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Response of Indeterminate and Determinate Soybean Cultivars to Defoliation and Half-plant Cut-off¹

The estimation of yield loss from defoliation and stem cut-off is based on the assumptions that soybean (Glycine max (L.) Merr.) cultivars do not differ appreciably in their ability to recover from plant injury, recovery is similar across locations, and percentage yield loss at all reproductive stages can be approximated from research involving only three of the stages. The objective of our study was to test the validity of the assumptions by comparing the response of indeterminate and determinate soybean cultivars to 100% defoliation and 100% half-plant cut-off at six reproductive stages from R2 to R7. Two indeterminate cultivars, ‘Hark’ and ‘Beeson’, were studied at Ames, Iowa, and Lafayette, Ind., and two determinate cuitivars, ‘Hill’ and ‘Lee’ were evaluated at Stuttgart, Ark.

The determinate cultivars had significantly greater yield reduction from 100% defoliation than did the indeterminate cultivars at all reproductive stages, except R7. Average yield reduction from defoliation for all stages was 59% for the determinate cultivars, compared with 39% for the indeterminate cultivars. Maximum yield loss from 100% defoliation occurred at R4 (86%) and R5 (88%) for determinate cultivars and at R5 (82%) for indeterminate cultivars. Average yield loss from half-plant cut-off was similar for indeterminate (34%) and determinate (33%) cultivars, but there was a significant interaction with stages. Yield loss for indeterminate cultivars increased from R2 to R5, then remained constant from R5 to R7. The yield loss from half-plant cut-off with determinate cultivars increased progressively rom R2 to R7. The determinateness of a cultivar should be considered when assessing yield reductidn from defoliation and half-plant cut-off during reproductive development. Damage from hail, insects, and other factors will be most accurately estimated by establishing separate loss values for indeterminate and determinate types.

There was no significant difference between Iowa and Indiana in the percentage of yield loss from 100% defoliation. For the cultivars and locations studied, location effects were not important in the estimation of yield loss from plant injury.

Yield loss from 100% defoliation at the six reproductive stages did not follow the relationship predicted from previous studies. Our data indicated that maximum loss for indeterminate cultivars occurred at R5, not R4 as has been assumed. Some yield loss occurred at R7 indicating that the description used to identify plants at physiological maturity was incorrect. A new description based on pod or seed maturation is needed to identify plants that have reached physiological maturity.

Key Words: Glycine max (L.) Merr. • Leaf removal • Physiological maturity • Seed size • Lead area • Stages of development

² Professor of agronomy, Iowa State Univ.: professor of agronomy, Univ. of Arkansas; and associate professor of agronomy, Purdue Univ., respectively.

Received for publication February 14, 1977.

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