Sicklepod (Cassia obtusifolia) Shoot Structure as Affected by Soybean (Glycine max) Interference

Abstract

Shoot growth and morphology of sicklepod responded to distance of sicklepod from the soybean row and relative time of emergence of crop and weed. Sicklepod was added to soybean as either uniform stands of 26 individuals m−2or as widely spaced individuals. Sicklepod shoot height, number of main-stem nodes, number of primary branches, and shoot dry weight were decreased at 12 wk after emergence when plants were situated closer to a soybean row. Sicklepod that emerged 7 d later than soybean were shorter statured and had fewer main-stem nodes. Virtually all sicklepod overtopped the soybean, but proximity to the soybean row or later emergence reduced shoot dry weight up to 60%. A doubling in branches originating at lower main-stem nodes on sicklepod 25 cm from the row, compared to those in the row (upper branch numbers were unchanged), was associated with a 67% increase in leaves. Sixty to 90% of lower branches reached the upper canopy and thus contributed to the number of leaves above the soybean. Early-season effects of soybean interference on sicklepod reduced lower branches and, as a consequence, leaf production above the soybean late in the season, when sicklepod has the greatest impact on soybean yield.