Effects of Continuous Application of Paraquat on the Species Composition and Volume of Weeds in the Upland Field

Abstract

We applied Paraquat herbicide to the same experimental fields (raising seedling fields of vegetable at the experimental farm of Faculty of Horticulture, Chiba University) three times a year for twenty years (1965-1984) consecutively (adding a mixture of herbicide 2, 4-D and Amitrole for the first seven years), and the effect of the herbicide application on the kinds and volume of weeds, that vegetated in the fields, was investigated (Tables 1, 2 and 5).
1. In the fields where weeding was practiced by man, no significant change was observed in the species composition and volume of weeds during the period of study (Table 3).
2. In the herbicide-treated fields, on the other hand, perennial and annual winter weeds disappeared after the continuous herbicide application for the twenty year period (Table 4).
3. The unique pattern of disapperance shown by each kind of weed was investigated. For perennial weeds, we assume the pattern might have been induced by such factors as the characteristics of the herbicide used and substantial time needed for the weeds to regenerate their plant body. For annual winter weeds, the pattern might be attributed to their specific vegetative period of winter.
4. As for annual summer weeds, the volume of vegetation was reduced to the 15% level of the initial value and remained unchanged for the rest of the period (Table 4).