Intergenotypic Competition Between Rows and Within Blends of Soybeans 1

Intergenotypic competition among soybean ( Glycine max (L.) Merrill) cultivars was evaluated in competing row plots and in two‐component blends. The objectives were to determine the importance of intergenotypic competition on soybean performance in unbordered plots and to evaluate the use of estimates of interplot competition for predicting intergenotypic competition in blends. Five soybean cultivars, ‘Chippewa 64’, ‘Hark’, ‘Corsoy’, ‘Provar’, and ‘Amsoy’, were grown in three‐row plots with 25, 50, 75, and 100 cm between rows. The center row was the test cultivar, and the border rows were either the cultivar itself (pure stand) or one of the four other cultivars. All possible pairs of the five cultivars were tested in a northern and a central Iowa location for 2 years. The average change in yield caused by intergenotypic competition increased from 2.6% in 100‐cm rows to 17.6% in 25‐cm rows. Significant yield changes occurred at all row spacings, particularly when Amsoy was either the test cultivar or the competing cultivar. Maturity, height, and lodging were not significantly affected by competition at any row spacing. Soybean yield tests with cultivars and growing conditions similar to Iowa should use bordered plots if no bias from intergenotypic competition can be tolerated. The soybean cultivars were grown in two‐component blends at the ratios 3:1, 1:1, and 1:3. The blends contained cultivars with different pubescence color so they could be harvested separately. The blends and pure stands of each cultivar were grown in bordered plots spaced 68 cm apart. The blend test was grown adjacent to the interplot competition test in all environments. Intergenotypic competition in blends was most closely predicted by competition between 25‐cm rows, but the relative competitive ability of cultivars in blends was not always the same as their relative competitive ability in adjacent rows.