作物と雑草との競争III : 雑草の許容限界に及ぼすオカボの種間密度効果

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In the previous papers (NUMATA and NIIYAMA 1953,NIIYAMA and NUMATA 1962), the authors found the existence of a critical time of weeding for cultivating the upland rice Oryza sativa var. terrestris (Norin No. 24), and a beneficial effect of weeds before the critical weeding time on the crop yield. This study is aimed at the further analysis of such beneficial effects of weeds on a crop. The field experiments were done according to the randomized block method with three repetitions, that is, Blocks : I, II, and III ; Sowing density treatments : A (5cm intervals), B (10cm intervals), and C (15cm intervals) ; and Weeding treatments : 1 (weed-free), 2 (no weeding during 5 weeks after sowing and thereafter weed-free), and 3 (no weeding during 7 weeks after sowing and thereafter weed-free). Here, 5 weeks after sowing is the maximum allowable period for weeds without decreasing the crop yield (NIIYAMA and NUMATA 1962). The plot size is 80×80cm and is divided into four squares (Fig. 1). The weed community in the experimental field is dominated by Digitaria adscendens and Th-D_4-R_5 type (Table 1). The growth of the weeds is, in genera, extremely depressed in the lowest density plots, C (Tables 3,6). The coverage of the weeds after June 19th is in the order of B>A>C. This fact suggests an optimum density of a crop for weed growth (Table 6). An extreme depression in the height growth of the weeds in C may not be caused by an effect of slender growth like in flax weeds and the extreme depression in the height growth of the upland rice on May 3rd be caused by a negative effect of the weeds growing longer over the critical weeding time. However, the recovery of crop growth in C3 is marked where a kind of crop-protective effect of weeds can be seen. Meanwhile, the dominant weed, Digitaria adscendens grows the most numerous adventitious roots on aboveground nodes in C. This may be a kind of adaptation in the growth form (Tables 4,5). Digitaria adscendens and Chenopodium album also grow best in C (Table 7). The distribution of the coverage of the weeds was most homogeneous on July 3rd in every plot (Fig. 2). The distribution of the weeds in a farmland comes, in general, to be most homogeneous in several weeks after sowing, through intraspecific and interspecific density regulation (NUMATA and SUZUKI 1958). The weeds affect the height growth and the number of heads of the upland rice in proportion to its sowing density (Tables 10,12). The growth of the upland rice becomes better after weeding in proportion to the lowering of density, particularly in C, and SDR (the summed dominance ratio) of the upland rice is larger in 2 than in 1 after August 7th (Table 13). The order of the heading time is contrary to the order of C>B>A in the plant height and the number of leaves of the upland rice in 3. This may be a kind of hunger phenomena. The crop yield in 2 and 3 increases as the density lowers (II in Table 15), and particularly the yield in C2 is larger than in C1. The total yield per plot indicates the tendency of 1>2>3 in A, but 2>1 in B and C. From the above-mentioned facts, the allowable limit of weeds (allowable quantity and critical weeding time) to a crop may be closely related with an interspecific density effect.
日本生態学会の論文
1969-08-01

作物と雑草との競争III : 雑草の許容限界に及ぼすオカボの種間密度効果

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