This study aims to estimate the long-term total cost function for wheat production under supplementary irrigation in Nineveh Governorate during the 2022–2023 season. Data were obtained through a questionnaire administered to a stratified random sample of 130 farms, representing 20% of the wheat farming population using supplementary irrigation. The cubic model was identified as the most suitable form of the total cost function, fulfilling key economic, statistical, and econometric criteria. The coefficient of determination (R2) indicated that 80% of the variation in total production costs could be attributed to changes in total wheat output, while the remaining 20% was due to other unexplained variables. Both the t-test for individual coefficients and the F-test for overall model significance confirmed statistical validity at the 1% level. The analysis found that the optimal production volume for minimizing costs and maximizing profits was 1,039 kg per dunum, with an optimal cultivated area of 97.4 dunums. The results also showed that 84.8% of farmers operated within the rational production stage (Louvouirt capacity zone), while 15.2% did not. Additionally, 58% of farms operated within the economic efficiency zone, whereas 42% did not. Of the inefficient farms, 40.8% produced below the efficiency threshold, and 1.2% were in the Louvoirt zone but still lacked economic efficiency. These findings underscore the importance of optimizing both output levels and cultivated area to improve efficiency and profitability in wheat production under supplementary irrigation.