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6. INDICATORS TO ASSESS ENVIRONMENTAL IMPACTS


6.1. Vegetation
6.2. Soils
6.3. Water
6.4. Socio-economic

In theory, it is possible to identify a large number of indicators from the previous sections that would enable assessment of the positive and negative impacts of crop-livestock interactions on environment. In practice, the use of any indicators will be influenced by the nature and scale of the environmental assessment and the resources available. Surveys of land use changes, populations of domestic livestock and wildlife, and the availability of crop residues are probably more relevant at country or regional level. For large-scale assessment of land use and animal populations, remote sensing is becoming an increasingly important technique (TUELLER, 1991). There are several sources of this type of information including aerial photography and satellite imagery. At local level, it will not be possible to quantify many indicators for technical, financial or logistical reasons. Therefore, only a few indicators that could be used realistically to assess the local impacts of livestock on environment are highlighted (Table 8).

6.1. Vegetation

Negative indicators for native vegetation:

(a) Botanical composition of lower layer (< 1 m high) i.e. a reduction in the percentage of better quality grasses or an increase in the proportion of poor quality grasses and weeds. Within a given plant community the desirable species will be known.

(b) Increase in the number of shrubs and trees (shrub canopy 1-3 m, tree canopy > 3 m high).

(c) Increase in bare ground. A measure of the potential erosion hazard.

Range can be classified as “excellent” if 76-100 percent of desirable indicator species are present, “good” for 51-75 percent, “fair” for 26-50 percent, and “poor” for 0-25 percent. As an example of the use of indicator species, in the thornveld of South Africa, NEL et al.(1993) classified grass species into ecological status groups according to their reaction to degradation. Data were collected from subjectively selected sample plots to represent different stages of degradation. Under-utilised grassland was dominated by Themeda triandra with a frequency of 86 percent; lightly grazed grassland was dominated by T. triandra (65 percent) followed by Bothriochloa insculpta (15 percent); moderately grazed grassland was dominated by B. insculpta (41 percent) and T. triandra (20.2 percent); severely overgrazed grassland contained Cynodon dactylon (21 percent), Urochloa panicoides (19.3 percent) and U. mosambicensis (13 percent). T. triandra was the best indicator for under-utilised and lightly grazed veld, while B. insculpta was the best indicator for moderately to heavily grazed veld, and U. panicoides the best indicator of severely grazed veld.

Undesirable changes in floristic composition will often be accompanied by encroachment of thorny, unpalatable woody species and the appearance of bare ground. In improved pastures, a decline in the sown species and the ingression of weeds are indicative of grassland degradation. In grass-legume pastures a range of 20-30 percent legume, on a dry matter basis, is regarded as optimal (STOBBS, 1976).

It should be emphasised that monitoring of vegetation needs to be undertaken over a considerable time period in order to detect long-term changes of a more permanent nature. Often, changes may be short-term, and not necessarily indicative of a trend towards irreversible land degradation. Recent studies over nearly a decade by ILCA in the Sahelian zone of West Africa have shown that vegetation is more resilient than was formerly believed. When climatic conditions are again favourable after drought, the original vegetation returns. However, any short-term deleterious changes can be regarded as warning-signals of a downward trend in range condition.

6.2. Soils

Suggested indicators for soils:

(a) Bulk density (mass of a unit volume of dry soil. Measured by the clod or core methods).

(b) Organic carbon (by wet digestion, a modification of the Walkley-Black procedure. The percent organic matter is derived by x by an empirical factor 1.72).

(c) Total nitrogen (Kjeldahl method).

(d) Available phosphorus (Bray or Olsen procedures).

(e) Cation exchange capacity (the sum total of exchangeable cations a soil can absorb. Measured by the ammonium acetate or silver thiourea methods. Base saturation percentage can be derived from the cation exchange capacity).

(f) Electrical conductivity (for soils under irrigation using conducimeter).

Run-off and erosion are both correlated with bulk density. Soils that are loose and porous have low bulk densities, and those that are more compact will have higher values. The bulk densities of clays, clay loams and silty loams normally may range from 1.00 to 1.60 g per cubic cm; sands and sandy loams 1.20-1.80 g per cubic cm. Very compact subsoils, regardless of texture, may have bulk densities as high as 2.00 g per cubic cm or even greater. Any increase in bulk density will indicate increasing compaction, and a potential erosion problem.

The amounts of organic matter in mineral soils can vary widely, from trace amounts up to 15 or 20 percent. Low levels of organic matter are often in the range 0-1.5 percent, medium 1.6-3.0 percent, and high >3.0 percent. Low values for total nitrogen may range from 0-0.08 percent, medium 0.081-0.15 percent, and high >0.15 percent. Low values for available phosphorus (heavy-textured soils) can vary from 0-5.0 ppm, medium 6.0-10.0 ppm, and high >10.0 ppm. Comparable phosphorus levels for light/medium-textured soils are 0-10 ppm, 10.0-20.0 ppm, and >20.0 ppm, respectively. Cation exchange capacity values for mineral soils lie between 15 and 40 mol/100 g soil, and for soils with a high humus content up to 300 mol/100 g. Any decline in these indicators would represent a decrease in soil fertility.

The salt content of saturation extracts of soils is commonly specified in practice by its electrical conductivity, which is a good index and easy to measure (POWELL, 1988). The US Salinity Laboratory grades water/soil solutions, based on its soluble salt content, into four classes. Those with an electrical conductivity below 0.25 d Sm-1 which do not contain enough soluble salts to cause any trouble and those with conductivities between 0.25 and 0.75, between 0.75 and 2.25, and above 2.25, for which special management is needed. These limits are now considered to be low and three ranges of <0.7, 0.7-3, and >3 have been proposed where the hazards are slight, moderate and severe.

6.3. Water

Suggested indicators for water quality:

(a) Nitrate content. The main sources of nitrate are the product of microbial breakdown of soil organic matter, manure and plant residues; nitrogen fertilisers; atmospheric nitrogen. Nitrate is found in many natural waters at concentrations between 1.0 and 10.0 ml/L. Higher levels would indicate contamination, particularily from nitrogenous fertilisers. European guidelines for nitrates in drinking water quote 50 mg/L as the maximum permissible concentration, and 25 mg/L as normal.

(b) Bacterial numbers and the presence of coliform organisms. Guidelines are given for microbial populations by RUMP and KRIST, 1988.

(c) Electrical conductivity. This has been mentioned in Section 6.2.

6.4. Socio-economic


6.4.1. Land competition
6.4.2. Competition for crop residues
6.4.3. Competition for labour availability
6.4.4. Negative impacts associated with the introduction of draught animal power
6.4.5. Impacts associated with the asset building function of livestock
6.4.6. Positive impacts associated with intensification of agriculture

6.4.1. Land competition

Competition for land between crops and livestock leads to intensification of crop production where inputs are available. Indicators of intensification will include:

a) Increased use of manures, fertilisers and higher yielding varieties.

b) Increased numbers of livestock held by cultivators to supply manure and consume crop residues.

c) Increasing demands for private ownership of land and enclosure of communal land for private use.

Suggested indicators for socio-economic changes caused by land competition:
a) Declining livestock numbers in pastoralist herds as a consequence of the reduced availability of grazing caused by an expansion of cultivated land, in particular into key dry season grazing resources (e.g. wetlands).

b) Changes in species composition of herds, e.g. from cattle to goats reflecting increased pressure on rangeland resources and changes in the vegetative composition of the range from grasses to shrubs (bush encroachment).

c) Declining food security of pastoral households, particularly during drought as a result of declining pasture availability.

d) Increased differentiation of pastoral households as the poorer members of society are forced to sell stock, particularly during drought, and are unable to re-establish their herds post-drought.

e) Declining revenues from protected areas as they increasingly become denuded of vegetation and wildlife and cease to be attractive to tourists.

6.4.2. Competition for crop residues

Suggested indicators for socio-economic changes caused by competition for crop residues:

a) Reduced availability of crop residues for pastoral groups will have similar consequences to the effects of increased competition for land (Section 6.4.1) and indicators will, therefore, be similar.

b) Privatisation of the use of crop residues to replace reciprocal arrangements will indicate increasing shortages of crop residues and declining production from grazing resources.

c) The development of markets for crop residues will indicate a shortage of this resource and grazing resources in general as a result of overgrazing and increases in livestock numbers.

6.4.3. Competition for labour availability

As agriculture intensifies and crop-livestock interactions increase so units of labour used per unit of output of crops and livestock increase. This may lead to temporary shortages of labour. Indicators of these changes will include:

a) Increased hiring of labour in mixed farming systems.

b) Increases in wage rates as a response to shortages of labour.

c) Increased adoption of animal or engine power to overcome labour shortages and increases in wage rates.
d) Increased adoption of other labour-saving technologies (fertiliser to replace manure) where available.

Suggested indicators for socio-economic changes caused by competition for labour availability as distances to pasture increase:
a) Declining availability of labour for crop production.

b) Declining availability of manure for crop production.

c) Declining availability of meat and milk for household consumption and sale, and fewer opportunities to market livestock, make distress sales etc.

6.4.4. Negative impacts associated with the introduction of draught animal power

Suggested indicators for socio-economic changes resulting from the use of draught animals:

a) Declining pasture availability will result in semi-arid regions as a result of expansion of cultivated area and increased land competition (see 6.4.1).

b) Declining yields and incomes/ha will indicate the negative consequences of shortening fallow periods.

6.4.5. Impacts associated with the asset building function of livestock

The building of herds or flocks by producers in mixed farming systems and their reluctance to market livestock should not be viewed as backward or conservative behaviour by those promoting agricultural development. These are positive indicators of

a) Capital accumulation which often plays a significant role in investments in crop production, survival and land improvements in crop-livestock systems.
Negative indicators will include:
a) Overstocking of communal pastures in the face of poor producer prices resulting from government policies to reduce prices for the benefit of consumers.

6.4.6. Positive impacts associated with intensification of agriculture

At a later stage in the agricultural development process (i.e. at higher population densities than the conditions described in 6.4.1. - 6.4.4.) under conditions of intensification of agriculture where human and livestock populations increase, indicators of environmental changes will include:

a) Increased investment in land improvements (water and erosion control).

b) Growth in livestock numbers as a result of intensification and the increased requirement for manure and increases in crop residue supply.

c) Increasing labour competition between crops and livestock, at least in the early stages of intensification.

d) Private ownership of land and further investments in land improvements and perennial tree crops.

e) Increasing returns per unit of land.

f) Increasing household incomes in the absence of further population growth which leads to farm subdivision in the absence of increasing employment opportunities in other sectors of the economy.


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