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Seasonal availability of water is a major consideration in the management and selection of plant materials for irrigated pastures in the Intermountain West, USA. Objectives were to evaluate the forage yield of 10 tall fescue ( Festuca arundinacea Schreb.) strains and cultivars across five irrigation levels and to elucidate the effects of the endophytic fungus Neotyphodium coenophialum (Morgan‐Jones & Gems) Glenn, Bacon & Hanlin on productivity and trends. A line‐source irrigation system was used in a 2‐yr study. Significant differences were detected among the tall fescue entries for dry matter yield (DMY), and differences were relatively consistent across water levels (WL) as indicated by the nonsignificant cultivar × WL interaction and significant correlations among WL. Trends in DMY across WL were largely curvilinear; however, linear trends were much more predominant during the late summer and fall. Stability parameters, based on regression of cultivar × WL × year means on their respective WL × year means, differed among cultivars in analyses including all harvests but were relatively uniform ( b ≈ 1.0) for most cultivars later in the season. Differences in DMY between ‘Ky 31’ tall fescue infected with the Neotyphodium endophyte and its endophyte‐free counterpart confirms earlier reports of the positive effect of this fungal organism on forage yield in tall fescue, particularly in water‐limited environments. Seasonal distribution of yield was primarily determined by water availability during the late summer and fall. The relative consistency in DMY of the cultivars across WL indicates that annual yield averaged across levels of water stress would be a logical criterion for selection of germplasm for irrigated pastures in the Intermountain Region.