Ecological Studies on Weed Damage of Rice Plants in Rice Cultivation. : I, II : On the mechanism of competition between rice plants and weeds.

Abstract

In order to establish the diagnostics of weed damage of rice plants synecological study is now in progress. This paper deals with the mechanism of competition between rice plants and weeds in rice cultivation. The experiment was carried out with three plots, in which the kind and population of growing weeds were regulated by sowing, thinning or additional transplanting of weeds early in July after the rice seedlings were transplanted in paddy field on June 15, 1955. The process of competition between rice plants and weeds were investigated. The results obtaind are summarized as follows : (1) The kind and population of growing weeds affected on the growth and yields of rice plants. In plot A where weeds growed scarcelly the highest yield of rice was obtained ; in plot B with many kinds and population of weeds yields of rice were 82.2 % of that of plot A ; and in plot C where weeds growed numerously in the kind and population yields were only 57. 1% of that of plot A. (Table 1 and 4) (2) Since the second ten days of July the growth of rice plants and weeds tended to be vigorous and the competition became serious. The nitrogen competition was the most serious in plot C and was moderate in plot B. (Table 2) (3) The productive structure of plant-communities were such as shown in Fig. 1 and the competition in light was the most serious in plot C at about full-tillers stage. (4) The difference in the competitions between three plots was owing to the difference of the growth habit and population of weeds. The gowth habit of weeds varied with species of weeds in height, dry weight, nutrient absorptivity, etc. Panicum Crusgalli L. was the most competitive and competitiveness of the same weed varied with the difference of community structure. (Tatble 3) (5) As above mentioned, the mechanism of competition between rice plants and weeds was complicate and dynamic, so that we discussed weed damage of rice plants by analyzing it into three factors from synecological standpoint ; that is, the total weight of community including rice plants and weeds, the ratio of rice plants to total weight of community ; and the ratio of grains to weight of rice plants. Each process of three factors was shown in Table 5. Of three factors the ratio of rice plants to total weight of community was the most remarkable in fluctuation. At about the full- titters stage of rice plants this ratio approached to the minimum value and thereafter varied scarcely. With other two factors the variation between plots was scarcely or less. (6) These results suggest that the early and accurate diagnosis of weed damage of rice plants in rice cultivation can be established by the further investigations of the process of the three factors under various conditions.