Ecological studies on Echinochloa crus-galli and the implications for weed management in direct-seeded rice

Echinochloa crus-galli, a C₄ grass, is one of the world’s most serious weeds. Weed management decisions for this species can be derived from knowledge of its seed biology. Studies were conducted to determine the effects of light on germination; seed burial depth and rice residue on emergence and growth; and flooding time and depth on emergence, survival and growth of this species. Light stimulated seed germination but it was not an absolute requirement for germination. The proportion of seeds germinating was greatest for seeds placed on the soil surface (92%), and emergence declined with increasing burial depth in soil; no seedlings emerged from the depth of 8 cm. A burial depth of only 0.4 cm reduced seedling emergence by 50%. Seedling emergence and seedling biomass were reduced by the addition of high level (6 ton ha⁻¹) of rice residue to the soil surface. Early and deep flooding significantly suppressed growth of E. crus-galli seedlings. In flooded conditions, with increased water depth the weed allocated more biomass to shoots at the expense of roots. The information gained from this study could contribute to improve weed control approaches. Soil inversion by tillage to bury weed seeds below their maximum depth of emergence, use of crop residue as mulch and early flooding of the crop could serve as important tools for managing E. crus-galli and other weed species with similar germination requirements. These management options, however, would need to be compatible with other crop management requirements.     

Field evaluation of Sclerotium rolfsii, a biological control agent for broadleaf weeds in dry, direct-seeded rice

The fungal pathogen Sclerotium rolfsii isolate SC64 is being assessed as the basis for a mycoherbicide for biological control of broadleaf weeds in dry-seeded rice fields. Species tested for susceptibility in the field included Cyperus difformis, Lindernia procumbens, Rotala rotundifolia, Ammannia baccifera and Eclipta prostrata. Following preliminary small plot field applications in summer 2008 and 2010, applications of fungus-infested solid substrates (mixture of rice hulls and bran) of 60-140 g m⁻² were conducted at two sites, Nantong and Rugao, in Jiangsu province, China in summer 2010. The sites included a one-year fallow field and a wheat-rice rotation field. Plant mortality was recorded 7 and 14 days after inoculation (DAI). Percentage mortality ranged from 50 to 89% and 30-71% in the 2008 and 2010 solarium small plot trials, respectively. At the Nantong site field trial, 30-60% plant mortality and 31-59% fresh weight reduction were recorded at 14 DAI when applied for the first time but the efficacy increased to 39-86% and 42-90% for plant mortality and fresh weight reduction at 14 DAI with a repeated application. Higher levels of plant mortality (42-77%) and fresh weight reduction (52-82%) were achieved at 14 DAI at the Rugao site with a single treatment, due to the lower weed density and more favourable temperature and humidity conditions at the time of pathogen application. Results confirmed that S. rolfsii SC64 is a potential biocontrol agent of some of the broadleaf weeds tested in dry-seeded rice.     

Effect of herbicides on weed management in dry-seeded rice sown under different tillage systems

In India, dry-seeded rice (DSR) production systems are rapidly replacing conventional rice production systems due to various advantages. DSR systems can be managed under zero-till (ZT) conditions or after a preparatory tillage, often referred to as conventional tillage systems (CONT). Although previous reports indicate the contribution of tillage to weed suppression, the effect of one-time preparatory tillage in a DSR system could vary depending on the dominant weeds in the system, vertical seed distribution and the weed seed dynamics. A study was conducted to test the efficacy of ZT and CONT and their interaction with herbicide treatments on the weed population dynamics and rice grain yield in 2010 and 2011. Tillage systems did not affect weed emergence, weed biomass, tiller production and crop yield. However, herbicide treatments varied in their efficacy on individual weeds. Hand-weeding treatments and pendimethalin combined with hand weeding did not effectively control Cyperus rotundus L. and Panicum maximum Jacq. (a perennial grass weed with underground parts). The herbicide combination of metsulfuron and chlorimuron was effective in controlling C. rotundus but not grass weeds. This indicates the need for sequential applications of herbicides for grass weed control or integration of hand weeding to achieve broad-spectrum weed control. Apart from hand weeding (three times), treatment with penoxsulam–cyhalofop and pendimethalin followed by (fb) hand weeding resulted in low weed density, high tiller production and grain yield. The study clearly indicates that tillage does not always lead to weed suppression compared with ZT, and herbicides must be chosen based on the dominant weeds in a system. The results of this study are pertinent as herbicide-resistant weeds are rapidly evolving under continuous herbicide selection pressure, which warrants studies on enhancing productivity through low-input, environmentally friendly and sustainable production technology.