In a study of intraspecific competition, using perennial and hybrid ryegrasses, three planting techniques were tried in three field experiments. The first experiment measured totals of groups of competing plants and the technique can be used for progeny testing in place of plot tests when seed supply or space is limited, and by varying the number of plants within groups, changes in competition intensity can be studied. The second and third experiments were intended for aiding selection work. Measurements were made on individual central plants in similar small groups and in larger groups, including some plants exposed to interspecific competition by alternate clover rows. It was concluded that single plant tests do not give precise information on the behaviour of the same plants under competitive conditions of varying severity. The deviations can amount to higl,ly significant interactions and their degree depends on the plant material used. The main factor in rye grasses seemed to be tillering habits and Fl hybrids showing heterosis effects were particularly subject to interactions of such characters with spacing. A new method for measuring tiller number was also evaluated here.
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