|Animal Husbandry - Initial Environmental Assessment Series No. 2 (NORAD, 1994)|
|Part I: General account|
|3 Possible environmental impacts|
Overgrazing is a widespread problem in many developing countries. The most serious impact is reduced feed supply, so that the growth and production of the animals are hindered. In areas where pastures are the communal property of a village or district, the risk of overgrazing is particularly great. A reform or new legislation can be required in order to make the utilization of communal land more efficient. A fee per grazer could be introduced, for example, as well as a quality grading system for meat. Should mitigative measures of this kind be called for, one also has to make assessments of socio-cultural conditions and consequences for, and of, these (cf. chapter 3.7). Land tenure (see also chapter 2).
Impacts of overgrazing can be illustrated by an example in which 100 hectares of good pasture caters for 40 -100 cattle. 1 hectare is presupposed to yield 5.0 tons of grass and the maintenance requirement per animal is 24.6 MJ.
This simple table shows that by an increase of the number of animals by 50 per cent from 40 to 60, most of the growth is lost. By doubling the number of animals from 40 to 80, nothing is left for growth and production. The animals lose weight instead of putting on weight. Research shows that cattle, goats and sheep graze on different plants and that the yield can be increased by letting different animal species graze together on the same land. In pastures under stress, only plant species that tolerate intense grazing will remain. Should there be a lack of feed, it is particularly important to adjust the number of animals according to the sustainability of the range. Since the plant production is dependent on the climate, however, this can be difficult. Should, for example, the expected precipitation fail to come, an unforeseen scarcity of feed may result. An emergency solution could be to move the animals to another area containing more feed, or transport feed to the animals. In areas with unstable precipitation, it may be necessary to take precautions by ensuring feed reserves in some way, ea. by means of standing hay or preserved feed.
Overgrazing and animal tracks can cause soil erosion, especially in steep terrain or in areas with much vulnerable vegetation and great seasonal differences in precipitation. Nevertheless, the risk of soil erosion in connection with grazing, especially extensive forms, seems to be generally less than previously assumed. Perennial plants in fields or pastures provide better protection against erosion than field plants that only partially cover the soil, and only in certain periods of the year. A measure for protecting exposed areas could be periodical or protracted enclosures. In some places, ea. in Tanzania, enclosure of pastures is practiced in order to keep them in reserve for periods of crisis.
Increased surface run-off will occur together with increased soil erosion, because the soil's capacity to retain water is dependent on the plant cover. This will lead to a reduced feeding of ground-water, and a reduced water flow in streams and rivers, so that erosion problems may also emerge here. A problem related to erosion is silting. Eroded matter being carried with the water in streams and rivers may cause a reduction in water quality and unwanted deposits and sediments in other parts of the watercourse. The supply of nutrients may exceed the natural level and thus contribute to eutrophication and overgrowing of water sources, so that breeding areas for fish may be damaged. Eroded matter which is retained in artificial reservoirs will be sedimented there. It will thereby reduce the longevity of the reservoir as well as the production capacity of any prospective power plant. Erosion, sediment transportation and depositing are complex processes, and the impacts of encroachments (ea. pastures) can be hard to predict. A precautionary attitude, therefore, is important in the planning and implementation of projects. (More about soil erosion, see booklet 7 Water supply).