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Crop Science Abstract -

Chemical Thinning of Soybean with Bentazon1


This article in CS

  1. Vol. 26 No. 3, p. 610-614
    Received: Apr 15, 1985

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  1. Daniel L. Jeffers2



Planting excessive amounts of seed to ensure adequate stands can sometimes result in dense stands that are susceptible to severe lodging. The objective of this research was to evaluate the feasibility of using chemical thinning of soybean [Glycine max (L.) Merr.] to adjust stands. Various seed mixtures of ‘Beeson 80’ (25 to 100%) and L75-6631, a genotype sensitive to bentazon [3-(l-Methylethyl)-lH-2,l,3-benzothiadiazin-4(3H)-one 2,2-dioxide], were planted in field plots of eight 38-cm rows, resulting in a density of about 40 plants m−2. At various times throughout the season from stages V3 to R4.5, the L75-6631 plants were removed by spraying the plots with bentazon. Yields were reduced if stands were thinned by more than 25% after first flowering. In a second experiment, seed mixtures of 70, 80,90, and 100% Beeson 80 (the remainder, L75-6631) were planted in densities of 35, 50, and 70 seeds m−2. Sparse rainfall precluded significant lodging at any density. Chemical thinning occurred at V3, V5, V6 and R1.5. Yields were similar for all treatments. In all cases L75-6631 plants were completely killed by treatment with 1.2 kg ha−1 bentazon applied during the vegetative stages. Later treatments (R3.5, R4.5) required a second spray using a one-half rate of bentazon. For practical application, thinning should occur during vegetative stages to minimize yield effects and maximize control of sensitive weeds that may occur. Use of chemical thinning will require breeding of bentazon-sensitive isolines of some common cultivars.

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