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Selected Determinate Soybean Cultivar Yield Responses to Row Spacings and Planting Dates¹

Previous studies to determine row spacing (RS)-planting date (PD) effects on yield of determinate soybeans [Glycine max (L.) Merr.] in the southern USA have been conducted north of 33° Lat, and have produced conflicting results. Current data on the effects of these planting variables are not available for a large portion of the Gulf Coast area that now supports a large and rapidly expanding soybean industry. The objectives OP this experiment were to study: 1) the effects of selected RS and PD on yield of selected cultivars of determinate soybeans; 2) the interaction of RS with PD; and 3) the interaction of cultivars with RS and PD.

Five cultivars, ‘Forrest,’ ‘Davis,’ ‘Lee 74,’ ‘Centennial,’ and ‘Bragg’ were planted in RS of 25, 50, and 100-cm on 6 PD between 15 April and 1 July in 1976, 1977, 1978, and 1979. The field plots were located on a Mhoon silty clay soil (Typic Fluvaquent). Averaged over years, the five cultivars evaluated produced significantly higher yields when grown in 25 and 50-cm RS compared with the 100-cm RS. The interaction between cultivar and RS was significant due to variability among cultivars in the yield increase when RS was reduced; 3 q/ha with Davis and Bragg, 4 q/ha with Lee 74, and 6 q/ha with Forrest and Centennial. The size of yield increases resulting from reduced row spacing was relatively consistent across PD. Because lower yields were recorded at all three RS at nonoptimum PD the yield increases, when expressed as a percentage of the yields obtained in 100-cm rows, were larger at the nonoptimum dates, however. Optimum PD was not affected by RS but differed among cultivars. Forrest and Lee 74 had highest yields when planted on 15 May while the other three cultivars had similar yields at PD from 1 May to 1 June. Results indicated that significantly higher yields are possible with current determinate cultivars when both RS and PD are selected for individual cultivars.

Key Words: Glycine max L. • Merr. • Narrow rows

² Associate professor of agronomy, N.E. Louisiana Exp. Stn., Louisiana State Univ., St. Joseph, LA 71366; professor of experimental statistics, Dep. of Exp. Statistics, Louisiana State Univ., Baton Rouge, LA 70803; and research associate, N.E. Louisiana Exp. Stn., St. Joseph, LA 71366.

Received for publication September 2, 1980.

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