Use of Nitrogen Stress to Demonstrate the Effect of Yield Limiting Factors on the Yield Response of Soybean to Narrow Row Systems1

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Abstract
Most research indicates yields of soybean [Glycine max(L.) Merr.] are increased as row width is narrowed and in general indicates that the optimum seeding rate per hectare increases. In spite of good management (appropriate seeding rates and good weed control), the anticipated yield increase from narrow rows is not always observed. It was postulated that this may be due to the effect of some other yield limiting factor or factors in the environment. In this study, N stress was used as a model to test this hypothesis. Three experiments were conducted to evaluate the effect of N stress on the yield advantage of a 17‐cm row system over a 75‐cm row system. In two of the experiments, a genetic non‐nodulated cultivar (‘Clark rj1,’) was compared with normal cultivars (‘Corsoy’ and ‘Williams’) in 17‐ and 75‐cm row widths across four rates (0, 55,110, and 220 kg ha−1) of added N. In the third study, a normal cultivar (‘Beeson’) was grown in similar narrow and wide row systems on soil (fine‐loamy, mixed, mesic Agric Fradgiudalf) with a low level of Rhizobium japonicum(Kirchner) Buchanan and four rates (0,8.8,17.6, and 26.4 kg ha−1) of granular R. japonicuminoculant were used. Results from all three studies dramatically demonstrated that the potential yield advantage (15 to 25%) of the 17‐cm row system over the 75‐cm row system was minimized or eliminated under N stress situations. These results corroborate other research results that indicate late season moisture stress has a similar effect and strongly supports the hypothesis that any yield limiting factor, such as lodging, moisture stress, N stress, or other nutrient stress can significantly reduce the yield response of soybean to a narrow row system. This relationship may help explain previously published data which appears to be contradictory, as well as some of the variability in yield response to narrow rows reported by soybean growers.