Bioeconomic optimization of zinc and iron agronomic biofortification in cowpea (Vigna unguiculata [L.] Walp.) in Mozambique under tropical field conditions

Abstract

Agronomic biofortification with zinc (Zn) and iron (Fe) has emerged as a promising approach to enhance crop productivity while improving food and nutritional security. However, fertilizer recommendations based exclusively on maximum biological yield may not maximize farmers' economic returns. This study integrated agronomic and economic analyses to identify the biologically and economically optimal Zn and Fe fertilization strategies for cowpea (Vigna unguiculata [L.] Walp.) under tropical field conditions in Mozambique. A randomized complete block design with a 3 × 4 × 4 factorial arrangement was used to evaluate three fertilization methods (soil, foliar, and combined soil + foliar) and four application rates of elemental Zn and Fe. Grain yield response was adequately described by a second-order polynomial regression model (R2 = 0.767), with maximum technical efficiency predicted at 18.92 kg ha ⁻ ¹ Zn and 19.17 kg ha ⁻ ¹ Fe. In contrast, maximum economic efficiency was achieved at lower application rates (16.94 kg ha ⁻ ¹ Zn and 10.09 kg ha ⁻ ¹ Fe), reducing Fe input by 47.4% while maintaining more than 97% of the maximum predicted grain yield and increasing net profit. Across the evaluated treatments, the combined soil + foliar application produced greater grain yield and agronomic efficiency than either application method alone. Economic robustness was further evaluated through break-even, sensitivity, and stochastic risk analyses, which indicated positive economic performance across the evaluated treatnebts under the agronomic, environmental, and economic conditions considered in this study. Within the agronomic, environmental, and economic conditions evaluated in this single-site, single-season study, the economic optimum was reached before the biological optimum, demonstrating that integrating agronomic and economic optimization can substantially reduce micronutrient inputs while maintaining high productivity and profitability. These findings provide an evidence-based framework for developing economically efficient Zn and Fe fertilization strategies for cowpea under tropical smallholder farming systems.