Water is the greatest limiting factor to plant growth in the northern Sahel, where rainfall averages 100 to 300 mm/year. The growing season for grasses on the northern rangeland does not exceed one month. Although the total standing biomass is less than 1,000 kg/ha, the protein content is high, at 10% to 20% with digestibility of 60% to 70%. (Protein content should be at least 7% for healthy, productive animals.)
The southern Sahel receives more rain (300 to 600 mm/year), but only a small fraction (10% to 15%) of this is actually used by the vegetation, due to low soil fertility and infiltration capacity. Also, a large proportion of the water that does infiltrate is lost as direct evaporation. The growing season is two to two and a half months, and produces 2,500 kg of biomass per/ha. The water supply outlasts vegetation growth, leaving 10% to 20% of infiltrated rainwater in the soil. At the end of the rainy season in this region the low availability of nutrients in the soil becomes problematic. The absence of nitrogen leads to low protein content in vegetation (3% to 5%) with digestibility of only 40%.
In the far southern savannah a rainfall of 600 to 1,200 mm per year produces a biomass of up to 4,000 kg/ha. However, poor soil nutrient content due to leaching leaves vegetation nearly void of protein. The tsetse fly (Glossina spp.) is another serious constraint to animal husbandry in the region. Advantages of the savannah include beneficial perennial grasses with a longer growth cycle and the availability of crop byproducts.
The transhumant system of animal husbandry benefits from good production possibilities in the northern Sahel during the rainy season and from the opportunities for survival in the south during the dry season. Twice a year herdsmen and their herds migrate hundreds of kilometers. At the beginning of the rainy season, herds migrate north to graze from July to October. Cattle grow at a rate of 0.5 kg/day per head and benefit from the high quality of protein-rich feed. The migration south, to floodplains like the central delta of the Niger or the savannahs, is prompted by the lack of drinking water, not by lack of food. While in the south, cattle at best maintain their weight. The only production then is calf birth and growth, and some milk. In the transhumant system net weight gains of 50 kg per head per year have been observed.
Improved land-management techniques such as erosion control may prevent further deterioration of land resource but are insufficient to bring about badly needed improvements. Increasing yield per unit area is necessary. Even if all available fodder resources are used for cattle production, the manure produced will maintain at most one-third of actual yields. Ongoing research is examining ways to integrate fertilizer use into indigenous farming systems through mixed cropping, use of leguminous species and agroforestry. Animal husbandry can benefit through the intensification of farming practices. Increased crop-residue quality and the halt of rangeland infringement may thereby preserve the pastoralist culture in the Sahel.
Dr. H. Breman, AB-DLO
P.O. Box 14
NL-6700 AA Wageningen