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.

Due to the looming water and labor crisis, farmers are adopting dry-seeded rice establishment, in which Chinese sprangletop is becoming a major weed. Concerns about the excessive use of herbicides in controlling Chinese sprangletop highlight the need for cultural weed management strategies. Such strategies require an adequate understanding of Chinese sprangletop response to rice plant density, nutrition, and water regime. Therefore, a greenhouse study was conducted to determine the effect of water regime (aerobic and saturated), nitrogen (N) fertilization (50 and 150 kg ha−1), and rice density (0, 160, and 640 plants m−2) on the growth and reproduction of Chinese sprangletop. Chinese sprangletop plants were taller than rice in aerobic conditions than in saturated conditions. All growth parameters (shoot, root, and inflorescence biomass and leaf area, leaf weight, and inflorescence weight ratio) of Chinese sprangletop were higher in aerobic conditions than in saturated conditions when grown without rice. However, no difference was observed for these parameters between water regimes when Chinese sprangletop was grown with rice. Chinese sprangletop growth and seed production was not affected, but rice growth was affected by N rates. Irrespective of N rate and water regime, Chinese sprangletop height (34 to 59%), tiller number (87 to 92%), leaf number (83 to 89%), shoot biomass (93 to 99%), and inflorescence biomass (95 to 99%) decreased as rice density increased from 0 to 640 plants m−2. The ability of Chinese sprangletop to grow taller and produce more plant biomass (107%) and inflorescence biomass (183%) under aerobic than saturated conditions suggests the need for integrated weed management strategies for controlling weeds under water-limited environments. Such strategies might include the use of weed-competitive and drought-tolerant rice cultivars, high seeding rates, and optimum rate of fertilizer application.

Studies were conducted in a screenhouse to determine the interaction of rice residue as mulch (0, 3, and 6 t ha−1) and herbicides (nontreated, oxadiazon at 0.5 and 1.0 kg ai ha−1, and pendimethalin at 1.0 and 2.0 kg ai ha−1) on seedling emergence and biomass of barnyardgrass, crowfootgrass, junglerice, and rice flatsedge. Regardless of the residue amount, crowfootgrass and junglerice were effectively controlled by all herbicide treatments. No seedlings of these weed species escaped the herbicides when applied in the presence of residue cover. There was no survival of barnyardgrass seedlings when both herbicides were applied on bare soil (without residue cover); however, some seedlings survived oxadiazon and pendimethalin when applied in the presence of residue cover. For rice flatsedge, the herbicide applications in the presence of residue cover resulted in lower weed control than in the absence of residue. These results suggest that some weed species can escape the application of PRE herbicides in conservation agriculture systems in which residue can bind soil-applied herbicides and result in lower efficacy.

Weeds are the main constraint in the adoption of zero-till, dry-seeded rice systems because weeds and rice emerge simultaneously in these systems, and there is no standing water at crop establishment to suppress weeds. A study was conducted during the wet season of 2011 and the dry season of 2012 at the International Rice Research Institute to evaluate the performance of different herbicides in mechanized-sown, zero-till dry-seeded rice. Among the treatments evaluated, oxadiazon followed by penoxsulam + cyhalofop followed by one hand-weeding at 42 d after sowing, provided 23 to 35% higher yield than the nontreated control. The yields in the oxadiazon-treated plots were similar to those in the weed-free plots. Some weed species, including eclipta and doveweed, were not controlled by the evaluated herbicides. Therefore, there is a need to evaluate additional herbicides and to integrate them with cultural weed management strategies to manage such problematic weeds.