Ludwigia is an important broadleaf weed of direct-seeded rice in Asia. Crop interference that relies on shading may have potential as a component of integrated weed management strategies but it requires understanding the extent to which rice can interfere with weed growth and how these weeds may respond. The growth of ludwigia was studied when grown alone and in competition with 4 and 12 rice (cv. IR72) plants. Rice interference reduced ludwigia height, number of branches, and shoot and root biomass. However, ludwigia showed the ability to reduce the effects of rice interference by increasing leaf weight ratio, increasing stem and leaf biomass in the upper half of the plant, and increasing specific stem length. At 11 wk after seeding, for example, ludwigia grown with 12 rice plants had 38% greater leaf weight ratio compared to plants grown alone. When grown with 12 rice plants, the weed had 82% of its leaf biomass in upper half of the plant compared to only 25% in weeds grown alone. The results showed that ludwigia responded to rice interference with a combination of adaptations typical of many weed species. Despite such plasticity, the control of ludwigia may be achieved by dense rice stands and increasing interference.

Two separate studies were conducted to determine the relative importance of root and shoot competition in dry direct-seeded rice growing with junglerice and ludwigia. By growing rice in pots placed within larger pots such that the roots of the plants were either separated from or free to mingle with those of neighboring weeds, or by growing rice in the same pots but in the absence of weeds, the relative importance of shoot and root competition can be described. When rice was grown together with either weed species, shoot competition reduced the growth and yield of rice more than root competition. Results suggest that shoot competition for light may be the primary mechanism determining competitive outcomes between dry direct-seeded rice and junglerice or ludwigia. Junglerice was more competitive than ludwigia, which may reflect the C4 metabolism efficiency of junglerice compared to the C3 metabolism of ludwigia. Rice grain yield was highly correlated with above- and belowground biomass. The results also suggest the importance of measuring the whole plant when seeking to understand differences in the competitive ability of dry direct-seeded rice.