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Plot Type and Treatment Method for Assessment of Soybean Response to Defoliation¹

Extensive labor is required to simulate soybean [Glycine max (L.) Merr.] defoliation in bordered row plots by removing a designated percentage from each leaf on all plants. We evaluated the use of unbordered hill plots and removal of all leaflets from a designated percentage of plants in a plot as a potential means of reducing the labor requirement. A determinate and an indeterminate cultivar were grown in an unbordered hill-plot experiment that was adjacent to a bordered row-plot experiment during 1981 and 1982. The leaf method of defoliation consisted of removing 0, 1, 2, or 3 of the leaflets from every leaf on all the plants in a plot, and the plant method consisted of removing all the leaflets from 0, 33, 66, or 100% of the plants in a plot. The treatments were imposed at the full-bloom (R2) and beginning-seed (R5) stages. The average yield loss from defoliation was 27.6% in the hill experiment and 17.7% in the row-plot experiment. Although the main effect of plot type was not significant when tested with the plot type by year interaction, the greater loss in hill plots was consistent across cultivars, defoliation methods, and years. The plant method of defoliation had an average yield loss of 22.6%, which was significantly greater than the 16.2% loss obtained by the leaf method. Significant differences for 100-seed weight were observed between defoliation methods, but not between plot types. Differences between plot types and defoliation methods for changes in maturity, height, and lodging were not significant. Hill plots should not be considered as substitutes for bordered row plots in estimating the yield response of a commercial soybean field to defoliation. The plant method of defoliation required less labor for imposing treatments than the leaf method, but the estimates may be biased by the effects of interplant competition in a mixture of defoliated and undefoliated individuals.

Key Words: Glycine max (L.) Merr. • Leaf removal • Yield reduction • Maturity • Lodging • Height • 100-Seed weight

² Graduate student and professor, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication December 27, 1983.