The effect of selective herbicide, MDBA( 2-methoxy-3, 6 -dichlorobenzoic acid dimethylamine), on the control of Rumex obtusifolius was examined in a permanent meadow at Kuju Agriculturural Research Center, Kyushu University during 1989-90. The plots (2×3 m) in the meadow were allocated to two treatments of application; once (late autumn) a year and 3 times (each post-harvest) a year, and to 3 sub-treatments of spraying rate; 75,100 and 200ml/100i water. The population of green Rumex obtusifolius was recorded monthly, and the growth was also observed. The results obtained were as follows : 1) The leaves of Rumex obtusifolius were turned red and wilted 2-3days after spraying with MDBA. However, the extent of the damage varied depending on the rate of spraying or the size of the plant, i. e. the plants sprayed at lower rate or in larger size were not so heavily damaged, so that they regenerated within a week. 2) The marked effect of spraying herbicide on Rumex obtusifolius was found when the plants had more leaves in spring and autumn during the year, whereas there was no effect in droughty summer. 3) The seedlings of Rumex obtusifolius emerged newly during the year were thoroughly destroyed by MDBA treatment. 4) The MDBA treatment did not damage to the growth of grasses such as Dactylis glomerata, Agrostis alba and Festuca arundenacea.
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