Iron deficiency chlorosis in soybean: Soybean seeding rate and companion crop effects

Iron deficiency chlorosis (IDC) is an important production problem in soybean [Glycine max (L.) Merr.] grown in the upper Midwest. Although IDC-tolerant varieties are available to producers, even tolerant cultivars suffer from IDC-related stresses. Several cultural methods have been utilized in an attempt to reduce IDC losses. Interplant competition from increased soybean seeding rates or companion crops appears to reduce IDC symptoms. Two related studies were established to examine increased seeding rates and the planting of companion crops to reduce IDC of soybean in eight IDC-prone environments. In one study, soybean was planted at five seeding rates from 432000 to 926000 seeds ha(-1). In a second study, a glyphosate-resistant soybean variety was planted as a sole crop or with companion crops of glyphosate-susceptible soybean or oat (Avena sativa L.). Seeding rate treatments affected IDC scores and yield, but these effects were confounded by environmental interactions. Overall, yield increases from higher seeding rates tended to be small and inconsistent, but among seven environments exhibiting mild to moderate IDC symptoms, increasing seeding rates from 432000 to 926000 seeds ha(-1) increased yields by 281 kg ha(-1) (16%). Blending of a glyphosate-susceptible with a glyphosate-resistant variety seemed to have little positive effect on yield of soybean (yield gains varied from -333 to + 566 kg ha(-1)). Likewise, using oat interseeded with soybean to reduce IDC symptoms and increase soybean yields is unlikely to be a profitable practice for soybean producers.