Field studies were conducted during the summers of 2007 and 2008 to determine the response of silage maize (Zea mays L.) to different levels of irrigation water to guide programs for the development of improved irrigation management practices for sub-humid zones. The experiments were carried out in Bursa, Marmara region, Turkey. Silage maize plants (cv. 'Ada-523') were subjected to different levels of irrigation using a drip system in the field on a clay-loam soil. Fully irrigated (FI = 100%) plants were irrigated at 100% pan evaporation (Epan) replenishment with 7-day intervals. In other treatments, irrigation was applied as excessive (EI = 125% Epan) and deficit (DI-75 = 75% Epan, DI-50 = 50% Epan, DI-25 = 25% Epan, NI no irrigation). Plant height, first ear height, stem diameter, number of ears per plant and net income decreased with decreases in the amount of irrigation, but the effect of soil water deficit on the number of leaves per plant and ear ratio in forage was minor. The highest forage yields were obtained with El (125% Epan), FI (100% Epan) and DI-75 (75% Epan) treatments. FT treatment produced the highest net income based forage yield. Severe soil water deficit substantially reduced forage yields and net income in both years. The results showed that full irrigation during the whole growing season is preferable for higher forage yield and net income. However, in regions of water scarcity, farmers should adopt the deficit irrigation (DI-75 = 75%, DI-50 = 50% and DI-25 = 25% Epan) approach to achieve economically sustainable crop production. As an alternative to full irrigation during the entire growing season, the irrigation at a rate of DI-75% Epan can be recommended as optimal level because it achieved irrigation water savings of 25%, an increase of 16% in forage yield irrigation water use efficiency, satisfactory crop morphological characters and an acceptable net income with a yield loss of only approximately 7% compared with full irrigation.