Simulating the yield of cowpea ( <i>Vigna unguiculata</i> L. Walp) by optimizing the sowing window using the CSM‐CROPGRO‐Cowpea model in the savannas of Nigeria

Abstract Determining optimal sowing windows for cowpea ( Vigna unguiculata L. Walp) in Nigeria's Sudan and Sahel savannas is critical for improving rainfed productivity. This study used the CROPGRO–Cowpea model in the decision support system for agrotechnology transfer to identify optimal sowing windows for improved cowpea varieties across these agroecological zones. The model was calibrated using nine field trials conducted between 2016 and 2021 in Abuja, Zaria, and Dambatta, with genetic coefficients estimated for FUAMPEA 1 and FUAMPEA 3 using GenCalc. Model performance was evaluated using five independent population density trials (2016–2018) in Dambatta and Zaria, employing root mean square error (RMSE), normalized RMSE (nRMSE), and the d‐index. The model showed strong performance, with low RMSE, nRMSE below 10%, and d‐index values ranging from 0.84 to 0.97. The validated model was applied to simulate cowpea performance across four locations in the Sudan (Azare and Kafin Madaki) and Sahel (Maigatari and Minjibir) agroecological zones using eight sowing windows at weekly intervals from June 24 to August 18 over a 36‐year period (1989–2024). Results showed that sowing between late June and early July consistently produced the highest yields across locations. FUAMPEA 3 generally outperformed FUAMPEA 1 under early sowing, whereas FUAMPEA 1 showed greater yield stability under delayed sowing. Yield reductions of up to 80%–100% occurred when sowing was delayed beyond mid‐July. Overall, late June to mid‐July represents the optimal sowing period across the savannas, and cultivar‐specific responses should inform climate‐smart sowing recommendations.