The aim of the study was to determine the effect of alternate irrigation method and deficit irrigation on radiation use efficiency and forage quality of foxtail millet (Setaria italica) under a double cropping system after barley (Hordeum vulgar). Conventional furrow irrigation (M1) and alternate furrow irrigation (M2) methods and different deficit irrigation levels including 100, 85, 70 and 55% of crop water requirement (V1, V2, V3 and V4) were tested in a field experiment on the west of Tehran, Iran for 2 years (2008 and 2009). Results showed that alternate furrow irrigation with 85% of crop water requirement (M2V2) had the highest Absorption Ratio of Photosynthetically Active Radiation (PARAR) and Cumulative Absorption of Photosynthetically Active Radiation (PARCA) in 2008 (P < 0.05). In 2009, M1V2 and M1V3 had higher PARAR than M2V1 (P < 0.05). Result difference in two years was probably due to that year 1 was drier than year 2 and the crop received more water in year 2 than year 1. Under 55% of crop water requirement, Leaf Area Index (LAI) and Radiation Use Efficiency (RUE) of conventional furrow irrigation method were reduced, while LAI and RUE of alternate furrow irrigation method were maintained. Alternate furrow irrigation had higher percentage of dry matter digestibility than conventional furrow irrigation. 55% of crop water requirement with alternate furrow irrigation resulted in increasing percentage of water soluble carbohydrates and decreasing crude protein percentage. The results indicate that alternate furrow irrigation should be a major consideration in crop production of arid and semiarid region.
