Simulated insect defoliation on soybean: Influence of row width

An ongoing change in soybean production gaining popularity in the United St ates is a reduction in row spacing. Plant canopy closure is quicker and lea f area index is greater, thus yield is usually higher. Because yield respon se to insect defoliation is primarily a function of how defoliation causes changes in light interception, the possibility exists that the insect-injur y-yield-loss relationship might differ among row widths. Soybean was grown in four states using similar methodologies. Insect defoliation was simulate d by picking leaflets based on an insect defoliation model. Plant growth me asurements were taken immediately following the end of defoliation. Numerou s independent variables were measured or calculated, including percentage l ight interception, leaf area index, percentage defoliation, and leaf area p er plot. Analyses of covariance were conducted on the resultant data to det ermine whether insect-injury-yield-loss relationships interact with row wid th. A significant interaction would indicate that the impact of the variabl es on yield was dependent on the row width, whereas a nonsignificant intera ction would suggest that the relationship between the variables and yield i s similar at all row widths. Few significant interactions were obtained, in dicating that the impact of the variables on yield is similar across row wi dths. Because of the lack of significant interactions, the insect-injury-yi eld-loss relationships previously developed should be usable across varying row widths. Thus, treatment decisions based on light interception and leaf area indices, both considered more appropriated measures of insect injury, should be applicable for all row spacings.