A Growth Retardant Improves Performance of Soybeans Relay Intercropped with Winter Wheat1

Abstract

When soybeans [Glycine max (L.) Merr.] are relay intercropped with winter wheat (Triticum aestivum L.), the soybeans may grow excessively tall and interfere with wheat harvesting. The objective of this study was to evaluate a chemical growth retardant, BTS44584, for its effects on soybean growth and yield. In relay intercropping the chemical was used at varying rates as a soybean seed treatment or as a foliar treatment at the second trifoliolate stage, either broadcast sprayed on both wheat and soybeans using a hand‐carried five nozzle boom or directly sprayed on soybean foliage only, using a single wand and nozzle. As a seed treatment the growth retardant was also evaluated for its compatibility with the fungicide, pyroxyfur, and for its growth and yield effects on monocropped soybeans. In relay intercropping seed or foliar spray treatments retarded soybean growth such that height at the time of wheat harvesting was significantly less than for untreated soybeans. The chemical treatments prevented substantial combine defoliation of intercropped soybeans as occurred with controls during wheat harvesting. Height of treated soybeans at maturity ranged from 11 to 32 cm taller than untreated soybeans resulting partly from this defoliation of untreated plants and partly from delayed development and continued vegetative growth of treated soybeans. Maturity of treated soybeans was delayed an average of 3 to 11 days depending on method and rates of application. Seed yields from treated soybeans were not significantly larger than those from the untreated in most instances, the exceptions being those receiving the 0.5 g/kg seed treatment and those receiving the directed foliar sprays. Seed treatment with 0.1 to 0.5 g/kg would be the preferred method on the basis of efficacy, ease of application and economy. Pyroxyfur and BTS44584 were found to be compatible as seed treatments. In monoculture soybean growth and yield were decreased by BTS44584 seed treatments ranging from 0.1 to 2.5 g/kg of seed.