Genotype by competition level interactions in alfalfa (Medicago sativa L.)

Competition among plants for available water and other resources seriously limits the persistence and productivity of alfalfa (Medicago sativa L.) and other forage plants on semiarid rangelands of western North America and si milar environments throughout the world. In alfalfa breeding programs, scre ening and selection are often done in space-planted nurseries, which permit s the breeder to maintain the identity of individual genotypes. However, co mparative differences among plants grown as spaced plants and under the lev els of competition characteristic of actual range conditions are not unders tood. A wagon-wheel design was employed at two field locations (D and E) ne ar Logan, Utah, to evaluate trends in dry matter yield (DMY) in 14 clonal l ines of NC-83-1 and Spredor 2 alfalfa under nine levels of intraspecific co mpetition. Differences among clonal lines within and across cultivars were significant at both locations and during each of the 2 yr of the study. Dry matter yield of clonal lines within each cultivar generally increased in a linear manner as plant spacings increased, particularly at location E, whe re no supplemental irrigation was applied. As indicated by ranges in mean v alues and genetic variances, opportunities for selection for DMY increased substantially at wider plant-spacing levels. Significant entry x spacing-le vel interactions and the magnitude of correlations among spacing levels ind icated, however, that relative differences among clonal lines were not cons istent at the different spacing levels. Based on these trends, we conclude that alfalfa breeding populations should not be screened at spacings substa ntially wider than those found under actual range conditions, particularly when making final selections in the development of a cultivar.