The present study was conducted to investigate the effects of different irrigation water salinity levels (0, 3, 5 and 8 dS/m) on hay yield, chemical composition, gas and methane production, metabolic energy (ME) and organic matter digestibility (OMD) of plants. Experiments were conducted in randomized blocks design with four replications for two years (2014-2015). Calcium chloride (CaCl2), sodium chloride (NaCl) and magnesium sulfate (MgSO4) salts were used to prepare irrigation waters with different salinity levels. Drip irrigation was used. Fresh and dry hay yields, plant height, acid detergent fiber, neutral detergent fiber and crude oil contents decreased with increasing salt doses. Tall fescue Olympus cultivar exhibited a moderate tolerance to salinity. Fresh hay yield decreased by 8.2% at 1.85 dS/m threshold salinity level and dry hay yield decreased 8.2% at 1.97 dS/m threshold salinity level. Crude protein content, gas and methane production, net energy lactation, metabolic energy and organic matter digestibility increased with increasing salt doses. Despite the decreases in hay yield, 8 dS/m did not result in any decreases in protein yields and resulted in low gas and methane production.