Drought stress is considered as the main environmental factor limiting crop growth and yield in the arid and semiarid regions. The possible increasing of drought risk as a result of climate change in these regions has forced field crop producers to search for drought tolerant plants. This research consisted of four studies. The first study was performed to investigate the effects of normal irrigation and drought stress (irrigation after 50 and 85% depletion of available soil water) and two nitrogen levels (0 and 112.5 kg N/ha) at four planting dates (June 22, 2015, July 6, 2015, June 14, 2016, and June 30, 2016) on biochemical and morphophysiological properties, yield, irrigation water use efficiency (IWUE), nitrogen use efficiency (NUE) and silage quality of two millet species, foxtail millet ( Setaria italica (L.) P. Beauv.) as Bastan cultivar and proso millet ( Panicum miliaceum L.) as Pishahang cultivar. Drought stress increased the contents of hydrogen peroxide (H 2 O 2) , malondialdehyde (MDA) while decreased leaf area index (LAI), chlorophyll content (Chl), carotenoid content (Car), membrane stability index (MSI) and relative water content (RWC) resulted in decreases in, the grain and biological yields and water and nitrogen use efficiencies. Moreover, ADF was reduced but leaf/stem ratio, digestibility, and silage quality improve in Pishahang as a result of drought while no significant effects observed on these traits in Bastan. The results showed that the application of nitrogen could mitigate the negative effects of drought stress on millet plants through motivating antioxidant systems and osmoregulation, alleviating lipid peroxidation and improving plant physiological traits. Nitrogen supply increased digestibility, grain and biological yield. In general, Bastan responsed to nitrogen fertigation far more than Pishahang. The positive effects of nitrogen supply on the studied traits were higher under normal irrigation than under water stress condition. Grain yield was higher in the plants sown on June 22, 2015 and June 14, 2016 under control irrigation condition, while under drought stress, July 6, 2015, and June 30, 2016 produced higher grain yields. The second study was performed to compare the quantitative and qualitative yield of millet, maize and sorghum on 14th and 30th of June, 2016, and under irrigation and nitrogen levels (similar to the previous experiment). There were no significant differences between millet (Pishahang) digestibility under stress condition with corn digestibility at both irrigation levels and sorghum digestibility in normal irrigation. Also, maximum biological yield was observed in sorghum, millet and maize, respectively. Third study was conducted to evaluate the effects of irrigation regimes (irrigation after 40 and 75% depletion of available soil water) and nitrogen levels (0, and 85 mg N kg -1 soil) on millet cultivars (Bastan and Pishahang) under two atmospheric CO 2 concentrations (390±50 and 700±50 µmol mol -1 ). Results showed that when plants were grown under combined conditions of drought with elevated CO 2 and nitrogen supply, the limitations observed under water shortage condition were reduced, H 2 O 2 and MDA productions were decreased while chlorophyll content and leaf area were increased and consequently the observed decreases in shoot dry weight were mitigated. In the fourth study, calibration and evaluation of the CSM-CERES-MILLET model was performed using field data in two crop years for two irrigation regimes and two nitrogen levels. Model evaluation results showed that NRMSE values at irrigation levels for biomass weight and leaf area index were less than 20% during growth period and Wilmot index (d) and coefficient of determinations (r 2 ) were more than 84%, which confirms the capability of the model in simulating millet yield under drought condition. Grain and biological yields will decrease in future climates in the years 2030, 2040, and 2050 compared to the last decade, with a decrease in RCP-8.5 greater than RCP-4.5. in sum, delayed sowing dates due to the lower drought effects associated, might be the proper alternative for achieving higher yields as well as improved IWUE and NUE in areas plagued with water shortage. The highest grain yields were obtained in Bastan but maximum biological yield belonged to Pishahang under both irrigation regimes. Accordingly, these two cultivars can be used for grain and forage production, respectively. Keywords Antioxidant compounds, Climate change, Digestibility, Water shortage, Millet, Nitrogen fertilizer management, DSSAT