Improving water‑use efficiency (WUE) in irrigated crops should increase productivity under less than optimum irrigation conditions. The objectives of this research were to compare WUE among alfalfa progenies, and to determine heritability and selection gain of this trait under three moisture levels (high, medium and low). Eight open‑pollinated progenies, previously studied under suboptimum flood irrigation, were evaluated in the field under a moisture gradient provided by a line source sprinkler irrigation system in 1988 and 1989. Water‑use efficiency was measured in terms of dry matter yield per water applied (irrigation plus rain). Soil moisture was not considered. The three moisture levels were considered variations of deficit irrigation because they were not enough to satisfy evapotranspiration of alfalfa during the experimental period. Progenies and progenies x level of irrigation interaction had significant effects on WUE. The WUE of progenies under sprinkler irrigation did not agree with their previous characterization under suboptimum flood irrigation. The low moisture level had the highest WUE. Heritability was higher at the low moisture level (h = 0.55) in 1988, and at the medium moisture level (h = 0.16) in 1989. The low moisture level had the highest selection gain in both 1988 (G = 0.78), and 1989 (G = 0.13). These results suggest that even under deficit moisture conditions the presence of interactions and the precision of suboptimum irrigation may produce different results during the plant evaluation process.