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Yield Reduction from Defoliation of Irrigated and Non-Irrigated Soybeans¹

Hail adjusters, entomologists, and others frequently use defoliation percentages to estimate soybean [Glycine max (L.) Merr.] yield loss caused by hail, insects, or diseases. Little information is available on the percent yield loss from defoliation under drought stress and adequate moisture. Thus, the objective of this research was to measure yield response of a determinate soybean cultivar ‘Lee 74’ to different levels of defoliation under irrigated and non-irrigated conditions. Field experiments were conducted on a Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualfs) at Stuttgart, Ark., during a 3-year period to study effects of four levels of defoliation (0, 50, 75, and 100%) applied at three stages of development (V5, R2, and R4) in 1974 and 1975 and two stages (V3 and R5) in 1976 on yield of irrigated and nonirrigated soybeans.

Irrigation significantly increased yield 7% in 1974, 38% in 1975, and 51% in 1976. Mean squares for levels of defoliation, stages of treatment, and the defoliation x stage interaction were highly significant each year. The least reduction in yield occurred when plants were defoliated at vegetative stages, V3 and V5, and the most at reproductive stages, R4 (full pod) and R5 (beginning seed). Average yield reductions for 50, 75, and 100% defoliation were 11, 17, and 37%, respectively.

Percent yield reductions under irrigated and non-irrigated conditions were similar because all interactions with irrigation treatments were non-significant. This same pattern existed regardless of whether the season was extremely dry as hi 1975 and 1976 or moderately dry as in 1974. Reduction in number of pods appeared to be the yield component primarily responsible for yield losses from defoliation. Results from these experiments indicated that percent reduction in yield from defoliation is similar for soybeans grown with adequate moisture or under drought stress.

Key Words: Glycine max (L.) Merr. • Stage of deevlopment • Drought stress • Leaf removal

² Professor of agronomy, Univ. of Arkansas, Fayetteville, AR 72701 and research assistant, Univ. of Arkansas, Rice Branch Exp. Stn., Stuttgart, AR 72160.

Received for publication July 16, 1979.

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