Selection in heterogeneous soybean populations: 2. Competitive ability

A main objection against individual plant selection at low plant population is a tendency to select strong competitors which don’t realize their yield potential in a genetically uniform and dense cenosis. A ‘green revolution’ paradigm implicates decreasing competitive ability to develop cultivars adapted to dense cenosis, while breeding progress is mostly attributed to increase of a harvest index. However, progress in soybean breeding in various countries is predominantly caused by total dry biomass increase rather than its redistribution, although the data are some inconsistent. Strong competitors better suppress weeds and possess greater yield stability in case of local irregularities of plant population. Interrelations of competitive ability and seed yield of early soybean genotypes were studied in model populations consisting of 000-00 maturity group cultivar mixtures. Competitive ability of genotype was expressed as a ratio of the given genotype’s average yield in the model population to its yield in a pure line crop. Competitive ability was nearly completely governed by a total node number per plant (R = 0.83) and was not in conflict with seed yield in the pure line crop (r = +0.09 to +0.18). The advantage of better yielding genotypes was not due to the harvest index (r = -0.29) but to the greater total dry biomass (r = +0.85). Thus, we consider plant selection in hybrid nurseries at low plant population to be reasonable in order to reduce the major component of genetically useless variance that is ecological (intragenotypic) competition.