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.
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 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.
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