Mixtures of Soybean Cultivars to Minimize Yield Loss Caused by Iron-Deficiency Chlorosis 1

Seed mixtures of soybean [Glycine max (L.) Merr.] Cultivars are an alternative to a pure stand of a high-yielding susceptible or a lower-yielding resistant cultivar for minimizing yield loss from iron-deficiency chlorosis. Our objective was to determine the relationship between the yield of mixtures and the frequency of component cultivars that differ in their level of resistance to iron-deficiency chlorosis. Seed of a highly resistant line A2 was mixed with each of four cultivars; ‘Weber’ (moderately resistant), ‘Wells’ (moderately susceptible), ‘Corsoy’ (susceptible), and ‘S1492’ (highly susceptible). The proportions varied by 10% increments from 90% A2 to 10% A2. The nine mixtures of each cultivar pair and a pure stand of each component were evaluated in Iowa on calcareous soil in replicated yield tests at seven environments. The cultivar pair ✕ component frequency interaction was highly significant because the yield of the mixtures varied with the amount of yield reduction sustained by the susceptible component. There were no significant differences in yield among frequencies for the A2-Weber mixtures at any of the seven environments. The results indicate that there would not be a benefit from mixing a high-yielding cultivar with moderate resistance and a lower-yielding cultivar with a high level of resistance. The average percentage of A2 required in a mixture to obtain a yield within 10% of a pure stand of A2 was 20% for A2-Wells, 33% for A2-Corsoy, and 39% for A2-S1492. Productivity of fields planted to a seed mixture, resistant cultivar, or susceptible cultivar depends on the percentage of calcareous area in the field, the yield difference between the resistant and susceptible cultivars on noncalcareous soil, and the level of chlorosis susceptibility for the cultivars being considered. Procedures are discussed for calculating when a cultivar or a mixture would be most productive.