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Soybean Recovery from Plant Cutoff, Breakover, and Defoliation¹

The current procedure for assessment of hail injury to soybeans (Glycine max (L.) Merr.) assumes that plant cutoff, breakover, and defoliation cause equivalent percentages of yield loss. Our objective was to compare the yield response of soybeans to the three types of injury in field experiments. Plants were cut off at half height and defoliated below the cut, broken over rit half height and defoliated below the break, or completely defoliated. The treatments were applied to 33, 66, or 100% of the plants at the V5, R2, R3, R4, and R5 stages of development at one location in Iowa and at the R2 to R5 stages at one location in Minnesota during 1981. Yield losses from the three types of injury were not the same. Removal of the top half of the main stem by the cut-off treatment was generally more severe than breaking it over or removing the leaves, especially when 100% of the plants were treated at later stages of development. At V5 and R2, breakover tended to cause greater yield loss than defoliation; at R3, the two treatments were the same; and at R4 and R5, defoliation caused greater yield loss than breakover. Reduction in seed weight and changes in time of maturity were generally small, except when 100% of the plants were treated. The treatments did not cause changes in lodging scores of practical importance when brokenover plants were ignored. Small reductions in plant height were observed for breakover and defoliation, but substantial reductions were observed for cutoff. The three types of plant injury should be considered independently in assessing damage caused by hail.

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

² Professor and graduate research assistant, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

³ Professor of agronomy and plant genetics, Univ. of Minnesota, St. Paul; professor of agronomy and plant genetics, Southwest Exp. Stn., Lamberton; and professor of soil science and superintendent, Southwest Exp. Stn., Lamberton, Minn.

Received for publication July 19, 1982.

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