The fate of crop residue and animal manure applied to soil is important for agriculture in the mixed farming systems of semi‐arid tropics. Organic matter (OM), nitrogen (N), and phosphorus (P) losses from a variety of crop residues and browse leaves were compared to the manures of sheep fed the same plant materials in the semi‐arid region of Niger in West Africa. All organic residues decomposition and mineralization were measured using litterbag techniques in a Labucheri soil (sandy siliceous, isohyperthermic Psammentic Paleustalf) during the dry, wet, and cool seasons. Losses of OM, N, and P were generally greater and more rapid during the wet than the dry and cool seasons. Greater initial losses of OM, N, and P occurred in manures than in most browse leaves, particularly during the dry and the cool seasons. Differences in the rate and magnitude of OM losses were related to seasonal effects. For manures, sheep diet affected net mineralization profoundly in the initial stages of decomposition, but differences were reduced over longer periods of times. The total amount of N released from manures generally increased in the order of Pennisetum < Ficus ≤ Pterocarpus ≤ Anogeisus ≤ Combretum ≤ Vigna ≤ Mitragyna ≤ Guiera ≤ Acacia diets. For crop residues, more N and P were mineralized from Vigna than Pennisetum leaves whereas net N and P immobilization occurred for browse leaves, especially during the dry and cool seasons. The passage of feed through ruminants is clearly an important regulator of nutrient cycles in this semi‐arid region. Predicting seasonal OM decomposition and nutrient release rates may provide crop residues and livestock management strategies to improve crop growth.