The pressure of a high animal population on limited grazing areas has led to progressive deterioration of native grasslands in developed and developing countries. As grazing pressures rise, the better forage plants lose vigour, give up the struggle and disappear, being replaced by inferior species (often poisonous or unpalatable) resulting in the production of less and less food for man. As grazing pressure increases, the reserve forage needed to carry animals through severe droughts and floods disappears. Overgrazing also disrupts cycles that assure relatively even distribution of water and nutrients, which prevent erosion, especially on semi-arid grasslands that easily degrade into deserts.
Overgrazing is a principal factor in the deterioration of rangeland. In general, overgrazing reduces grazing duration during the growing season as well as during the dry season, often forcing settled farmers to buy food supplements. For nomads, overgrazing causes a decrease in the milk production of their animals, which forces them to increase the number of animals, further increasing the pressures on the range. In the long run, both farmers and nomads may decide to leave the region or settle around deep boreholes. A special disease may appear among cattle – botulism in the Sahelian areas is one example. Finally, in areas where water erosion occurs and where wells exist, sand dunes may fill up the wells.
In Southern Africa, escalating land degradation over the past decade has been caused by increased livestock. Overgrazing causes more than half the soil degradation in the sub-region. In Namibia, livestock production subsidies actually encourage farmers to raise more livestock than if they had to meet the full costs themselves (Byers 1997). With new economic policy changes under way in the region of the Southern African Development Community (SADC), including the removal of such subsidies, stocking rates are expected to decline over the next decade.