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Chapter 8
Management of Silvopastoral Resources (continued)

Agroforestry System Combining P. juliflora and Buffel Grass in the Brazilian Semi-Arid Region

Preliminary Results

Jorge Ribaski
Forester, embrapa
Agriculture and Livestock Research Center for the Semi-Arid Tropic
Petrolina

Introduction

The exploitation of the plant resources in the Brazilian semi-arid region boils down to extracting timber for farming use and for commercial purposes, as well as to using native fodder species for feeding livestock.

This exploitation is performed without any management criteria, thus bringing about degradation of the plant cover.

Cattle farming, predominant in the region, shows low productivity as a result mainly of the scant fodder available during the dry season. During this period, the pastures cultivated with grasses, mainly buffel grass (Cenchrus ciliaris L.) frequently do not offer a protein level sufficient for the animals to maintain or gain weight.

Prosopis juliflora (Sw) dc is grown in the region as a fodder tree and for afforestation. The advantages this xerophyte offers for afforestation are its precociousness, drought hardiness, good-quality wood for a variety of end uses, production of highly palatable and nutritive pods, and bearing of fruit during the dry season.

The use of this leguminous tree in afforestation for timber and fodder production purposes through silvopastoral systems constitutes an economically and socially important alternative for the region.

The purpose of this research was to study the technical viability of a silvopastoral system combining P. juliflora and C. ciliaris cv. Gayndah.

Review of Literature

In silvopastoral systems, it is important to stress the fact that the tree, on account of the functions it performs, must be the basic structural element. The tree component constitutes an important soil stabilization factor, as it affords protection against direct raindrop impact, sunlight, water runoff and wind erosion, minimizing the damage caused by leaching (Galvão, 1978). In these systems, the tree cover can modify the microclimate, allowing better nutrient recycling by natural processes, through organic matter originated from dead plants and animal faeces (Weaver, 1979).

According to Azevedo (1982), Prosopis juliflora in association with grasses produces beneficial effects. Soil temperatures are lower as a result of shading, reducing thereby the humus oxidation rate. Leaf litter and root nodules contribute to increasing nitrogen content and of other minerals in the soil.

When the tree cover is not very dense in terms of canopy, thereby permitting solar radiation to reach the ground, as in the case of P. juliflora, the grasses beneath the canopy keep their protein levels longer and are more digestible than those out in the open (Karlin and Ayerza, 1982).

Christie (1975) reports that cultivation of Cenchrus ciliaris cv. Gayndah under an overstory of adult Eucalyptus populnea in low-fertility soils of the semi-arid region of Queensland, Australia, produced a threefold increase in dry matter yield as compared to that of grass planted among the trees, outside the natural micro-habitat existing beneath the trees. This was explained by the different concentration of phosphorus in each case. The forest species contributed, under these conditions and through nutrient recycling, to increased soil fertility, particularly as regards phosphorus content.

The presence of a tree component in silvopastoral systems contributes to reducing evapotranspiration from the soil and leaves of plants cultivated in combined crops, as well as reducing extreme temperature oscillation. By reducing evapotranspiration, the water economy of the system is generally improved, a very important factor in regions affected by scant water resources.

Theoretically, better use of water and soil nutrients can be achieved with a silvopastoral system when the combined species have different growth habits, i.e. water consumption peaks at different times of the year and different nutritional requirements. This benefit can also result from the arrangement of the root systems in the different strata of the soil profile, enabling better water tapping and nutrient uptake.

Work performed at the Brazilian semi-arid region found that the introduction of Cenchrus ciliaris cv. Gayndah into an area planted with Eucalyptus camaldulensis and Mimosa caesalpiniaefolia was not significantly detrimental to either growth or survival rates of the tree species. On the contrary, this intercropping system contributed to a better utilization of the soil's productive capacity, offering greater fodder supply to animals during the dry season (Ribaski, 1985).

The forest stocking rate in silvopastoral systems bears upon the greater or smaller fodder output and, consequently, on the grazing pressure to be exerted on the area. As the forest develops, integration with animals can become problematic, as the fodder supply decreases in line with the increase in basal area of the plantations (Clary et al., 1975)

Another important factor to be considered in intercropping is competition which, in global terms, refers to a decrease in the availability of total water, nutrients, light, carbon dioxide, etc., for every individual. Competition between crops and weeds is more serious when the crop is young, and is stronger among individuals or species with similar characteristics in terms of growth habit, production, etc. This competition decreases when the requirements of each species are different (Oliva, undated).

The use of silvopastoral systems must be tailored to every local bioclimatic condition, and establishing one requires basic knowledge on the species to be used for intercropping. If not managed properly, they can bring about failure of the whole system.

Material and Methods

This research project was carried out in an experimental plot belonging to the Agriculture and Livestock Research Center for the Semi-Arid Tropic (cpatsa), of the Agriculture and Livestock Research Agency (embrapa) in Petrolina, Pernambuco.

Geographical location

The experimental area is located at the Petrolina district, Pernambuco, at 09° 09' south lat., 40° 22' west long., at an altitude of 365.5 m above sea level.

Trial establishment

The trial was established in January 1983, with a wholly randomized linear experimental layout with five replications and four treatments:

  1. P. juliflora planted in an area free of buffel grass, using normal cultural procedures.
  2. P. juliflora planted in an area containing buffel grass, hoeing a 2-m-diameter clear space around the seedling.
  3. P. juliflora planted in an area with buffel grass, hoeing a 1-m-diameter clear area around the seedling.
  4. P. juliflora planted in an area with buffel grass, without any cultural practices.

Buffel grass establishment

Buffel grass was sown in November 1981, at the onset of the rainy season, in fifteen 192-m2 plots (12 m × 16 m), leaving the remaining five plots, of equal size, without sowing. Sowing was made manually in furrows and at a spacing of 0.5 m, using 10 kg of seeds per hectare, applying no fertilizer.

P. juliflora planting

After buffel grass establishment, P. juliflora was planted using seedlings raised in the cpatsa forest nursery, previously selected with a view to the greatest homogeneity in terms of healthiness and vigor.

Fertilization and phytosanitary treatments were performed at planting, applying 100 kg of npk in 5–14.3 proportion and 100 g of aldrin, to prevent termite attack. Both the fertilizer and the prophylactic element were applied in pits dug for the purpose in plots with and without buffel grass.

The spacing used was 3 × 4 m, with 16 seedlings per experimental unit, but only the four central ones were measured later for statistical analysis purposes.

Cultural practices

Approximately 20 days after P. juliflora planting, cultural practices were carried out, with manual hoeing and weeding, with the purpose of defining the treatments set forth. Thereafter, systematic hoeing was practiced every four months.

Data collection

Data collection for P. juliflora was made quarterly starting on the date of trial establishment and up to twelve months of age. Thereafter, measurements were made at 19 and 30 months of age, when the trees were felled to quantify the biomass produced. In all these periods survival assessments were made, measuring also collar diameter, height, crown diameter and number of stems and branches up to ⅓ of total plant height.

The buffel grass was mown thirty months after trial establishment, measuring its biomass.

Some of the mineral elements contained in buffel grass and P. juliflora biomass were also measured, performing also a water balance.

Results and Discussion

P. juliflora behavior during the first year

Table 1 shows the quarterly survival rates for P. juliflora, up to 12 months of age. Table 2 shows the dendrometric data of the species, at three and six months of age.

TABLE 1
P. juliflora Survival Rates 3, 6, 9, and 12 Months After Establishment in an Area with Buffel Grass, Using Different Cultural Practices

TreatmentsSurvival (%)
Three monthsSix monthsNine monthsTwelve months
Normal cultural practies    
(Area without buffel grass)100100100100
2-m-diameter hoeing    
(Area with buffel grass)100959085
1-m-diameter hoeing    
(Area with buffel grass)100503020
No cultural practies    
(Area with buffel grass)10010105

TABLE 2
Mean Values for P. juliflora Collar Diameter (Cd), Height (H), and Crown Diameter (CD) at 3 and 6 Months of Age

 TreatmentsThree monthsSix months
Cd
(cm)
H
(cm)
CD
(cm)
Cd
(cm)
H
(cm)
CD
(cm)
Normal cultural practices      
(Area without buffel grass)1,4921292,1105225
2-m-diameter hoeing      
(Area with buffel grass)0,873671,08288
1-m-diameter hoeing      
(Area with buffel grass)0,561300,56126
No cultural practices      
(Area with buffel grass)0,458160,45817

Figure 1 illustrates the water balance for the 12-month period, beginning on the date of trial establishment.

From the water balance data, it may be seen that total rainfall in 1983 was 553 mm, concentrated in January, February and March. Thereafter, a marked water deficiency period set in, lasting until year's end. It eased off somewhat in November, when a total of 100 mm rainfall occurred. The trial area soil had sufficient moisture storage capacity to make moisture in its profile available for plant use during the months of April, May and June. That year was characterized by absence of hydric excess.

Figure 1

Figure 1. Water balance of the experimental site in 1983.

Comparing the data in Table 1 with the water balance (Figure 1), it may be seen that as the water availability in the soil decreased, P. juliflora survival rates were also affected. With the exception of the treatment were P. juliflora was not associated with buffel grass, all the other treatments exhibited a drop in survival rates, particularly in the treatment exempt from cultural practices.

At three months of age, 100% of the initial seedlings still survived in all treatments. This was also the period with the highest rainfall. However, the dendrometric data in Table 2 show that in spite of the cultural practices performed on the associated crops, the maximum increases attained by P. juliflora were always lower than those for P. juliflora planted alone. The same table shows that, between three and six months of age, the parameters measured for P. juliflora in the treatments less favored with cultural practices exhibited no increase.

Table 3 shows the effect of the treatments on survival rates and P. juliflora development in terms of collar diameter, height, crown diameter, number of stems and number of branches up to ⅓ of total plant height, at thirty months of age.

The analysis of the data in Table 3 show that single P. juliflora keeps 100% survival at 30 months of age. When associated with buffel grass, only those seedlings where hoeing had been performed in a 2-m-diameter area around the plant survived, with 15% mortality. Despite the high survival rate in this treatment, the data obtained from measuring the different parts of the plant were always smaller and statistically different from the treatment where P. juliflora was planted alone, with the exception of the number of stems.

Table 4 shows dry matter production for P. juliflora and buffel grass and the amounts of nitrogen produced 30 months after trial establishment.

TABLE 3
Mean Values for P. juliflora Survival (S), Collar Diameter (Cd), Height (H), Crown Diameter (CD), Number of Stems and Number of Branches up to ⅓ of Total Tree Height, at 30 Months of Age

TreatmentsS
(%)
Cd**
(cm)
H*
(m)
CD**
(m)
Number of stemsNumber of branches
Normal cultural practices      
(P. juliflora planted alone)1008.0 a3.78 a3.89 a2.2 a14.4 a
2-m-diameter hoeing      
(P. juliflora × buffel grass)  854.9 a2.58 b2.58 b2.0 a  7.6 b
1-m-diameter hoeing      
(P. juliflora × buffel grass)   0
No cultural practices      
(P. juliflora × buffel grass)   0

* & ** Figures followed by different letters are significantly different as per T test at 5% and 1% probability level, respectively.

TABLE 4
P. juliflora and Buffel Grass Biomass Production and Amount of Nitrogen Exported Under the Different Treatments

 TreatmentsDry matter (kg/ha)Nitrogen (kg/ha)
P. julifloraBuffelTotalP. julifloraBuffelTotal
P. juliflora planted alone      
(Normal cultural practices)17,66917,669160.4160.4
P. juliflora × buffel grass      
(2-m-diameter hoeing)2,7578,73611,49327.059.886.8
P. juliflora × buffel grass      
(1-m-diameter hoeing)9,8719,87170.470.4
P. juliflora × buffel grass      
(No cultural practices)10,05010,05059.059.0
Buffel grass planted alone*7,3737,37344.044.0

* Data gathered at an area adjacent to the experimental plot.

The results obtained for P. juliflora biomass production (Table 4) evidence the sensitivity of this species to competition when associated with buffel grass. Meanwhile, in nutritional terms, nitrogen does not appear to be one of the limiting elements for P. juliflora development in this association, as, despite the differences in the amount of this element produced by P. juliflora planted alone (160.4 kg/ha) and in association (27.0 kg/ha), the proportion of N for both biomasses was similar, staying around 1%.

The production of buffel grass biomass (Table 4) follows certain logic; the smaller the clearing around P. juliflora, until total absence of any cultural procedure, the greater the buffel grass biomass production, resulting from the increasingly greater area it then occupies.

The treatment with no cultural practices showed the highest dry matter output (10,050 kg/ha), but P. juliflora presented 90% mortality after the first six months (Table 2). The proportion of N found in the biomass was similar to that found in the plot with buffel grass alone.

In the combined crops with clearings 1 and 2-m in diameter around P. juliflora, respectively, the proportions of nitrogen found in the biomass were higher that in the trial with no cultural procedures.

The findings presented in Table 4 suggest that P. juliflora contributed considerably to increasing the N content in buffel grass biomass, thereby increasing its nutritive value.

General Considerations

It may be seen from these findings that P. juliflora showed sensitivity to competition when cultivated in association with buffel grass. This competition appears to be more marked during the first year after plantation establishment, mainly for the water available.

Other variables are under study, such as soil moisture content and fertility, basal coverage of buffel grass, nutrients produced by P. juliflora and buffel grass biomass, and the root system of both species, with the purpose of better explaining P. juliflora performance in intercropping and verifying the feasibility of this silvopastoral system.

References

azevedo, c.f. de, 1982: “Potencialidade da algaroba no Nordeste,” In: Algaroba, Empresa de Pesquisa Agropecuária do Rio Grande do Norte, Natal, rn, pp. 283–299.

christie, e. k., 1975: “A note on the significance of Eucalyptus populnea for buffel grass production in infertile semiarid rangelands,” Tropical Grasslands, 9 (3): 243–6.

clary, w.p.; kruse, w. h. and larson, f. r., 1975: “Cattle grazing and wood production with different basal areas of Ponderosa pine,” Journal of Range Mangement, 28 (6): 434–437.

galvao, a.p.m., 1978: “Agrosilvicultura,” In: Proceedings, Encontro Nacional does Reflorestadores, 5. Canela, pp. 27–28.

karlin, u.o. and ayerza, r., 1982: “O programa da algaroba na República Argentina,” In: Algaroba, Empresa de Pesquisa Agropecuária do Rio Grande do Norte, Natal, rn, pp. 146–197.

oliva, m.: “Ecofisiología vegetal,” (no further data) 250 p.

ribaski, j., 1985: “Pesquisa agro-silviculturais em andamento na região semi-árida brasileira (resultados preliminares),” In: Florestas plantadas no Neutrópico como fonte de energia, Proceedings of Symposium ufv/mab/iufro; iufro si, Working Group, February 1983. Universidade Federal de Viçosa, Minas Gerais, pp. 286–295.

weaver, p., 1979: “La agri-silvicultura en la América Tropical,” Unasylva 31 (126): 2–12.

Management of Prosopis in Livestock Production Systems in the Dry Chaco, Argentina

Ricardo Ayerza
Agronomist, Coordinator for the Argentine Subtropical Arid Zone
Secretaría de Ciencias y Técnica de la Nación Argentina (secyt)

Raúl Díaz
Agronomist, Professor, Coordinator for Pastures Management

Ulf Karlin
Agronomist, Professor, Coordinator for Marginal Agroecosystem Management
Universidad Nacional de Córdoba, Argentina

Introduction

The potential offered by Prosopis tree species in the Dry Argentine Chaco, a forest region traditionally used for livestock farming, is discussed within the framework of furthering regional development through the establishment of silvo-pastoral production systems.

An abundant tree cover dominates the environment, by modifying sunlight intensity and quality, changing the water balance and the nutrient cycle. Its influence is evidenced by changes in the soil, and in the amount, composition and quality of the species under tree canopies. In a fragile region such as the Dry Chaco, these features, if well managed, can have great relevance in increasing and sustaining productivity.

Characteristics of the Dry Chaco

Location and Boundaries

The southwestern portion of the “Great Chaco” is known here under the name of Dry Chaco (also Arid Chaco and Southern Chaco), and covers flatlands in several Argentinian provinces: the northwest of San Luis, west and northwest of Córdoba, East of La Rioja, Central Depression and southwestern Catamarca, and the southwest of Santiago del Estero (see Map).

Although it shares some characteristics with neighboring phytogeographic regions (Monte and Semi-Arid Chaco) (Cabrera, 1976), it has features of its own, on account of which it can be treated as a unique ecosystem. It is important to consider its rainfall gradient (east-west) which has bearing on productivity. It covers from 6 to 8 million hectares, located between 28° and 34° S. lat. and 65° to 68° W. long. (Morello et al., 1977; Ragonese and Castiglioni).

Argentine Arid and Semi-Arid Subtropical Ecosystems

Climate

The Dry Chaco has high temperatures in the summer and moderate temperatures in the winter, with occasional frosts (5 to 10 days/year). Rainfall is concentrated in summer (70% during the four warmest months), being almost nil during the winter. Mean annual rainfall ranges from 500 mm in the East to 300 mm in the West (Anderson et al., 1980; Morello et al., 1977).

Droughts (less than 75% normal annual rainfall) occur one out of every six years.

Vegetation

Only traces of the climax plant cover remain, which has been reconstructed by observing less altered areas (Díaz and Karlin, 1984; Karlin and Díaz, 1984).

The predominant vegetation in terms of area —and pressumably climactic— was as follows:

Excessive felling operations (for fuelwood and timber extraction purposes) and overgrazing transformed the climax plant cover into a new structure called “Fachinal”, made up mostly of shrubs from the genera Larrea, Cercidium, Cassia and Opuntia, and annual grasses of the genera Aristida and Boutelava (Morlans, 1985).

This shrubby cover competes strongly with the herbaceous cover and with second-growth trees, precluding rapid recovery (Figure 1) (Karlin and Díaz, 1984).

Soils and Topography

Soils are generally sandy, with scant organic matter and nitrogen, good content of phosphorus and other nutrients; pH alkaline. The land is flat, sloping softly towards the bottom of wide valleys; soil texture becomes progressively finer towards the central lowlands.

Alkalinity and salinity also increase towards the central lowlands.

Actual and Potential Production

The Dry Chaco is a fairly unstable system, unable therefore to support continuously traditional crops without additional water. Nevertheless, it is possible to raise evergreen crops, particularly of forage species, using suitable management techniques.

The present and basic production system is extensive livestock farming (cattle and, to a lesser extent, goats), with sporadical extraction of forest products (Calella, 1986).

The real output of the region is low, with 4–10 kg beef/ha/year (15 to 20 ha/cattle head, 40% to 50% steers of 120 kg at weaning).

Potential production can reach 100 kg beef/ha/year (2 ha/cattle head, 80% steers at weaning, with 160 kg) (Díaz and Karlin, 1984; Seia, 1985).

In high-technology managed subsystems at La Magdalena and El Desafío farms, production attained 300 kg/ha/year. Production levels show an East-West gradient dictated by rainfall (Ayerza, 1984).

Forest production is low (1 to 2 ton/ha/year), with very inefficient exploitation, no technified or ordered forest production systems (Alessandria et al., 1978).

Socio-Economic Characteristics

Local demand for these products is scant, as a result of low population density (1 to 2 inhabitants/km2), and to low and erratic outside demand, particularly for forest products. This results from unstable supply, outside competition, lack of regional infrastructure and marketing channels, etc. (Bronstein and Karlin, 1986; Zaffanella, 1986).

The present trend in Argentina is to raise productive crops in the Pampas and in other “suitable” ecosystems (Semi-Arid Chaco), and to displace livestock production towards marginal regions (Dry Chaco).

This trend will bring about a stronger pressure onto the system, not only by cattle raising operations, but by forest undertakings as well, as the forest resource becomes depleted in other regions. Some indications of this are already apparent, with Prosopis wood being sought for carpentry.

To accomodate this pressure, it is necessary to find production systems and management schemes suitable for optimizing resource utilization without degrading the ecosystem.

These new systems must be analyzed from an economic standpoint, and the feasibility of implanting them must be considered in light of the region's existing socio-cultural structures.

The land is generally under private ownership, with little organization among the producers.

Most land owners do not live in their estates, being these under the administration of a “puestero”. The owners let them raise a few animals in their property, in exchange for their taking care of the estate's assets, which are periodically “harvested” (Bronstein and Karlin, 1986; Zaffanella, 1986). Land estates range usually from 500 to 5,000 ha.

Another land ownership scheme is the “comuneros,” whereby each family owns small plots of land (10 to 100 ha) and share “open” lands (5,000 to 10,000 ha) without clear titles to ownership; other families occasionally raise claims of rights to the same piece of land. The community organization is loose (Nogués, 1985).

This region receives low investment, which entails sparse productive infrastructure, derived from the fact that the benefits from traditional operations are low. Both owners and potential investors do not believe productivity can be improved substantially.

The local population has no real power to bring about change, and decisions are made from outside the region by individuals or institutions not familiarized with its realities.

The Dry Chaco is a “population producer,” on account of its high birth rate. Most inhabitants eventually emigrate to other areas of the country where prospects are better.

Development Strategies

Management models must aim at:

  1. Recovering both fodder and forest renewable natural resources. There are different strategies to achieve this, as will be discussed below. The recovery of the various forest components should be given high priority, so as to count on better production alternatives, with productive flexibility and stability. This entails devising multiple-use systems (Saravia, 1977).
  2. Maintaining self-sufficient productive structures by cost-reduction and maximum benefit, and not through maximizing profit in capital-intensive schemes.

The production systems proposed must be compatible with the local realities and with those of the decision-making centers (markets, credit organizations, etc.).

The strong traditional cattle-raising mentality must be reckoned with, as it tends to disregard the possibilities of forest production under rational management.

Therefore, the forest concept must be reinforced:

  1. Forests do not compete with cattle, but produce fodder.
  2. Forests can be beneficial to livestock production.
  3. Forests can provide direct benefits (timber, firewood, etc.).
  4. Forests can occasionally provide better benefits than livestock.
  5. A good combination of forest and livestock provides the maximum benefit.

Importance of Prosopis Trees

The Dry Chaco has vast areas where Prosopis dominance and density are important, particularly where livestock operations have been conducted or where phreatic or runoff water is available.

The predominant species is Prosopis nigra, followed by P. alba and/or P. chilensis.

Other Prosopis shrub species also occur, such as P. torquata and P. sericantha, but their behavior and strategies are different from those of Prosopis trees.

Another relevant forest species in the Dry Chaco is Aspidosperma quebracho blanco, predominating in large areas. This species must be taken into consideration in any silvo-pastoral scheme.

P. nigra enhances forage production under its canopy, the exact reasons for which have not been wholly ascertained in Argentina; the possible factors behind this might include higher nutrient and organic matter contribution, more favorable water balance, shading, or a combination of factors creating a more favorable environment.

Setaria leucopila growing under Prosopis nigra canopy. Dry season, eastern Catamarca, Argentina.

The Prosopis nigra umbrella-shaped canopy is large and of medium density, offering ideal characteristics for general silvopastoral combinations. It has few low branches, and it often has several stems starting at or near the base.

It nodulates abundantly when young, with greener hues and vigorous seedling development (Oliva, 1986). It is not known, however, whether it continues to nodulate —and, consequently, fix nitrogen— as an adult or under low moisture conditions.

This species is native to the area, therefore being adapted to the local environmental conditions, pests and diseases.

It has also good forest aptitudes, its timber being used for furniture, poles, etc. Its wood does not become deformed, and therefore can be worked while green. It also produces gums, nectar for syrup, alkaloids, etc.

It is a fodder species per se, being both its leaves and fruit palatable to animals.

It is very suitable for reclaiming degraded lands, poor in nutrients and in organic matter, either alkaline or saline. Research ought to focus more on improving soils with Prosopis species rather than on developing salt-tolerant or alkali-tolerant fodder species (Karlin and Díaz, 1984).

Contribution of Prosopis

Effects of Prosopis Canopies on Forage Yields

The optimum forage supply entails fodder output quantity, quality and stability throughout the year and year after year.

Prosopis canopies contribute to establishing a system closer to the ideal forage supply.

Effects on Grass Production

Measurements have shown that grass output is higher under a Prosopis overstory than in the open, as shown in the Table below:

TABLE 1
Accumulated Production of C. ciliaris (Buffel grass Texas 4464) Planted 1976

 Under canopy of P. nigraIn the open
Output4,3002,000 (4.14.83)
(kg DM/ha/year)3,9002,600 (4.10.84)

In 1983 there was drought at the end of the rainy season, while 1984 was a normal year; this would account for the smaller difference between yields under canopy and in the open (Díaz et al., 1984).

Several studies show significant differences in favor of indigenous grasses growing under the canopy.

The difference is very marked at the limit of crown projection in dry years, and less so in normal years, where similar values were observed between trees close to one another (3,800 kg dry matter/ha/year, on April 10, 1984). This shows the importance of establishing an optimum canopy coverage, keeping a balance among nutrient contribution, organic matter, etc., and the pasture's sunlight requirement.

In areas degraded by overgrazing, plants recover faster under tree canopies than in the open (accumulated fertility, grazing habits, etc.). Thus, a ten-year livestock exclusion from degraded areas produces the following results:

TABLE 2
Under P. nigra Canopy

 Under canopy of P. nigraIn the open
Fodder output1,200Annuals
(kg DM/ha/year)Setaria leucopila(no value in winter)

If recovery is accelerated by intercropping with Cenchrus ciliaris, higher values than with native species are obtained (3,000 kg dm/ha/year; April 1984) under tree canopies.

Cenchrus ciliaris (buffel grass var. Texas 4464) deserves a special mention. It was introduced into the Dry Chaco and showed good production and quality levels, with proven establishment and management practices. Seeds are also available commercially. It shows very good performance under shade, therefore being very suitable for silvopastoral schemes. Other grasses are not ruled out, but they must be tried out, as was the case with C. ciliaris (Ayerza, 1981).

Effects on Forage Supply Stability

In degraded areas (shrub cover), herbaceous forage output is low, the values depending on the shrub species occurring, their stocking rate, utilization records, etc.

Land clearing —total removal of woody species— boosts production by 300% to 500%. This explosive increase in forbs is due to the removal of competition, and greater light, nutrient and water availability.

Production increase after thorough clearing in medium-degraded areas decreases gradually due to nutrient and nitrogen depletion, organic matter decrease and less water efficiency, ending up with values lower even than the initial values (Figure 2).

Figure 2

Figure 2

Under tree canopies the production decreases less, as nutrient contribution is sustained and organic matter degradation is slower.

TABLE 3
Soil Values Under Adult Prosopis alba Area closed to Grazing for Over 15 Years

 Depth of sampleO.M.
%
Nitrogen
%
UnderSuperficial1.830.26
canopy  2–10 cm1.570.23
 20–40 cm0.700.05
In theSuperficial0.900.13
open  2–10 cm0.940.08
 20–40 cm0.620.06

The differences are highly significant, decreasing with depth (Karlin, 1983).

TABLE 4
Saline Soil Values Under P. nigra Complex Sample

 In the OpenUnder Canopy of P. nigra
Sample depth (cm)0–100–10
Organic matter (%)0.551.00
CaCo3 (%)0.370.12
pH paste8.307.15
pH hydrolitic9.007.60
C.E. sat. extr. (mmho/cm)0.401.80

Very significant differences are observed for organic matter, nitrogen, pH, etc. (Karlin, 1983).

Penetrability (a measure of soil compaction) is greater under the trees than away from them.

Five years after total land clearing, pastures planted with C. ciliaris and with deferred grazing showed absence of grass seedlings, lower plant density, clear signs of sheet erosion, greater soil compaction and lower fodder output. This indicates that the physico-chemical conditions of the soil are now poorer (Karlin, Miraflores, Catamarca Central Valley).

Influence on Sustained Fodder Output

Trials with nitrogenated fertilizers (urea at 0–50–100 kg N/ha/year) on C. ciliaris gave the values shown in the table below.

TABLE 5

 Control (no fertilizer)50 kg N100 kg NUnder canopyAway from canopy
kg DM/ha/year2,5002,9003,7003,9002,000
Cleared area, Feb. 1983Collection in April, 1983

The measurements under P. nigra canopies gave values similar to 100 kg nitrogen. No significant response was found for fertilization with phosphorus (15 ppm and 25 ppm), as the soils contain acceptable P levels (12 ppm) (Díaz, 1984).

Despite the higher production outputs when nitrogen is added, the economic analysis (clearing + fertilizing) does not justify the effort.

Other measures, such as soil scarification, produce significant increases, doubling buffel grass output every three years; this procedure, however, must be evaluated for longer periods, as soil exhaustion is likely to occur (Ayerza).

Trials with legumes (Macroptilium atropurpureum-siratro var. seca and scabra; Stylosanthes hamata var. verano) to date have not given satisfactory results (Ayerza).

Effect on Pasture Quality

It is well known that pasture quality decreases as it matures. In the figures below, quality refers to digestibility and/or percentage of crude protein, both correlated in grasses.

Figure 3

Figure 3

Figure 3

The quality drop causes important losses in terms of animal weight gain and on fertility. Losses of up to 300 and 400 g/day were recorded between June and September (Ayerza and Karlin, 1984).

These losses can be reduced if crude protein values under Prosopis canopies are considered.

TABLE 6
Crude Protein Evolution (%, 1984) in C. ciliaris Planted in 1976 Under Prosopis nigra Overstory

 04/0907/0509/18
Under canopy8.66.96.7
In the open4.83.93.7

Differences are also found with other grasses and in other years:

TABLE 7
Crude Protein (%) in C. ciliaris 1983 Under P. nigra Canopy

 04/1506/14
Under canopy8.58.8
In the open6.05.4

TABLE 8
Crude Protein (%) in Native Grasses Under P. alba Canopy

 UnderIn the open
Setaria sp.13.78.8
Trich. pluriflora10.99.3
Dig. californica10.08.3

TABLE 9
Crude Protein (%) in Native Grasses Under P. nigra Canopy

 UnderIn the open
Set. leucopila10.3
Dig. californica4.8

The difference in crude protein values under the canopy and in the open is clear, even with different values according to season, year, and species.

Canopy influence can be detected even some distance away from it, as shown in the table below.

TABLE 10
Crude Protein in C. ciliaris Under P. nigra Canopy

 Under canopyAt 2 mAt 5 mNo influence
C. ciliaris8.65.65.04.7

Under Aspidosperma quebracho blanco overstory, the other important tree species in the Dry Chaco, quantity and quality values are similar to those under Prosopis, as shown below for C. ciliaris.

TABLE 11
C. ciliaris Under Aspidosperma quebracho blanco Canopy

 UnderIn the open 
kg DM/ha/year3,8002,000(04-14-83)
CP (%)7.55.4(06-14-83)
CP (%)7.03.9(07-05-84)

The disadvantage with Aspidosperma quebracho blanco lies in its smaller canopy diameter, as compared with Prosopis.

Quality Drop with Land Clearing

Not only fodder quantity decreases after land clearing, but also its quality, as shown below.

TABLE 12
Trials with C. ciliaris

 Under canopy
CP (%)
In the open
CP (%)
1 year after clearcutting12.411.5
8 years after clearcutting6.93.9

Silvopastoral Schemes

From the above data and observations, management strategies can be set forth for the Dry Chaco.

The initial situation is a land degraded by clearcutting and severe grazing (A in Figure 4), where the plant cover has become a low-productivity shrubland with few grasses, mostly annuals, and the target is to arrive at a productive structure made up of trees and perennial grasses with few shrubs, similar to the climax structure. Both recovery and maitenance of the productive structure must be of low cost (Anderson et al., 1980).

There are several technological strategies conditioning recovery and maintenance techniques; the most common are the following:

  1. “Natural” recovery: Achieved by closing up the land to grazing or by reducing grazing and logging activities. Recovery of the fodder and forest resources is slow, particularly in the more degraded areas. Recovery and maintenance is low-cost, but the system continues to be unstable for many years. Productivity increases slowly (4 to 12 kg beef/ha/year, with 10-years recovery) (Anderson et al., 1984; Vera, 1984).

  2. Quick recovery: Achieved by removing woody species and sowing grasses. Recovery is very fast, but expensive. Productivity soars but then decreases gradually due to nutrient depletion. Sustaining the productive system is costly and not always possible.

  3. Selective recovery: Achieved by removing undesirable woody plants (shrubs), leaving the trees and their second-growth, and desirable shrubs. Resowing and/ or recovery through management techniques. Recovery is fast, and cost is intermediate (50% or less). Productivity is maintained, with stable, low-maintenance-cost structures (Seia Goñi, 1985; Díaz and Karlin, 1984).

Forest Recovery

If the existing forest mass is poor, the following must be carried out:

Reforestation

Using conventional methods, direct sowing with normal or scarified seeds, or through seedlings.

Prosopis alba nursery, Central Valley, Catamarca, Argentina.

Prosopis can also be sown by making the seeds pass through the digestive tract of animals.

Trials with 2,500 P. nigra individuals/ha (maximum initial spacing 2 × 2 m). Individuals at shorter distances exhibit signs of little vigor due to competition (Bronstein et al., 1985).

Regeneration of the Existing Forest Mass

Seed trees must be left at regular intervals, protecting the second growth shoots and eliminating competing shrubs.

Severe competition has been observed from Larrea divaricata against P. nigra second growth (Karlin, 1986).

The technique to be employed depends on the productive scheme aimed at (livestock-oriented, forestry-oriented, silvopastoral).

In any of these productive systems, the afforested areas can be used for grazing as of the first year after establishment, provided grass availability is not limiting for livestock voluntary intake.

Trials conducted at Chancaní, West Córdoba, with very high carrying loads (10 cattle head/ha during 15 days) at the time of year of lowest grass palatability (November) did not damage Prosopis nigra and P. chilensis second growth (Bronstein et al., 1985).

Early grazing of afforested areas boosts economic profitability, prevents eventual fires and decreases forb competition against Prosopis seedlings.

A forestry-oriented scheme for degraded areas in need of afforesting, in Argentina, is as follows:

  1. Land preparation (clearing and previous grazing);
  2. Dense planting rate (2,500 plant/ha) in order to obtain long stems;
  3. First thinning (1,250 tree/ha) between years 6 and 10, obtaining firewood and stakes;
  4. Second thinning (625 tree/ha) between years 15 and 20, obtaining firewood, stakes and poles;
  5. Nearly total felling (550 tree/ha) between years 30 and 40, obtaining firewood, stakes, poles and timber. There remain 100 trees/ha, the minimum stocking rate for silvopastoral purposes.

This scheme is profitable, more so if grazing is introduced as of the first year after establishment, with an internal return rate of 16% per annum, 200% superior to a well-managed pastoral scheme (Tártara and Coirini, 1986).

Measuring Prosopis nigra biomass in western Córdoba, Argentina.

The combination of Prosopis spp. with grasses insures productivity increases, but other combinations are also feasible, such us Prosopis and Atriplex. Atriplex cordobensis occurs naturally in areas with clayey-saline soils or in sandy soils.

Atriplex cordobensis growing under Prosopis nigra canopy, Catamarca, Argentina.

Atriplex cordobensis can occur as large bushes with up to 5 m in diameter and 2 m high, and it is necessary to lop them or trim them for better use by cattle.

This species tolerates very well shade from Prosopis, and in trials with livestock it has proved to be very palatable in critical seasons, with 15% crude protein content in winter, insuring good livestock farming levels.

With selective felling and trimming of the Atriplex plants, good protein “banks” can be obtained for the critical seasons.

Another species that grows in association with Prosopis is Justicia sp., an Acanthaceae between 20 and 40 cm high, forming pure communities under the Prosopis. Its fodder value approaches that of alfalfa, with crude protein contents of up to 17% (Peuser and Carranza, 1986).

Other Contributions of Prosopis to Livestock Farming

The effects of Prosopis canopies on fodder production are remarkable. The following additional advantages can be mentioned:

  1. Direct fodder contribution (leaves and fruits): Natural browsing of these Prosopis by cattle is negligible, and it attains certain relevance in dry seasons or years in fairly poor fields.

    A promissing alternative is hay production in plantations suitable for constituting protein banks.

    Fruit utilization by cattle under natural conditions is of little importance in the Dry Chaco, although fruit yield of some species is very abundant, if erratic. Fruit bearing depends on environmental conditions. Fallen fruit is not very relished by cattle, preferring grass during the fruit-bearing season, as fallen fruit decays very fast.

    The best possibility for using this resource is with selected trees under adequate management, and storing the fruit for consumption during critical seasons (Karlin, 1983; Karlin and Díaz, 1984).

  2. Contribution in terms of animal behavior: The ample and well-distributed shade afforded by the trees increases cattle well-being during the hottest hours of the day (10–18 h) in summertime, which in turn increases voluntary fodder intake with consequent weight gains.

    Another aspect is better grazing distribution, as the pasture is not only grazed at sites adjacent to water holes (Karlin, 1985).

  3. Contribution to livestock farming infrastructure: One of the most costly items in a livestock farming operation is construction of fences, with around 30% to 50% incidence in normal improvement outlays. If trees are used as “live fences” or hedges, combined with single live-wire fences, costs drop substantially, to about 1/3 of traditional barbed wire fences in the Dry Chaco (Tártara, 1986).

Discussion

The preliminary data presented justify continuing the studies on the role played by arboreal Prosopis. The second stage has been started, implementing silvopastoral production systems to evaluate economic performance (livestock and forest output, implementation cost, system recovery and maintenance).

Actually functioning production systems are the most suitable for rapid technology transfer. They also serve to evaluate the cause-effect relationships (hydric dynamics, nutrient balance, etc.) and to pinpoint the necessary adjustments to improve efficiency. They also help to develop technologies for efficient management, such as planting techniques, fences, type of livestock, etc. (Tomalino, 1984).

They are also the best school for training managers and operators of silvopastoral systems.

The following aspects call for further research in Argentina:

  1. Longer data gathering time to account for environmental variations and ecosystem trends.
  2. Use and management of Prosopis foliage as fodder.
  3. Use of Prosopis fruit as fodder, selecting trees with a view to increasing and stabilizing fruit production, management and conservation.
  4. Selection and management aimed at improving timber quality and productivity.
  5. Combination of Prosopis with other non-grass fodder resources (Atriplex, Justicia, Opuntia, etc.).

Production models other than silvopastoral schemes, such as apiculture support, gums, alcohol, food source for human consumption, etc., where Prosopis play an important role, are of course not ruled out (secyt, 1986), although in this paper emphasis was laid on silvopastoral schemes.

These other production models are not incompatible with the silvopastoral systems, and they can form part of multiple-use schemes.

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