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Plant competition for available resources is important for most crops, but it is particularly critical for grasses growing in water-limited rangelands of the western United States and similar environments. A wagon-wheel design was used to study the responses of 30 crested wheatgrass (Agropyron desertorum [Fisch. ex Link] Schultes) clonal lines to eight intraspecific competition levels. Plots were established at two locations, D (irrigated) and E (nonirrigated). Differences among clonal lines and competition levels were significant (P < 0.01) for dry matter yield when expressed as g plant(-1) (DMYP) and Mg ha(-1) (DMYH). Genetic variances and heritability values for DMYP increased markedly as the level of competition decreased; however, trends for genetic variances were less consistent for DMYH. A largely linear increase was found in DMYP under wider plant spacings, particularly under more arid conditions with no supplemental irrigation at location E. Maximum DMYP occurred at spacing level 8 (area = 1.28 m(2)) under dryland conditions at location E; however, with irrigation at location D, DMYP tended to level off at spacing 6 (area = 0.7 m(2)). A curvilinear response to increasing levels of plant spacing was found for DMYH at both locations. Entry X spacing-level interactions indicated that screening of crested wheatgrass breeding populations at spacings substantially wider than that found under actual range conditions would not be effective, particularly during the final stages of a breeding program or when genetic resources are limited.