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Intraspecific Competition, Competition with Weeds, and Spacing Response in Rice¹

Intraspecific-intergenotypic competition, competition with weeds, and spacing response were studied in 25 rice (Oryza sativa L.) cultivars with different growth habits in Lambayeque, Peru. The actual yield of genetically mixed rice populations was always below the yield of the better component genotype in pure populations. The yield of the mixtures seldom exceeded the average yield of the pure populations of the component genotypes. Thus, yield reduction in one genotype as a result of competition was not compensated by the yield increase in another.

Intraspecific competition, competition with weeds, and spacing response were highly intercorrelated with each other, suggesting that these were controlled largely by the same genetic factors through the same physiological process. Vegetative vigor, large leaf area, a high rate of N absorption in early growth stages, and plant height were the most significant characters related to competitive ability. Tillering itself was not related to either competitive ability or to spacing response. Tall, vigorous genotypes with a long growth duration were adapted to extremely primitive agronomic conditions. The evolution of cultivars has been accompanied by the evolution of cultural methods and the loss of competitive ability.

Key Words: Genetic mixture • Pure stand • Competitive ability • Nitrogen response • Vegetative vigor • Growth duration

² Rice breeding specialist, NRP, and Visiting Assistant Professor, NCSU (presently Associate Plant Breeder, Centro International de Agricultura Tropical, Apartado Akreo 67-13, Cali, Colombia); and Rice Breeder, and Weed Agronomist, NRP, CRIAN, Ministerio de Agricultura, Chiclayo, Peru; respectively.

Received for publication April 24, 1974.

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