The nutritional value of the forage characterized by the concentration of energy, digestible nitrogenous matter, minerals and vitamins. In the majority of cases and especially in the case of perennial alfalfa, the essential limiting factor of the nutritional value is the energy value. The water supply promotes the production of crude fiber for all cultivars. Gabes is a cultivar more adapted to the pedoclimatic conditions of Chlef than the other cultivars, it makes better use of irrigation water. The results of our work show that the digestibility of the organic matter evolves linearly with the samples with values ranging from 43.63 to 50.73% whatever the water regime. Digestibility (dMO) always increases gradually even in water deficit conditions with less steep slopes whatever the cultivar studied. The absorption of water and mineral salts seems affected by the age of the tissues formed. The use of irrigation reasoned to play between the different components of the nutritional value of alfalfa in relation to economic profitability.