cerrados) |
Manuel C. M. Macedo |
|
| Forages in cropping systems |
Anibal de Moraes |
|
| Forages in dairy cattle systems |
Duarte Vilela |
|
| Forages in beef cattle systems |
Ademir H. Zimmer |
|
| Forages in small ruminant systems |
César H. E. C. Poli |
|
| Forages in horse systems |
João R. Dittrich |
|
| Amazonian forage system |
Antônio P. de Souza |
|
| Cerrados forage system |
Valéria P. Euclides |
|
| Caatinga forage system |
Magno J. D. Cândido |
|
| Pantanal forage system |
Sandra Aparecida Santos |
|
| Mata Atlântica forage system (northern part) |
Domício Nascimento Jr. |
|
| Mata Atlântica forage system (southern part) |
Ricardo A. Reis |
|
| Pampa forage system |
Aino V. A. Jacques |
Key institutions
Empresa Brasileira de Pesquisa Agropecuária: www.embrapa.br
EMATER : www.emater.tche.br/
Universidade Federal do Rio Grande do Sul: www.ufrgs.br
Universidade Federal de Viçosa: www.ufv.br
Universidade Federal de Lavras: www.ufla.br
Universidade Federal Rural do Rio de Janeiro: www.ufrrj.br
Universidade de Brasília: www.unb.br
Universidade de São Paulo: www.usp.br
Universidade Estadual Paulista: www.unesp.br
Universidade Estadual de Maringá: www.uem.br
Instituto de Zootecnia: www.iz.sp.gov.br
Fundação Estadual de Pesquisa Agropecuária: www.fepagro.rs.gov.br
Sociedade Brasileira de Zootecnia : www.sbz.org.br
Conselho Nacional de Pesquisa e Desenvolvimento Tecnológico. www.cnpq.br
Coordenadoria de Aperfeiçoamento de Pessoal de Nível Superior:www.capes.gov.br
Allem, A.C., Valls, J.F.M. 1987. Recursos forrageiros nativos do Pantanal Mato-grossense. Embrapa, 339 p.
Andrade, R. P., Barcellos, A. O., Rocha, C.M.C. 1995. Pastagens nos Ecossistemas Brasileiros: Pesquisas para o Desenvolvimento Sustentável. In: XXXII Reunião Anual da Sociedade Brasileira de Zootecnia, Brasil, Proceedings, 200 p.
Assis, A.G.1997. Milk production under grazing in Brazil. In: International Symposium on Animal Production under Grazing, Viçosa, Proceedings, p.31-59.
Atlas Climatológico do Brasil. 1969. Ministério da Agricultura, Rio de Janeiro. 100 p.
Brazil. First National Report for the Convention on Biological Diversity. http://www.biodiv.org
Cândido, M.J.D., Araújo, G.G.L., Cavalcante, M.A.B. 2005. Pastagens no ecossistema semi-árido brasileiro: atualização e perspectivas futuras. In: Simpósio sobre pastagens nos ecossistemas brasileiros: alternativas viáveis visando a sustentabilidade dos ecossistemas de produção de ruminantes nos diferentes ecossistemas, 2005, Goiânia, Proceedings, CD-Rom.
Confederação Nacional da Agricultura. http://www.cna.org.br/cna/index.wsp
Cordeiro, A. 2000. Sustainable Agriculture in the Global Age: Lessons from Brazilian Agriculture. Swedish Society for Nature Conservation - Report. 28p.
Corsi, M., Martha Jr., G.B., Nascimento Jr., D., Balsalobre, M.A.A. 2001. Impact of grazing management on productivity of tropical grasslands. In: XIX International Grassland Congress, Brazil, Proceedings, p.801-805.
Dias-Filho, M.B., Andrade, C.M.S. 2005. Pastagens no ecossistema trópico úmido. In: Simpósio sobre pastagens nos ecossistemas brasileiros: alternativas viáveis visando a sustentabilidade dos ecossistemas de produção de ruminantes nos diferentes ecossistemas, 2005, Goiânia, Proceedings, CD-Rom.
Instituto Brasileiro de Geografia e Estatística. http://www1.ibge.gov.br/
Joly, C.A., Aidar, M.P.M., Kilnk, C.A., McGrath, D.G., Moreira, A.G., Moutinho, P., Nepstad, D.G., Oliveira, A.A., Pott, A., Rodal, M.J.N., Sampaio, E.V.S.B. 1999. Evolution of the Brazilian phytogeography classification systems: implications for biodiversity conservation. Ciência e Cultura, v.51, p. 331-348.
Macedo, M.C. 1997. Sustainability of pasture production in the savannahs of tropical America. In: XVIII International Grassland Congress, Canada, Proceedings, p.5-15.
Macedo, M.C. 2005. Pastagens no ecossistema Cerrados: evolução das pesquisas para o desenvolvimento sustentável. In: Simpósio sobre pastagens nos ecossistemas brasileiros: alternativas viáveis visando a sustentabilidade dos ecossistemas de produção de ruminantes nos diferentes ecossistemas, 2005, Goiânia, Proceedings, CD-Rom.
Maraschin, G. E. 2001. Production potential of South American grasslands. In: XIX International Grassland Congress, Brazil, Proceedings, p.5-15.
Maraschin, G. E., Jacques, A.V.A. 1993. Grassland opportunities in the subtropical region of South-America. In: XVII International Grassland Congress, NZ-Australia, Proceedings, p.2014-2015.
Ministério do Meio Ambiente. 2000. Avaliação e ações prioritárias para a conservação da biodiversidade da Mata Atlântica e Campos Sulinos. 40p.
Nabinger, C., Moraes, A., Maraschin, G. 2000. Campos in Southern Brazil. In: Grassland Ecophysiology and Grazing Ecology. CABI. p.355-376.
Pallarés, O.R., Berretta, E., Maraschin, G.E. 2005. The South American Campos ecosystem. In: Suttie, J.M., Reynolds, S.G., Batello, C. Grasslands of the world, FAO, p.171-220.
Peixoto, A.M., Moura, J. C., Faria, V.P. 1986. Pastagens na Amazônia. In: Congresso Brasileiro de Pastagens, Piracicaba, Proceedings, 99 p.
Peixoto, A.M., Moura, J. C., Faria, V.P. 1986. Simpósio sobre Manejo da Pastagem. In: Anais do Congresso Brasileiro de Pastagens, Piracicaba, Proceedings, 542 p.
Santos, S.A., Crispim, S.M.A., Comastri Filho, J.A. Pastagens no ecossistema Pantanal: manejo, conservação e monitoramento. In: Simpósio sobre pastagens nos ecossistemas brasileiros: alternativas viáveis visando a sustentabilidade dos ecossistemas de produção de ruminantes nos diferentes ecossistemas, 2005, Goiânia, Proceedings, CD-Rom.
Silva, S. C. ; Carvalho, P. C. F. 2005. Foraging behaviour and herbage intake in the favourable tropics/subtropics. In: Mc Gilloway, D. A. (Org.). Grassland: a global resource. Wageningen, p. 81-96.
Zimmer, A.H., Euclides Filho, K. 1997. Brazilian pasture and beef production. In: International Symposium on Animal Production under Grazing, Viçosa, Proceedings, p.1-30.
Zimmer, A.H., Macedo, M.C.M., Kichel, A.N., Euclides, V.P.B. 2004. Integrated agropastoral production systems. In: Guimarães, E.P., Sanz, J.I., Rao, I.M. et al. (Eds.). Agropastoral systems for the tropical savannas of Latin America, CIAT, p.253-290.
| This profile was prepared by Paulo
César de Faccio Carvalho. His research area is the plant/animal
relations and integrated crop-livestock systems and since October
1997 he has lectured at the Faculdade de Agronomia, Universidade
Federal do Rio Grande do Sul, in Porto Alegre - RS, in Southern
Brazil. |
|
To prepare a Country Pasture Resource Profile for such a large and diverse country as Brazil has not been a simple task. The author wishes to thank important contributors and has much appreciated the comments received from various reviewers. Contributions are still welcome. Arrangements are being made for local revision and updating.
[The profile was prepared in April/May 2002 and was edited by S.G. Reynolds and J.M. Suttie in June 2002; it was further modified by S.G. Reynolds in October 2005, and a major revision undertaken by Paulo C. de F. Carvalho in January, 2006].