Alfalfa Persistence and Yield in High Density Stands
The impact of competition on plant persistence and plot yield has not been well characterized in alfalfa (Medicago sativa L.). In an even aged stand the effects of density, and thus competition, at three levels were elucidated with respect to rate of plant mortality and changes in the plant and plot yields. The genetic materials used were three semidormant alfalfa populations with very different percentages. The replicated experimental plots, each with 483 plants, were space planted in the field, an Orr loam (fine-loamy, mixed, mesic Aridic Argixeroll), to stand 10, 17.3, and 30 cm on centers. The relative densities were in a ratio of 9:3:1 per unit area. Over 2 yr of data were collected on yield and plant loss, and expressed in terms of the lowest density to adjust for environmental changes over the growing seasons and to isolate genetic differences among the populations from the effects of density. The rates of thinning were linear over the life of the experiment, but different for each density. The yield, when measured on a per plant basis, increased faster than that explained by the rate of thinning for the highest density, and the yield per plot had a net gain for both the medium and high density plantings in all three populations. Thus, it appeared taht competition eliminated some of the less vigorous genotypes; this suggested that selection through competition could be effcetive in a space planted nursery with plants at 17.3 cm on centers or less, and if the trait called competitiveness is heritable.
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