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Competition Thinning of Alfalfa Planted at Three Densities

USDA-ARS, Crop Sci. Lab., Mississippi State, MS 39762-5367

^* Corresponding author.

Persistence in alfalfa (Medicago sativa L.) is a critical component in long term yield but little is known of the relationships among stand density, competition, and stand thinning. This study was conducted to determine if thinning of different alfalfa populations at different densities was a simple function of competition (distance between plants). Three alfalfa were space planted in a honeycomb design in the field, an Orr loams (fine-loamy, mixed, mesic Ardic Argixeroll), at 10, 17.7, and 30 cm on centers for high, medium, and low densities, respectively. After each of nine harvests, the dead plants in each of the replicated plots of 483 plants were mapped and classified by how many of the six surrounding equidistantly spaced plants were also dead. With random death the number of dead plants with 0,1...,6 adjacent plants that are also dead should have binomial distribution. Significant (P < 0.05) deviations from the binomial distribution occurred for 67,37, and 88% of the test at high, medium, and low densities, respectively. For two populations, significant (P < 0.05) deviations at both low and high densities were due to an excess of larger numbers of adjacent dead plants. Disease may have been an important cause of death in these cases. The complex interactions between density and population over time indicated that increasing stand density does not simply equate to an increase in selection pressure for survival in competitive environments.

Received for publication November 3, 1988.