Nutrient Cycling in the Sahel

In the West African Sahel an erratic and declining rainfall pattern combined with rocketing population growth are straining the natural resource base. Crop yields are declining. In order to maintain production levels, farmers devote more land to crops and less to grazing. Consequently, livestock become more dependent on forage from croplands. Cycling of nutrients between rangeland, cropland and ruminant livestock has long been critical to sustained agricultural productivity in this region. However, the nutrient transfer mechanisms are poorly understood. Sustaining productivity in the Sahel requires a better understanding of these nutrient cycles and the development of new and innovative animal and land management strategies.

Sandy Sahelian soils have low levels of soil organic matter (SOM) and low buffering and nutrient exchange capacities. They are deficient in nitrogen (N) and phosphorus (P). The soils are susceptible to crusting and have low water-holding capacities.

Livestock enhance soil productivity by recycling biomass into excreta that fertilize the soil. Manuring increases SOM and nutrient availability, improves water-holding capacities and augments crop yields. However, livestock can contribute to soil degradation by grazing and trampling crops, leaving soil exposed. The resulting increased soil temperatures, wind erosion and sand blasting of plants limit production.

Manure Management

Sahelian farmers distribute manure on cropland in two ways: corralling animals in fields at night, and/or hauling manure from animal stalls to crops. Corralling animals returns both manure and urine to the soil and requires little additional labor. First-year results of a six-year trial showed that yields where animals were corralled (manure plus urine) were, on average, 52% greater than yields where just manure was applied. In areas where land is intensively cultivated and animals are stall-fed, manure must be handled, stored, transported and spread on fields. By the time manure is applied, it has lost about 50% of the original total N. Corralling livestock at night on cropland is perhaps the most efficient, traditional practice for maximizing nutrient cycling.

Diet and Nutrient Cycling

Seasonal differences in the diet of grazing animals can greatly influence quantity and nutrient content of manure. During the rainy season manure output (2.2 kg/animal/day) is doubled that produced in the dry season, and is higher in N and P. Although wet season manure is more abundant and of higher quality, it is largely unavailable to cropping as animals are usually outside the cultivation zone during the wet season.

The total amount and proportion of N excreted in urine and manure depends on animal diet. Feeds that have high intake levels and passage rates through the rumen, low protein solubility and small particle size generally are less N-efficient for ruminant use. Feeds with high levels of soluble phenols (e.g., browse) decrease the digestibility of N, resulting in a relatively high proportion of excreted N.

The urine and manure from animals fed highly digestible diets is more susceptible to N loss than excreta from diets higher in roughage. Although in feeding trials growth rates were similar for animals supplemented with cowpea hay and Acacia tortilis pods. More than twice the amount of N is excreted as urine by animals fed cowpea hay than from animals fed A.tortilis pods or no supplement. Much of this urine N is lost via ammonia volatilization. Manure from animals fed A. tortillis pods contains greater amounts of structural carbohydrates and, therefore, decomposes slower than manure produced from other diets. This results in greater build-up of SOM, which increases nutrient cycling efficiency and sandy soil productivity .

Gains in nutrient cycling may be possible by developing feeding strategies that not only satisfy the nutritional needs of animals, but also produce animal excreta less susceptible to nutrient loss.

Crop residues are an important source of feed for ruminant livestock during the dry season in the Sahel. However, the animals remove far more biomass and nutrients from the cropland than they return to it as manure; an exception is data from Burkina Faso (Table 1).

Because crop residues, animal feed and manures have different chemical compositions, field level management of organic materials impacts nutrient availability. Organic matter decomposition and nutrient release should be synchronized with crop nutrient demands.

Sustainable Nutrient Cycles

Sustainable increases in productivity can be achieved through a combination of external inputs, such as fertilizers, with inputs common to low-input agricultural systems, such as N-fixing legumes and organic nutrients sources. The efficiency of fertilizer use increases dramatically when combined with crop residues and manures. Forage legumes in fallow systems and dual-purpose grain legumes can increase soil productivity while providing high-quality forage. The introduction of leguminous browse as windbreaks can increase the sustainability of mixed farming systems by providing food, fodder, wood, soil amendments and erosion control.

Sustainable increases in agricultural productivity in the Sahel must come from optimal use of manures and fertilizers in combination with crop and animal management strategies that minimize nutrient losses. Corralling animals on cropland is one such viable strategy, but it must be practiced with the knowledge that crop residues also provide valuable nutrients, biomass and erosion protection for the soil. The rotation of corralled animals on cropland, the occasional tilling of crop residues, and the planting of forage legumes as fallows and leguminous browse as wind breaks could be useful practices toward balancing soil nutrients and maintaining soil health.


Mark Powell
RD 1, Box 6B
Wyalusing, PA 18853 , USA
Phone: (717) 746-1121
Fax: (717) 746-1595