Simulated insect defoliation on soybean: influence of row width.

An ongoing change in soybean production gaining popularity in the United States is a reduction in row spacing. Plant canopy closure is quicker and leaf area index is greater, thus yield is usually higher. Because yield response to insect defoliation is primarily a function of how defoliation causes changes in light interception, the possibility exists that the insect-injury-yield-loss relationship might differ among row widths. Soybean was grown in four states using similar methodologies. Insect defoliation was simulated by picking leaflets based on an insect defoliation model. Plant growth measurements were taken immediately following the end of defoliation. Numerous independent variables were measured or calculated, including percentage light interception, leaf area index, percentage defoliation, and leaf area per plot. Analyses of covariance were conducted on the resultant data to determine whether insect-injury-yield-loss relationships interact with row width. A significant interaction would indicate that the impact of the variables on yield was dependent on the row width, whereas a nonsignificant interaction would suggest that the relationship between the variables and yield is similar at all row widths. Few significant interactions were obtained, indicating that the impact of the variables on yield is similar across row widths. Because of the lack of significant interactions, the insect-injury-yield-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.