Abstract: The aim of this study was to determine the potential nutritive value of sainfoin (Onobrychis viciaefolia) hays harvested at flowering stage from Afsin, Tekir, Pazarcik, Baskonus and Kapicam using chemical composition and in vitro gas production technique. Chemical composition including Crude Protein (CP), Neutral Detergent Fiber (NDF) Acid Detergent Fiber (ADF), ash Condensed Tannin (CT), Organic Matter Digestibility (OMD) and Metabolisable Energy (ME) were calculated. Gas productions were determined at 0, 3, 6, 12, 24, 48, 72 and 96 h incubation times and their kinetics were described using the equation Y = a+b (1-exp^-ct). The CP, NDF, ADF and CT content of sainfoin hay ranged from 11.39-17.70, 43.31-47.64, 35.61-43.30 and 4.19-9.95%, respectively. The sainfoin obtained from Afsin and Baskonus had significantly (p<0.001) higher crude protein contents than those of sainfoin hays from Tekir, Pazarcik and Kapicam. The NDF content of sainfoin hay obtained from Pazarcik and Kapicam was significantly (p<0.001) higher than those of sainfoin hays from Afsin and Tekir the ADF content of sainfoin hay from Kapicam was significantly (p<0.001) higher than those of sainfoin hays from Afsin, Tekir, Pazarcik and Baskonus. The CT content of sainfoin hay from Pazarcik was significantly (p<0.001) higher than those of sainfoin hays from Afsin, Tekir, Baskonus and Kapicam. The gas production from slowly soluble fraction of sainfoin hay from Afsin was significantly (p<0.001) higher than those of sainfoin hays from Tekir and Kapicam. The ME and OMD ranged from 6.86-7.79 MJ kg‾¹ DM, 47.82-53.17%, respectively. The ME and OMD content of sainfoin hay from Afsin was significantly (p<0.001) higher than those of sainfoin hays from Tekir and Kapicam. As a conclusion, the growing site had a significant effect on the chemical composition in vitro gas production, ME and OMD of sainfoin hay harvested at flowering stage. There are considerable variations in chemical composition of sainfoin hay harvested at flowering stages among growing sites. The ME and OMD values of sainfoin hay are negatively correlated with cell wall and CT contents but positively correlated with CP content.