Abstract
A field experiment was conducted in Fukuoka, Japan for evaluating effects of plant top and root competition on DM yield, N yield, and N_2-fixation. There were seven treatments replicated three times: three mono-croppings of cowpea (Vigna unguiculata, TVu 4557), tall corn (Zea mays, Koh-1), and dwarf corn (TX-74) and four cowpea/corn inter-croppings of two levels of plant top competition×two levels of plant root competition. Inter-cropping cowpea with tall corn or with dwarf corn facilitated two levels of plant top competition. Burying wooden partitions in between rows of corn and cowpea or at right angle with these rows facilitated two levels of plant root competition. Results obtained were as follows: 1) When compared with other coppings, the tall crorn/cowpea inter-cropping created favourable canopy structure for increasing DM yield, showing even vertical distribution of leaves throughout the canopy. 2) Dry matter yield was higher in the tall corn/cowpea intercropping, followed by the dwarf corn/cowpea inter-cropping, showing relative yield totals of 132 and 124, respectively. Root competition created favourable effects on increasing DM production in the tall corn/cowpea but not in the dwarf corn/cowpea intercropping. 3) Nodule formation and N_2-fixation changed significantly with combinations of cowpea and corn. Among the factors comprising N_2-fixation (C_2H_2 reduction), nodule number and nodule weight decreased as light environments on cowpea become worse while specific nodule activity increased. N_2-fixation decreased with root competition in the tall corn/cowpea but not in the dwarf corn/cowpea inter-cropping. 4) Nitrogen yield in the inter-croppings were higher than in the corn mono-croppings and lower than in the cowpea monocropping, showing lower relative yield totals. Relative yield totals of N were reduced with root competition. With the results obtained above, discussion was directed towards depicting the mechanisms in which higher DM yield was obtained in the tall corn/cowpea inter-cropping when root competition existed. A conclusion of this experiment is that plant top competition for light is functioning more significantly in determining DM yield than the competition for soil nutrients. Finally, a suggestion was made for the ideal types which maximize productivity in the corn/cowpea inter-cropping, viz., tall stem with leaves on the top layer for corn and no-climbing type for cowpea.
